Lecture Notes

for

National Programme for Capacity Building for Engineers

in

(NPCBEERM)

DEPARTMENT OF EARTHQUAKE ENGINEERINGINDIAN INSTITUTE OF TECHNOLOGY ROORKEE

EARTHQUAKE RISK MANAGEMENTEARTHQUAKE RISK MANAGEMENT

December, 2006

Sponsored by

National Disaster Management DivisionMinistry of Home Affairs

Government of India

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CONTENTS Preface1. Earthquake Engineering: An Overview 1-14

- D.K. Paul2. Elementary Seismology 15-24

- M. L. Sharma3. Basic Concepts of Vibration 25-32

- D.K. Paul4. Performance of Building in Past Earthquakes : Lessons Learnt 33-60

- D. K. Paul5. Lessons on Detailing from Past Earthquakes 61-72

- Pankaj Agrawal6. Geotechnical Considerations in Earthquake Resistant Design 73-88

- B.K. Maheshwari7. Philosophy and Principles of Earthquake Resistant Design 89-98

- Yogendra Singh8. Earthquake Resistant Design, IS:1893-2002 Code 99-122

- D. K. Paul9. Earthquake Resistant Low Strength Masonry Buildings 123-130

- Pankaj Agrawal10. Earthquake Resistant Design of Masonry Buildings 131-154

- Pankaj Agrawal11. Earthquake Resistant Design and Detailing of RC Structures 155-161

- Yogendra Singh12. Architectural Considerations and Guidelines for ERD of Buildings 162-180

- Yogendra Singh13. Seismic Vulnerability Assessment of Existing Buildings 181-208

- Yogendra Singh and D. K. Paul14. Assessment of Existing Multistoried Buildings for Desired Seismic Performance 209-218

- D. K. Paul15. Retrofitting of Masonry Buildings 219-232

- Yogendra Singh and D. K. Paul16. Retrofitting of RC Buildings 233-264

- Yogendra Singh and D. K. Paul17. Retrofitting Material 265-274

- Yogendra Singh and D. K. Paul18. Quality Control of Construction 275-286

- Anand S. Arya19. Fire Safety of Buildings 287-300

- Yogendra Singh20. Improving Wind/ Cyclone Resistance of Buildings: Guidelines 301-334

- Anand S. Arya, Prem Krishna & N.M. Bhandari21. Proposed Amendment in Town and Country Planning Legislations, Land use 335-354

Zoning Regulations, Development Control Regulations & Building Bye-laws- Anand S. Arya

22. Do’s and Don;ts 355-35623. Essential Details in Structural Drawings 357-358

- Anand S. Arya

Lecture Notesfor

National Programme for Capacity Building for Engineers

in

EARTHQUAKE RISK MANAGEMENT(NPCBEERM)

Sponsored by

National Disaster Management DivisionMinistry of Home Affairs

Government of India

December, 2006

DEPARTMENT OF EARTHQUAKE ENGINEERINGINDIAN INSTITUTE OF TECHNOLOGY ROORKEE

Disclaimer

This lecture notes are being printed for the use of teachers belonging to State ResourceInstitutions identified under NPCBEERM for their use in training the practicingEngineers from the Government Departments, private undertaking etc. No commercialuse of these notes is permitted and copies of these will not be offered for sale in anymanner.

PREFACEIt has been realized that most of the casualties and loss of property is mainly due to the widespreaddamage/ collapse of the buildings. This is mainly due to the construction practice in our countrywhich is not well regulated and buildings are being constructed without earthquake resistantconsideration i.e. the IS code of practice is not followed. Civil Engineers passed out fromvarious engineering colleges do not study Earthquake Engineering and therefore not trained todesign earthquake resistant structures.

The Ministry of Home Affairs is the nodal Ministry for Disaster Management in the countrywhich has taken many important initiatives to build the capabilities at all levels necessary forpreparing and handling all types of disasters. Some of the important initiatives are (i) ensuringthat BIS Codes on disaster safety construction are followed; (ii) development of model buildingbyelaws incorporating the disaster prevention consideration for adoption in States and UnionTerritories; (iii) introduction of Earthquake Engineering concepts in Engineering EducationCurricula etc. This has suddenly created demand for Structural Engineers and EarthquakeEngineers. Unless a large-scale capacity building programme is taken in the country, this cannotbe achieved. Therefore Ministry of Human Recourse (MHRD) has initiated National Programmeon Earthquake Engineering Education (NPEEE) for training the teachers in Engineering collegesand Ministry of Home Affairs has undertaken National Programme on Capacity Building ofEngineers and Architects in Earthquake Risk Management. These two programmes are fullyfunded by the respective Ministries. The Ministry has identified number of resource institutionssuch as IIT’s; IISc; SERC Chennai; CBRI Roorkee; and BITs Pillani. Therefore, it is importantto develop suitable training material, which can be used for such programmes. This volume isone such effort in this direction.

The course covers basic formulation of engineering seismology, theory of vibration applied tostructural dynamics, common damage to buildings during earthquakes, philosophy and principlesof earthquake resistant design and construction, earthquake resistant measures in masonrybuildings, ductility provisions for better seismic performance, Indian standard codes of practice,seismic analysis and design of multi-storey buildings, nondestructive testing methods and repair,restoration and retrofitting of buildings, fire safety of buildings and guidelines for cyclone resistantbuildings.

This volume is a compilation of the lecture notes delivered by experienced faculty membersfrom Department of Earthquake Engineering, Indian Institute of Technology Roorkee, Roorkee.We hope that these lecture notes will be extremely useful in Capacity Building Programme onEarthquake, Cyclone and Fire Resistant construction in the country.

The inspiration received from Dr. A.S. Arya, National Seismic Advisor in bringing out thislecture volume is gratefully acknowledged. The financial assistance received from MHA forprinting this volume is also gratefully acknowledged.

D.K. Paul Prof. & Head

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EARTHQUAKE ENGINEERING: AN OVERVIEW

D. K. PaulProfessor, Department of Earthquake Engg., IIT Roorkee, Roorkee, 247 667

INTRODUCTION

Earthquake Engineering deals with innovative ideas and knowledge in design and construction,which are put in practice to safeguard structures from seismic forces and prevent earthquakehazard from becoming a disaster.

The earthquakes are un-preventable and unpredictable. Earthquake causes vibratory groundmotion caused by waves originating from a source of disturbance inside the earth. These aregenerally associated with active tectonic features. Large numbers of earthquakes occur but onlythose earthquakes, which affect structures and disrupt the normal way of life, are of engineeringimportance. The loss of life and property occurs directly from failure of structures and may alsotake place due to indirect causes such as failure of water supply, fire caused by short circuitingof electric wires or kitchen fires, release of poisonous gases, release of radiation, flooding throughfailure of dams and embankments or due to tsunamis. The energy contained in different wavesof different frequencies varies significantly. When such seismic wave strikes a structure restingon ground causes it to vibrate in horizontal and vertical directions. Intensity of vibration dependson the relative frequencies of ground motion waves and the structure, and the energy contentassociated with the frequencies. The vibratory ground motion causes additional moment andshear in the structure. If a structure is not designed for the additional forces, the structure maybe severely damaged/ collapsed.

In this article, a brief historical development of Earthquake Engineering in India and world ispresented. The various factors contributing in the development of Earthquake Engineering arealso presented.

HISTORY OF EARTHQUAKE ENGINEERING

History of Earthquake Engineering is not well documented. It is a recent development. Most ofthe major developments have taken place in the last 50 years only. The first World Conferenceon Earthquake Engineering was held in 1956 in the city of Berkeley, California; second in Tokyo/Kyoto, Japan (1960); third in Willington, New Zealand (1964); fourth in Santiago, Chile (1968);fifth in Rome, Italy (1972), sixth in New Delhi, India (1977), seventh in Istanbul, Turkey (1981),eighth in San Francisco, USA (1985); and the ninth in Tokyo/Kyoto, Japan (1989), the tenth inMadrid, Spain (1922), the eleventh in Acapulco, Mexico (1986), the twelfth in Auckland, NewZealand (1999), and Vancouver, Canada(2004).

Chapter 1

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In the 19th Century, a number of English engineers developed keen interest in earthquakes.These include Robert Mallet (1810-81), an Irish Civil engineer and John Milne (1850-1913), aMining engineer. In the last Century no distinction was made between Seismology and Earth-quake Engineering. The word "Seismology" was derived from the Greek Word "Seismo" means"Shaking". It was coined by the engineer Mallet and covered all aspects of earthquakes. In factthe name "Seismo-logy" means "Shaking-Knowledge" and could have been assigned to Earth-quake Engineering itself but during the later developments, the name was used for non-engineer-ing aspect of the subject.

Mallet in 1846 presented his first paper before the Irish Academy "On the Dynamics of Earth-quakes" which later appeared in the Transactions of the Irish Academy, Vol. 2, 1848. This paperdescribed the earthquake effects and considers seismic waves and tsunamis. He also describeshis invention of the electro-magnetic seismograph. Mallet also invented the "rocking blocks" (orfalling pins) intensity meter. He also compiled a seismic map of the world, which was in use formany years. The destructive Naples earthquake of December 16th, 1857 provided him theopportunity to make an extensive field studies of seismic effects and wrote a detailed reportentitled "The Great Neopolitan Earthquake of 1857". He also compiled a 600-page catalog ofearthquakes. Robert Mallet was therefore responsible for the birth of both the subject of mod-ern Seismology and the Earthquake Engineering.

Minle, Ewing and Gray together with Seikei Sekiya (1855-96), the world's first officially ap-pointed professor of Seismology organized Seismological Society of Japan in 1880 and this earth-quake society was the fore runner of the many National Societies of Earthquake Engineeringthat make up the International Association for Earthquake Engineering. Minle was appointedprofessor of Mining and Geology at the Imperial College of Engineering in Tokyo in 1882. Hedesigned the sensitive Seismographs. The October 28, 1891 Mino-Owari devastating earth-quake provided him the opportunity to make extensive field studies and concerned with therelationship between the wave motion and the damage. This has resulted in the creation of theImperial Earthquake Investigation Committee to look into the ways of predicting earthquakes,measures to reduce such disaster by choice of suitable methods of construction, building materi-als and building sites.

Minle had carried out experiments on models to test various conclusions on design and construc-tion so as to withstand the earthquake forces better. Minle and Japanese seismologist, ProfessorOmori tested high columns of bricks on a metal frame trolley. He also tested the idea of baseisolation, now much discussed in Earthquake Engineering and actually designed a building stand-ing on cast iron balls held between metal plates which separated the building. His instrumentsshowed that slow moving earth movements were transmitted to the building while sudden shockswere not. These examples are sufficient to indicate that Minle often designated as the "father ofmodern Seismology" made great contribution to what came to be called Earthquake Engineering.

Messina, Italy Earthquake of December 28, 1908: The 83,000 death toll of the Messinaearthquake was the greatest number ever from an European earthquake. The government ofItaly had appointed a special committee composed of nine practicing engineers and five profes-sors of engineering to study the earthquake and make recommendations. The recommendationof this committee appears to be the first engineering recommendation that earthquake resistant

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structures be designed by means of the equivalent static method (%g method). This contributionis appears to have been made by M. Panetti, Professor of Applied Mechanics in Turin. Herecommended that the first storey be designed for earthquake forces equal to 1/12 the weightabove and the second and third storeys to be designed for 1/8 of the building weight above. ADanusso, Professor of Structural Engineering at Milan, won a prize with his paper, "Statics ofAnti-Seismic Construction". The method recommended by Panetti and explained by Danusso,gradually spread to seismic world.

On January 1, 1943, the city of Los Angeles changed its earthquake requirements so that theseismic coefficient varied over the height of the building and was also a function of the totalheight (i.e. the period of structure). This was the first time that the seismic requirement ofbuilding code took into account the flexibility of building as well as mass. These requirementswere based on dynamic analysis of structures carried out by Martel and his students.

The first accelerographs were installed by the Seismological Field Survey of the U.S. and Geo-detic Survey in late 1932, just in time to record the first ever strong ground shaking of thedestructive earthquake of March 10, 1933 Long Beach Earthquake. This was the most impor-tant step in the development of Earthquake Engineering. For the first time engineers could seethe nature of strong ground shaking, the amplitude of motion, the frequency characteristics andthe duration of shaking. This was also the crucial information for dynamic analysis of engineeredstructures.

EARTHQUAKE SCIENCE AND ENGINEERING IN INDIA

Earthquake Hazard

In the first half-century, six mega earthquakes of magnitude 8+ had occurred in India. Theywere the 1819 Kutch earthquake, 1897 Shillong earthquake, 1905 Kangra earthquake, 1934Bihar-Nepal earthquake, 1941 Andaman earthquake and the 1950 Assam earthquake. In thesecond half of this century, such large earthquakes have not occurred. Out of these three earth-quakes have occurred in Himalaya and is considered prone to great earthquakes of magnitude 8or more. The earthquakes of importance which caused damage were Anjar(1956), Kapkote(1958),Badgam(1962), Koyna (1967), Baroach(1970), Kinnaur (1975), Pithoragarh(1980), Silchar (1984),Dharamshala(1986), Shillong(1986), N.E. India (Indo-Burma Border (1987), Indo-Bangladeshboundary (1988), N.E. India (1988), Bihar-Nepal(1988), Uttarkashi (1991), Latur (1993),Jabalpur(1998), Chamauli (1999), Bhuj(2001), Kashmir (2005), Sumatra & Andaman(2004) andSikkim(2006).

Oldham in 1883 published the first authentic catalogue of Indian earthquakes from the earliesttime to the end of 1869. Indian Society of Earthquake Technology has brought out a "Catalogueof Earthquake in India and Neighbourhood" from historical period upto 1979. A new catalogue isunder preparation which will include recent data. The earlier history of seismological setup inIndia is given in Tandon (1959). The very first Milne type seismographs were installed in India in1898 at the Colaba Observatory, Bombay.

During the past earthquakes, the main damage was due to residential houses of non-engineeredconstruction. These construction still dominate in severe seismic zones of the country and there-

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fore wide spread damage would repeat if a major earthquake strikes again. The initial impetuson the development of Earthquake Engineering was seen after the Koyna earthquake andmajor initiatives were taken in earthquake disaster mitigation and management in the countryafter the Bhuj earthquake.

Koyna Earthquake of Dec. 11, 1967

After the Koyna earthquake of Magnitude 6.5, Dec. 11, 1967 the Earthquake Engineering stud-ies has made steady progress in this country. For the first time, a strong motion accelerogramwas recorded in one of the abutment blocks. Some cracks were developed in the Koyna dam inthe non-overflow section. A host of studies were made to understand the effects of this earth-quake [ Jai krishna et al.(1969), Chandrasekaran et al.(1969), Saini et al.(1972)]. The State ofMaharastra carried out a program of check analysis for the various dams. The Koyna dam wasstrengthened by adding buttresses on the downstream side of non-overflow sections. Analysisindicated very high tensile stresses in the concrete. It was also realized that it is not possible todesign such dams on no-tension basis for strong motion and necessarily tensile stresses are to bepermitted in mass concrete or masonry.

Bhuj Earthquake of January 26, 2001

The major earthquake of Magnitude 7.7 of January 26, 2001, created lot awareness. After thisearthquake, there has been a paradigm shift in focus from 'reactive' relief to 'proactive' mitiga-tion and preparedness. Numbers of initiatives were taken to strengthen disaster managementsystem in the country to reduce the effect of earthquake disaster. The focus is basically to buildup the capabilities at all levels necessary for preparing for and handling disasters. A road mapwas drawn to reduce the vulnerability to disasters and to upgrade capabilities at all levels forresponding to disasters. Some of the important measures like amending the building bye-laws toimplement the codal provision on earthquake resistant construction; evaluation and retrofitting oflifeline buildings and carrying out awareness generation campaigns have been undertaken atnational level.

Initiation of Earthquake Engineering Studies

The officers of the Geological Survey of India (GSI) have been studying all major earthquakesand publishing their findings in the GSI memoirs. The publication include Oldham's classicalmemoir "On the great Assam Earthquake of 1897" which gave a great impetus to seismologythroughout the world. This was perhaps the first earthquake for which a description in somedetail is available. Other important GSI memoirs are on "The Kangra Earthquake of 4th April1905" and "The Bihar-Nepal earthquake of 1934" which are full of valuable information.

A landmark paper on Earthquake Engineering Problems in India was published by Jai Krishna in1958 wherein he described the damageability of Indian buildings in an earthquake and methodsof improving present engineering practice and proposed design rules for earthquake resistantstructures [Jai Krishna(1958)]. With the initiative of Professor Jai Krishna an Earthquake Engi-neering research cell was created at University of Roorkee. Professor D.E. Hudson of the

5

California Institue of Technology, Pasedena visited the centre as Visiting Professor. Togetherwith George Housner, he assisted University of Roorkee in establishing the programme in Earth-quake Engineering. The very first Seminar on Earthquake Engineering was held at Roorkee in1959 within 3 years after the first World Conference on Earthquake Engineering in 1956 atBerkeley. The Seminar paved the way for establishing the School of Research and Training inEarthquake Engineering well known as SRTEE. The status of Earthquake Engineering in Indiaaround this period can be found in the papers of the Seminar volume.

Earthquake Engineering in an organized manner was first introduced in India in 1960 at theSchool of Research and Training in Earthquake Engineering (SRTEE), University of Roorkee ina modest way. As resolved in the Second Indian National Conference on EarthquakeEngineering in 1962, a P. G. Diploma and Master of Engineering Courses including EngineeringSeismology were started in 1963 at the University of Roorkee, which have been continuouslyeducating the civil and/or mechanical engineers and seismologists upto now, thus producing welleducated personnel who are serving various Government department in the Centre and States aswell as major Industrial consulting organizations in the country.

Initiatives Towards Capacity Building in Earthquake Engineering

Training programmes were being organized by SRTEE for different groups such as teachers ofengineering colleges, field engineers from PWD, bridge engineers, engineers from consultingorganization such as Engineers India Ltd. NTPC, NHPC, having durations of 1 to 3 weeks. Suchtraining programmes started as early as 1959 and are still continuing. A large number of engi-neers have thus been trained in the school (Converted to Department of Earthquake Engineering(DEQ) since 1971 and IIT Roorkee since 2001. Department is participating actively in MHRD'sNational Programme on Earthquake Engineering Education (NPEEE) and MHA's NationalProgramme for Capacity Building of Engineers in Earthquake Risk Management (NPCBEERM)and National Programme for Capacity Building of Architects in Earthquake Risk Management(NPCBAERM).

Earthquake Disaster Mitigation through Consultancy Services

The School of Research and Training in Earthquake Engineering ( SRTEE) and laterDepartment of Earthquake Engineering (DEQ) have been providing consultancy to major engi-neering projects in the country including all major dams, major bridges, petro-chemical works,building projects, industrial undertakings including atomic power plants to the extent that com-plete know-how has been developed in the country to carryout earthquake resistant design andconstruction without seeking foreign consultancy. When the Koyna dam was cracked during theearthquake in 1967, SRTEE provided details of rehabilitation and retrofitting of the dam in con-sultation with the Central Water Commission. SRTEE/DEQ have carried out site specific studiesbased on deterministic and probabilistic approaches taking into account historical seismicity, neo-tectonic activity, potential seismic features, geological surveys, remote sensing photography, faultmovement monitoring and satellite imageries for developing the design seismic parameters formore than 170 projects in India.

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Research and Development

The research and development work have been carried out in all aspect of Earthquake Engi-neering and have done some pioneering work in Earthquake Engineering Studies and catered tothe need of the country. The Structural Response Recorders (SRR) and Accelerographs weredeveloped at the Department of Earthquake Engineering and these were deployed in the seismicregion of the country those have provided valuable earthquake data. Several small and largemechanical shake tables were fabricated and used to carryout testing of structures. A shocktable facility constructed using three railway wagons was found to be very useful in testingmasonry structures. Major landmark in the development of Earthquake Engineering in India isthe establishment of earthquake testing facility at Earthquake Engineering Dept., University ofRoorkee which includes a digitally controlled 20 t biaxial 3.5x3.5m computerized shake table totest models as well as some full scale testing. It was commissioned in 1984 which greatly helpedin the seismic qualification studies of equipment and structures. The department has awarded 60Ph.D's and currently, 30 research scholars are pursuing research leading to Ph.D degree.

Establishment of Indian Society of Earthquake Technology (ISET)

The Indian Society of Earthquake Technology was first established in 1964 with headquarter atUniversity of Roorkee. With a membership of about 1500 geologist, engineers and seismologist ithas provided a forum to scientist and engineers, publishing a journal and disseminating theinformation on earthquake technology. The Society and SRTEE along with Institution ofEngineers (India) organized the Sixth World Conference on Earthquake Engineering in Delhi inJanuary 1977. The Executive Committee of the society has been recognized as the NationalCommittee on Earthquake Engineering for membership of the International Association ofEarthquake Engineering in which India has held the position of President as well members ofBoard of Directors almost without a break

Standardization in Earthquake Engineering

Through very hard work put in by GSI, IMD and SRTEE, the first code of practice of the IndianStandards Institution on the Criteria for Earthquake Resistant Design of Structures was broughtout in 1962 as 1893-1962. Rather than using a constant seismic coefficient over the height ofstructures, the concept of response spectrum was introduced in the Code. The seismic zoning ofthe country was brought out in the Code, demarcated a major part of peninsular India as non-seismic. Then through the combined effort of SRTEE, CBRI, MES and CPWD the first code onNon-Engineered Buildings (Masonry and Wooden) was published in 1967 as IS:4326-1967. Thiseffort continued in revising and updating the codes upto 1984. Through largely the effort ofDepartment of Earthquake Engineering (Since 1971), University of Roorkee and utilizing theinternational experience brought out in guidelines for Non-engineered construction (IAEE-1986),the Code IS: 4326 was developed into four Codes and Guidelines in 1993.

Strong Motion Instrumentation

Strong motion data is the basic input used for seismic design of structures. SRTEE was the firstInstitution to design and build 'strong motion accelograph' and 'multiple structural responserecorders' and install them in the length and breadth of the country. On the initiative of SRTEE

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and in collaboration with NSF, USA, an array of strong motion accelographs was installed inShillong Plateau. SRTEE was taken as a member of the International Strong Motion Council andrecognized as focal point by Department of Science and Technology, Government of India forstrong motion studies in India and provided large scale funding for the same. As a result addi-tional strong motion arrays have been installed in Uttaranchal hills, Himachal hills and NorthBihar. All these arrays have provided very important data on attenuation of accelerations inthese areas. These were first installed particularly in some hydroelectric projects.

It may be mentioned that presently there are 159 analog ( SMA-1 -135 and RESA-V -24)accelographs, 54 digital (SSA-1 -3, SSA-2 -20, GSR-18 -31) accelographs and 338 multiplestructural recorders located in Seismic Zone IV & V. The region wise distribution: N.E. IndiaSMA-1 -45, SSA-2 -17; Himachel Pradesh SMA-1 -50, SSA-2 2, U.P. Hills SMA-1 40, SSA-11 and rest in Nothern Bihar and river valley project sites. There are also 338 Structural Re-sponse Recorder (SRR) installed in high seismic region of India. The strong motion measure-ment in the form of dense arrays in Shillong (NE India), Himalayas in Kangra (Himachel Pradesh),Western Uttar Pradesh and North Bihar is carried out by Department of Earthquake Engineer-ing, University of Roorkee, supported by Department of Science and Technology (DST). Theyhave provided valuable information of engineering importance.

Recently, under the Mission mode project DST has entrusted 300 accelerographs to IIT Roorkee(formerly University of Roorkee) for installation in Zone III, IV and V. Out of installationscarriedout in District Headquaters about 180 accelerographs are being networked using V-SATwhereas others are being networked through telephone lines.

Twelve buildings located in different part of the country like Delhi, Hydrabad, Ahmedabad, Puneetc. have been completely instrumented to understand the behaviour of buildings. Many recordshave been obtained such as the records of response of Post office building in Ahmedabad duringthe Bhuj earthquake of January 26, 2001.

In addition to these some strong motion instruments have been installed in special structures likedams and multi-storeyed buildings. So far more than 100 three component strong motion recordshave been obtained. They have provided valuable information of engineering importance. Thesegive an idea of attenuation of energy as waves travel from source, peak ground acceleration(PGA), predominant frequency and the response spectra. The introduction of digital instrumentsfor acquiring strong motion data is one of the major achievements, which record waves of widerange of frequencies in a digital form with their range adjustable. These instruments have alsobeen fitted with absolute time recording mechanism making them immensely more useful toseismology. The Department of Earthquake Engineering has brought out an Atlas of StrongMotion Data in a CD.

The widespread installation of strong-motion accelerographs, together with the development ofpowerful computers, has provided large amounts of data and this has posed problems of dataacquisition, data analysis, data storage, data retrieval and also data interpretation.

Micro-earthquake Studies

The micro-earthquake monitoring in a region assesses the seismic activity of known tectonic

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faults or locates unknown ones lying buried deep. However, prediction of size of future majorearthquakes from studies on micro-earthquake is still far away. The first telemetered seismicnetwork of instrument has been installed around Tehri dam by the Department of EarthquakeEngineering to monitor the seismic activity in the region of the Tehri dam. It has already collectedvaluable data for more than 15 years. The monitoring is still going on and likely to providevaluable data during the reservoir filling. The telemetered network provides the earthquake dataat a central recording station which can be processed in shorter possible time. It helps in estimat-ing the source parameters, study the seismicity and seismotectonics of seismically active regions(e.g. plate boundaries, seismicity of major faults, rift zones), and attenuation characteristics ofthe region, site selection, studying the reservoir induced seismicity, study of earthquake predic-tion and determination of ambient tectonic stress of the region from the stress drop of the locallyrecorded earthquakes. The digital telemetry is considered superior to analog telemetry on ac-count of higher dynamic range, digital transmission in the form of bit stream, and ease with whichthe digital data can be processed employing modern computing facilities. Micro-earthquakestudies have also been carried out for many river valley projects by the Department.

Other Important Events and Contributions

The sixth World Conference on Earthquake Engineering was held in New Delhi in January 1977.India has been represented in various international bodies dealing with Earthquake Engineering.Four yearly national symposiums are held at Roorkee, twelfth one was held in Dec. 1998. De-partment of Earthquake Engineering has provided consultancy to many projects in India andprojects in SriLanka, Nepal, Iran, Yemen, Libya, Bangladesh and Bhutan. It has also participatedin UNESCO activities in Yugoslavaia and South and Southeast Asia region.

EARTHQUAKE PREDICTION

In case of earthquake, precise prediction is not possible as prediction has no meaning unless thecombined prediction "where", "when", "what size" of earthquake is precisely made. Earthquakeprediction is uncertain and can only be possible partially for certain faults. Research in earth-quake prediction has shown abnormal ground deformation preceding a major earthquake. Otherparameters were also studied but none of them proved reliable one for prediction. Further, evenif is able to predict earthquake and consequently all the population is evacuated safely, the poor/faulty and weak construction is bound to fail and therefore there is no substitute for earthquakeresistant construction. Successful earthquake prediction cannot eliminate earthquake hazard.From engineering point of view, prediction of earthquake is based on importance and type ofstructure, and its usable life.

Estimation of Earthquake Parameters

Major and important structures have to be protected from future strong motion earthquakeswhich may occur during the life time of structure. As earthquakes cannot be predicted accu-rately, at best earthquake parameters (i.e. magnitude, epicentral distance and focal depth) canbe predicted on the basis of available seismological and geological information about the pastactivity. Improved method of estimating the earthquake parameters based on deterministic andprobabilistic approach, historical seismicity, neotectonic activity, active faults or potential seismicfeatures, improved technique of mapping, geological surveys, remote sensing photography, fault

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movement monitoring, satellite imaginary have made the estimation of earthquake parametersmore reliable. Estimation of earthquake parameters requires both knowledge and judgment, andit should be done only by qualified and experienced persons.

Seismic Zones

For ordinary structures it is not feasible to undertake a special development of earthquake crite-ria for each building, instead, general design criteria are presented in the building code. Forordinary structures, engineers may use general prediction in the form of seismically gradedzones i.e. zones of severe, moderate and light seismicity as shown in seismic zoning map ofIndia. Similar risk is expected in a seismic zone and accordingly earthquake force can bepredicted.

EARTHQUAKE RESISTANT DESIGN AND CONSTRUCTION

Seismic design concept of a structure in many ways different because of the uncertainty ofearthquake loading. The earthquake loading can not be estimated accurately at a site and thattoo is uncertain whether it would be subjected in the lifetime of the structure. Therefore, a limiteddamage is allowed without permitting the collapse of the structure when subjected to the mostsevere earthquake expected at the site thus ensuring safety of lives. Accepting the possibility ofdamage, on the basis that it is less expensive to repair when hit by an earthquake rather thanmaking the structure earthquake damage proof. This concept results in an economical design,which will be susceptible to earthquake damage but will not collapse in an event of severeearthquake. These design criteria are also based on considerations of allowable stresses, per-missible inelastic strain, desired factor of safety against collapse, acceptable damage etc. Intel-ligent framing system, careful design and construction detail can vastly improve the performanceof structure to resist earthquake

The major developments in basic philosophy and principles of seismic design, development ofnormalized shape of response spectra and the multiplying factor based on the attenuation rela-tionship to obtain the design response spectra, site dependent artificial earthquakes to match theshape of response spectra, design for strength and ductility, developments in 2D/ 3D mathemati-cal modeling, simultaneous excitations in three cardinal directions, evaluation of modal damping,influence of missing modes, modal combinations, soil-structure interaction and dynamic analysiswere developed in the last four decades.

For very important structures/projects such as nuclear power plants, high dams, high-rise build-ings, long span bridges, etc. and their high cost requires high degree of safety than the ordinarystructures and therefore requires special design criteria. . Seismic design of important and com-plicated structure is now largely possible in the country. The procedure for evaluation of seismicparameters for Nuclear Power Plants is very stringent and comprehensive as it assumes theworst scenario closet to the site. Atomic Energy Regulatory Board has brought out Code ofpractice for Seismic Design of Nuclear Power Plants and fairly good guidance is availablethrough its practice. Nuclear power plant work gave impetus to analysis and design of equipmentlocated in building at various floor levels. As a consequence, seismic analysis and design ofnuclear power plants in seventies, the development of Earthquake Engineering was acceleratedby the need of these special projects. During this period, Nuclear power plants came up at

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various sites: Narora in U.P., Kakrapar in Gujarat, Kaiga in Karnataka. It can be seen from theforegoing that the setting of design criteria involves many elements that cannot be evaluatedprecisely and therefore, it is necessary to rely on judgment. Good judgement, based on experi-ence, should lead to a near optimum solution of the Earthquake Engineering problem.

Dams and Bridges

For dams also site dependent earthquake parameters are evaluated which was very similar toNuclear Power Plants. Finite element technique is used extensively in the analysis of gravity andfill dams. Foundation-structure-reservoir interaction is considered. Various appurtenant struc-tures like intake tower, powerhouse, spillway bridge, retaining walls, shafts are also designed forearthquake effects. Stability of reservoir rim is also carried out. In bridges, the development ofnew bearings to reduce forces on super structures is of significance. Only for a few majorbridges, site dependent earthquake parameters were used. Major industrial structures are de-signed for site specific earthquake parameters.

Buildings

For buildings, codal provisions are sufficient. The old type indigenous construction developedover a long time through trial and error which have performed well like bamboo-ikra constructionin Assam, NE India, Dhajji Diwari in Himachal Pradesh and Kashmir; wood frame constructionwith brick noggin in Himachal Pradesh and bhunga construction in Kutchch. If possible, woodenconstruction should be restored in high seismic regions.

In 1930 earthquake at Dhubri, the phenomenon of friction base isolation was first observed whenthe building resting on rocks which could slide were much less damaged then those which werefixed to the ground.

Response control systems are gaining popularity. It aims in controlling the response i.e. reducingthe response by design. There are two methods of control, passive and active. Seismic baseisolation is a passive device, which has emerged as a major technique to protect buildings, andsome basic research work has been done at DEQ. It is hoped that in near future this techniquewould be used widely in seismic regions. Special damping devices, active control systems aresome of other ideas are also gaining popularity.

Finite Element Method in Earthquake Engineering

The finite element method of analysis has also been important development for EarthquakeEngineering. Large numbers of sophisticated finite element softwares are now available andbeing developed which are capable of solving difficult large size field problem. This has in-creased the capability of an earthquake engineer.

EARTHQUAKE DISASTER MITIGATION

Earthquakes are one of the nature's greatest hazards to life and property. As the developmenttakes place, city and the population grow with new construction. The number of structures andpopulation exposed to earthquake hazard increase. The earthquake hazard to life and property isalmost entirely with man made structures except for earthquake triggered landslide. Earthquake

11

hazard can be minimized with better understanding of earthquake behaviour of structures and bycareful planning, design and construction. The dissemination of knowledge to masses is also veryimportant in the mitigating of the earthquake hazard. The responsibility or mitigation of thesehazards rest on the joint efforts of professionals from instrumentation, geology, seismology, struc-tural, soil and dynamics. It helps in earthquake preparedness measure also.

India faces a potential disaster during future earthquakes particularly in Himalayas, plains ofUttar Pradesh, plains of Bihar adjacent to Nepal and in plains of Northeast India. There are widespread use of non-engineered construction in residential buildings of common man which arevery prone to earthquake damage. Since earthquake do not occur frequently, people tend toforget about them so that the non-engineered construction undertaken by the common populationhas shown little improvement and as result in future earthquake a greater disaster may occur.Also many people cannot afford the additional inputs required for improving strength and theyleave it to the fate. This is because earthquake resistant construction practices are not very wellknown to the commoner even though the technology is available. Any earthquake hazard mitiga-tion program has to address this seriously. There is a need for earthquake resistant constructionparticularly for lifeline structures like hospitals, water supply and emergency shelters. Still mucheffort is needed to educate people about earthquake resistant construction and earthquake pro-tection methods. Although, the code specifies how a project should be designed however lot ofseismic hazard can be minimized at the initial planning stage of the project. Considerable advan-tage can be gained by choosing the best site/spot from the earthquake hazard point of view or thebest type of structure for that site. The local geological structures, active faults and the soilcharacteristics together with the economic and social consequences of destructive earthquakesdetermine the suitable location.

Though, the Code of practice is generally followed in organized sector of construction, it is not atall followed by small private parties. Therefore, there is a need for municipal by-laws, whichshould regulate earthquake resistant construction. There is a need for pre-disaster earthquakesurvey and retrofitting of important public buildings having inadequate seismic strength and com-munication links. The technical know how is available but what is need is allotment of funds.Regions that are earthquake disaster prone must be identified and without creating scare, disas-ter mitigation plans must be drawn and executed.

Earthquake disaster management in the country was greatly helped with the Ministry of HomeAffairs as the nodal Ministry for Disaster Management. The establishment of Authorities likeGujarat State Disaster Management Authority (GSDMA), Orissa State Disaster ManagementAuthority (OSDMA), Disaster Mitigation and Management Centre (DMMC) in Uttartanchalhave greatly contributed in reducing the vulnerability and in preparing to counter disasters.

CONPUTERS AND EARTHQUAKE ENGINEERING

Since 1960 massive development of both the Earthquake Engineering and the computer technol-ogy has taken place. The development of computers has been very important for development ofEarthquake Engineering. These made possible the practical analysis of accelerograms, the de-velopment of response spectrum of earthquake motions and the design spectrum which haveplayed important roles in Earthquake Engineering. Computers have also made possible the cal-

12

culations of the dynamic response of structures to earthquake ground shaking and this has greatlyhelped in our understanding of structural dynamics.

ORGANIZATIONS INVOLVED IN EARTHQUAKE ENGINEERING STUDIES

There are several important organizations apart from Department of Earthquake Engineering(DEQ), Indian Institute of Technology Roorkee formerly Univ. of Roorkee which are involved invarious aspects of Earthquake Engineering. The Geological Survey of India (GSI) deals withseismotectonic activities of various geological features and post disaster survey to draw isoseismals.

The Seismology Directorate of India Meteorological Department (IMD) maintains nationwideseismological observatories, which record earthquake events. The National GeophysicalResearch Institute (NGRI), Hydrabad, Wadia Institute of Himalayan Geology, Dehradoon andCentral Water and Power Research Station (CWPRS) -Pune, Earth Science Department,University of Roorkee and Bhawa Atomic Research Centre study local micro-earthquakeactivities. Some river valley projects and Nuclear Power Plants monitor micro-earthquakeactivity of the project sites.

Some analytical and experimental research work are carried out at Central Water and PowerResearch Station at Pune and at various IIT's, Indian Institute of Science- Bangalore; BHEL-Hydrabad, BHEL ARP -New Delhi, Structural Engineering Research Centre (SERC)- Madras;National Building organisation- New Delhi; Central Building Research Institute (CBRI)- Roorkee,HUDCO- New Delhi; and BMTPC- New Delhi.

There is now greater awareness in design and construction organizations in India about Earth-quake Engineering such as Public Works Departments (PWD) in centre and states, Indian Rail-ways, Nuclear Power Corporation (NPC), National Thermal and Hydro Power Corporations,Bharat Heavy Electricals, Engineers India Limited, Tata Consulting Engineers, DevelopmentConsultants etc.

Through internet service National Informatics Centre for Earthquake Engineering (NICEE)excellent work in promoting earthquake engineering in the country.

SUMMARY

Over the past fifty years world over, there has been remarkable progress in EarthquakeEngineering studies and research. In India Earthquake Engineering studies were initiated in 1960in a modest way with the establishment of School of Research and Training in EarthquakeEngineering, at University of Roorkee. Koyna earthquake, construction of Nuclear Power Plants,and Dam projects in Himalayas provided great fillip to Earthquake Engineering. Bhujearthquake has provided great fillip in the Disaster Mitigation and Management activities in thecountry.

The knowledge of earthquake ground shaking and earthquake vibrations of structures hasundergone a great expansion. Earthquake resistant design and construction practices haveadvanced substantially. Great improvements in building codes for design and construction havebeen made worldwide and this will go long way in reducing loss of life and property in futureearthquakes. We are now wiser to identify where to build and how to build so that structure iseconomically feasible and safe. The confidence and expertise developed over the years has

13

attained the level that it is self sufficient in matters of earthquake resistant design. However, theknowledge has yet to be disseminated widely. The foregoing developments have primarilyresulted through foresight and effective planning, training of manpower, development ofcurriculum for education, research, field investigation and consultancy as required to supportprofessional needs. Much more has to be done in the area of earthquake disaster mitigation andmanagement to achieve substantial disaster reduction.

REFERENCES

Basic Concept of Seismic Codes, International Association for Earthquake Engineering,Gakujutsu Bunken Fukyu-Kai, Tokyo 152

Bolt, Bruce A., Seismology as a Factor in Earthquake Engineering, Post Conference volumeon VIII World Conference on Earthquake Engineering, July 21-28, 1984, San Francisco,California, USA.Catalouge of Earthquake in India and Neighbourhood, from historical period upto 1979 (1983),Indian Society of Earthquake Technology, Roorkee.Chandrasekaran, A.R. Srivastava, L.S. and Arya, A.S(1969), Behaviour of Structures inKoyna Earthquake of December 11, 1967, Indian Concrete Journal, Vol. 43, no.12, Dec.1969.

Dewey, J. and P. Byerly, The early history of seismology, Bull. of Seismological Society, Feb.1969.

Housner, G.W., Historical view of Earthquake Engineering, Post Conference volume on VIIIWorld Conference on Earthquake Engineering, July 21-28, 1984, San Francisco, California,USA.IS:1893-1962, Recommendations for Earthquake Resistant Design of Structures, Indian Standards Institution, New Delhi.Jai Krishna (1958), Earthquake Engineering Problems in India, J. of the Institution of Engineers (India), Vol.XXXIX, No.1, Pt1, September, 1-31.Jai Krishna, Chandrasekaran, A.R. and Saini, S.S.(1969), Analysis Koyna Accelerogram ofDec. 11, 1967, Bull. Seismological Society of America, Vol. 59, No.4, Aug. 1969.Memoir GSI, The Kangra Earthquake of 4th April, 1905, Memoirs of the Geological Surveyof India.Memoir GSI , Report on the Bihar-Nepal Earthquake of 1934, Memoirs of the GeologicalSurvey of IndiaMemoir GSI, Report on the Great Earthquake of June 12 1897, Memoirs of the GeologicalSurvey of India.Paul, D.K.(1984), Behaviour of Buildings During Earthquakes, Proc. Int. Symp. on Creationof Awareness about Earthquake Hazards and Mitigation of Seismic Risks, ISET RoorkeeChapter, Roorkee, November 28-29, pp 31-44.Paul, D.K.(1997), Indian Experience in Earthquake Disaster Mitigation, J. of Indian Buildings Congress, Vol.IV, No.1, pp. 67, 78.

14

Paul, D.K.(1993), Engineering Aspects of The Uttarkashi Earthquake of October 20, 1991,National Workshop on Synthesis of Uttarkashi Earthquake of Oct.20, 1991 and Seismotectonicsof Garhwal-Kumaon Himalaya, Nov. 12-13

Paul, D.K.(1993), Earthquake Resistant Stone Masonry and Cement Concrete Block Buildings, Workshop on Earthquake Resistant Low Cost Housing, Srinagar (Garhwal), 12-14 March(also in Hindi).Richter, C.F., Elementary Seismology, Freeman, 1958

Saini, S.S., Jai Krishna and Chandrasekaran, A.R.(1972), Behaviour of Koyna Dam in Dec. 111967 Earthquake, J. Str. Div., ASCE, Vol.98, ST7.

Tandon, A.N.(1959), Development of Seismological Organisation in India, Seminar on Earthquake Engineering, Univ. of Roorkee, Roorkee, Feb.10-12.

15

ELEMENTARY SEISMOLOGY

M. L. SharmaAssoc. Prof., Department of Earthquake Engineering, IIT Roorkee, 247667

INTRODUCTION

An earthquake is a series of vibrations on the earth's surface caused by the generation of elastic(seismic) waves due to sudden rupture within the earth during release of accumulated strainenergy. Faulting may be considered as an immediate cause of an earthquake. Earthquakes areone of the most powerful natural forces that can disrupt our daily lives. The science dealing withearthquakes is called seismology. The Earthquakes occur due to finite physical sources buriedbelow the surface of the earth. These sources generate band-limited signals, which are recordedat the surface of the earth by seismic instruments. The medium through which these signalspropagate, i.e., the Earth, acts as a filter (Fig.1.1).

[Input: Band Limited signal] [Band Pass filter] [Output signal]

Fig. 1. Earth as a filter

CAUSES OF EARTHQUAKES

Earthquakes are usually caused when rock below the earth’ssurface suddenly breaks along a fault. This sudden release ofenergy causes the seismic waves that shake the ground. Whentwo blocks of rock or two plates rub against each other, theystick partially. In other words they don't slide smoothly and catchon each other. The rocks while pushing against each other donot move. After a while, the rocks break up because of theintense pressure built up. Eventually when the rocks break, theearthquake occurs. During the earthquake and afterwards, theplates or blocks of rock start moving, and they continue to moveuntil they get stuck again. The Ried's theory of elastic reboundexplains the cause of earthquakes as in fig 2.

Earth / medium of propagation

Seismogram

Earthquake Source

Fig. 2 Theory of elastic rebound

Chapter 2

16

The spot below the surface where the rock breaks is called the focus (Fig. 3) of the earthquake.The region right above the focus (on top of the ground) is called the epicenter of the earthquake.The epicenter is the point on the earth's surface vertically above the hypocenter (or focus), thepoint in the crust, where a seismic rupture begins.

Fig 3. Epicenter and focus of earthquake

SEISMOTECTONICS OF INDIA

Tectonics is a greek word meaning force. In tectonics, the forces of the nature, which areresponsible for the tectonic and geological set up of the region and result in form of earthquakes,are studied. The tectonics associated with the seismicity of the region is called as seismotectonics.Plate tectonics deals with the earthquake occurrence, which are due to the movements of thelithospheric plates. More than ninety percent of the seismicity is due to the movement of thelithospheric plates.

Earthquakes occurrence over the globe is not uniformly distributed but occur predominantly inwell-defined narrow seismic zones. These narrow zones mainly consist of the circum-Pacific,the Alpine-Himalayan belt and the world-circling oceanic ridges. The occurrence of earthquakescan be explained with the help of Plate tectonics theory. Plate tectonics provide valuable insightinto the mechanisms by which the earth's crust and mantle have evolved. Plate tectonics is aunifying model that attempts to explain the origin of patterns of deformation in the crust, earth-quake distribution, continental drift, and mid-ocean ridges, as well as providing a mechanism forthe Earth to cool. Two major premises of plate tectonics are: firstly, the outermost layer of theEarth, known as the lithosphere, behaves as a strong, rigid substance resting on a weaker regionin the mantle known as the asthenosphere. And secondly, the lithosphere is broken into numeroussegments or plates that are in motion with respect to one another and are continually changing inshape and size.

These seismic zones divide the lithosphere laterally into tectonic plates (Fig. 4). There are 12major plates (Antarctica, Africa, Eurasia, India, Australia, Arabia, Philippines, North America,South America, Pacific, Nazca, and Cocos) and few minor plates (e.g., Scotia, Caribbean, Juande Fuca, etc.). The parental theory of plate tectonics, seafloor spreading, states that new lithos-

17

phere is formed at ocean ridges and moves away from ridge axes with a motion like that of aconveyor belt as a new lithosphere fills in the resulting crack or rift. The mosaic of plates, whichrange from 50 to over 200 km thick, are bounded by ocean ridges, subduction zones, and trans-form faults (boundaries along which plates slide past each other).

Fig 4 The major and minor lithospheric plates

There are three main plate tectonic environments: extensional, transform, and compressional(Fig. 5). These environments are also called normal, reverse and strike-slip faults respectively.Plate boundaries in different localities are subject to different inter-plate stresses, producingthese three types of faults that cause earthquakes. Each type has its own special hazards. Thecrust moves along cracks called faults. A fault is a break in the earth's crust. The earth can movein different directions depending on the type of fault.

Tension, a pulling force that causes the plates to move apart, can create a normal fault. Therocks above a normal fault move downward as the plates below the fault move upward. Whenthe earth's plates come together, they produce compression forces that push on rocks fromeither side. Sometimes the rocks bend. In other cases, they break and one rock slides up over theother. In a reverse fault the rock above the fault slides up over the rock below the fault. At astrike-slip fault, the rocks on either side of the fault slide past each other. This sliding force iscalled shearing. As the plates slide past each other, the forces bend and twist the land.Sometimes the land gets caught as it slides. When it releases or breaks, an earthquake occurs.

18

Fig. 5. Different types of faults due to which earthquakes occur (In each fault are shown twoblocks of earth surface rubbing or pushing over each other)

Tectonic framework of the Indian subcontinent covering an area of about 3.2 million sq.km isspatio-temporally varied and complex (Fig. 6). Three distinctive morphotectonic provinces canhowever be generalised as i) Himalaya and the Tertiary mobile belts of the east (Indo-Burmarange) and west (Suleiman-Kirthar fold belt), ii) the Indo-Gangetic Foredeep and iii) the Penin-sular Shield, all of which are characterised by distinctive stratigraphic, tectonic and deep crustalfeatures with wide ranging tectonic histories. The

Himalayan region dominated by compressional tectonics marks the largest active continent-continent collision zone that has witnessed four great earthquakes during the last century. ThePeninsula, in marked contrast is a mosaic of Archaean nucleus with its peripheral Proterozoicmobile belts sutured and cratonised during late Proterozoic, followed by development of latePaleozoic intracontinental rift related basins along Precambrian sutures. Cretaceous volcanismand formation of rift-drift Mesozoic passive coastal basins have added to the complexity of thePeninsular shield.

19

Fig. 6 Tectonic Map of India and Neighboring Areas (Eremenko and Negi (1968) andValdiya (1973)).

SEISMIC WAVES

Seismic waves are the waves of intense energy caused by the sudden breaking of rock withinthe earth or an explosion. They represent the energy that travels through the earth and isrecorded on seismographs. The two main types of waves are body waves and surface waves.Body waves can travel through the earth's inner layers, but surface waves can only move alongthe surface of the earth like ripples on water. Earthquakes radiate seismic energy as both bodyand surface waves.

20

BODY WAVES

The first kind of body wave is the P wave or primary wave (Fig. 7). This is the fastest kind ofseismic wave. The P wave can move through solid rock and fluids, like water or the liquid layersof the earth. It pushes and pulls the rock, it moves through, just like sound waves push and pullair. P wave reaches the seismogram first and is recorded as the first seismic recording. Hencethe detection of P waves for seismic warning systems is of utmost importance.

Fig. 7. P-wave or primary wave (The arrow shows the direction in which the wave is moving).

The second type of body wave is the S wave or secondary wave (Fig. 8), that is the secondwave felt in an earthquake. An S wave is slower than a P wave and can only move through solidrock. This wave moves rock up and down, or side-to-side.

Fig. 8. S-wave or secondary wave (The arrow shows the direction in which thewave is moving )

21

SURFACE WAVES

The first kind of surface wave is called a Love wave (Fig. 9), named after A.E.H. Love, aBritish mathematician who worked out the mathematical model for this kind of wave in 1911. It'sthe fastest surface wave and moves the ground from side-to-side (shown in Fig 9 with smallarrows).

Fig. 9 Love wave (The arrow shows the direction in which the wave is moving).

The other kind of surface wave is the Rayleigh wave (Fig.10), named after John William Strutt,Lord Rayleigh, who mathematically predicted the existence of this kind of wave in 1885. ARayleigh wave rolls along the ground (shown in Fig. 10 with rotating circle) just like a wave rollsacross a lake or an ocean. Because it rolls, it moves the ground up and down and side-to-side inthe same direction that the wave is moving. Most of the shaking felt from an earthquake is dueto the Rayleigh wave, which can be much larger than the other waves.

Fig. 10 Rayleigh wave (The arrow shows the direction in which the wave is moving).

22

SEISMOLOGICAL INSTRUMENTATION

Seismologists study earthquakes by observ-ing the site of occurrence, assessing thedamage caused by the earthquakes, and byusing seismographs. A seismograph is aninstrument that records the shaking of theearth's surface caused by seismic waves.

Most of the seismographs used today areelectronic devices, but a basic seismograph(Fig. 11 ) is made of a drum with paper onit, a bar or spring with a hinge at one or bothends, a heavy mass, and a pen. The oneend of the bar or spring is bolted to a pole ormetal box that is bolted to the ground. Asthe drum and paper shake next to the pen,the pen makes squiggly lines on the paper,creating a record of the earthquake. Thisrecord made by the seismograph is called aSeismogram. .

The seismograms are used to locate theearthquakes and to estimate the energyreleased from the event in terms ofmagnitude of the earthquake. The earthquake hypocenter is located based o the arrival ofprimary and secondary waves at different stations recording the earthquake.

EARTHQUAKE MAGNITUDE

The size of the earthquake is measured in terms of magnitude; generally the magnitude reportedis Richter magnitude, which is defined as the log10 of the maximum amplitude, recorded (inmicrons) at a distance of 100 km on Standard Wood Anderson Seismograph.

It is a number that characterizes the relative size of an earthquake. Several scales have beendefined, but the most commonly used are (i) Local magnitude (ML), commonly referred to as"Richter magnitude," (ii) Surface wave magnitude (MS), which is measured based on surfacewave amplitudes (iii) Body -wave magnitude (MB), which is measured based on body wavemagnitudes and (iv) Moment magnitude (MW) which is measured based on the fault area and theslip.

Earthquake magnitude is a measure of the amount of energy released during an earthquake.Depending on the size, nature, and location of an earthquake, seismologists use several differentmethods to estimate magnitude. Since magnitude is representative of the earthquake itself, thereis thus only one magnitude per earthquake. But magnitude values given by different seismologi-cal observatories for an event may vary depending on the magnitude scale used.

Fig. 11.The Basic Seismograph

23

EARTHQUAKE INTENSITY

The effect of earthquake at any place is measured in terms of its intensity on a XII point MMIscale (Modified Mercalli Intensity). Thus the Richter scale measures the energy released in anearthquake by measuring the size of the seismic waves and the Mercalli scale measures theresults of an earthquake, such as the shaking and damage that people actually feel and observe.

STRONG GROUND MOTION

The instrument which records the strong groundmotion is called as accelerograph as shown inFig. 12.

The record produced is known as accelerogram. The characteristics of accelerogram are shownin Fig. 13.

Fig. 13. Strong Motion record - Accelerogram

An accelerogram is a time history of acceleration composed of non-periodic sequences of accel-eration pulses. The maximum amplitude of the pulses is often taken as a measure of severity ofground shaking. An accelerogram is composed of pulses of different durations and therefore, notonly the peak of amplitude but also the frequency content of the record is necessary in charac-terization of accelerogram. The temporal evolution of accelerogram is composed of three partsnamely, rise time, strong motion and decay time. The effect of ground shaking is mostly depen-dent on duration of strong ground motion part. The accelerograms are richer in high frequenciesas we go nearer to the causative fault. The high frequency components attenuate faster than thelow frequency components, therefore the contribution of high frequency component is reducedin the accelerograms at larger distances. The amplitude of ground acceleration decreases withincreasing distance from the earthquake source. The ground velocity and the displacement canbe obtained by direct integration of the accelerogram.

Fig. 12 Strong motion accelrograph

24

25

BASIC CONCEPTS OF VIBRATION

D.K. PaulProfessor, Department of Earthquake Engineering, IIT Roorkee, Roorkee, 247667

INTRODUCTION

Every structure vibrates under external excitation and mostly depends on its mass, stiffness,damping and boundary conditions. All of these parameters can be expressed by a single param-eter frequency ' f 'or time period ' T 'of vibration. The mass of a structure is obtained by dividingweight 'W' of structure, by acceleration due to gravity 'g'. The stiffness 'k' is a structuralproperty defined as force 'F' per unit deflection 'δ ' as shown in Fig.1.

DEGREE OF FREEDOM OF VIBRATION

The vibration of structure depends upon the degree of freedom of vibration. The number ofindependent deflections required to define the complete vibration of a structure is called thedegree of freedom of structure. The vibration of a structure shown in Fig.1 can be define by asingle displacement 'δ ' and therefore can be defined as Single independent Degree of FreedomSystem (SDOFS).

δ

gW

mmass =,

δF

kStiffness =,

Fixed or pinned BoundaryCondition

F

Fig.1 Mass-spring system

Chapter 3

26

TIME PERIOD OR FREQUECY OF VIBRATION OF A PENDULUM

Time period of vibration of pendulum i.e the time taken by the pendulum to complete onecomplete cycle is given by

gl

Tπ21= (1)

where, 'T' is the time period of vibration in sec; l is the length of the string and 'g' is theacceleration due to gravity.

Figure 2 shows the vibration of a pendulum of length l and mass m . The time period is notinfluenced by the mass of the pendulum. The frequency of the pendulum is related to time periodof vibration as

Tf

1= cycles/ sec (2)

and the frequency p in radian/sec is given as

fp π2= radians/sec (3)

extreme positionl

mmm

Time

Am

plitu

de

Fig.2 Vibration of a pendulum of length

FREQUENCY OF VIBRATION OF AN OVERHEAD TANK

Vibration characteristics of structures can be worked out by idealizing the structure as a spring-mass system. For example an overhead water tank can be idealized as a Single Degree ofFreedom System (SDOFS) as shown in Fig.3.

27

pile providing

Heavy mass

Stiffness of circular shape

Mass of tank with water andpart column mass

Assumed fixed at foundationlevel

mmm

C.G.

Fig.3 Vibration of a overhead tank

The overhead water tank supported on circular shaft can be idealized by simple mass lumped atthe c.g of the tank and by a continuous beam as shown above. The full weight of the overheadtank, part of the weight of circular staging shaft (1/3 of the total weight of the shaft) and theweight of the water inside the tank are assumed to be lumped at the c.g of the tank. Since thetank is resting on pile foundation, the shaft can be considered fixed at the top of the pile cap. Sothe lumped mass 'm' at c.g. can be worked as:

gWWWm watershaftk )31

( tan ++= (4)

where, kWtan is the weight of the tank, shaftW is total weight of the supporting shaft, waterW is

the weight of water contributing to the vibration.

The stiffness 'k 'of the cantilever circular shaft fixed at the base can be worked out as

3

3hEI

k = (5)

where E is the modulus of elasticity, I is the moment of inertia and h is the height of thecantilever shaft.

The undamped frequency of vibration can therefore can be worked as

mk

fπ21

= (6)

28

Say, the spring constant is

cmkgk 338.0=The mass m is

cmkg

m2sec

9205.4 −=

5.4920338.0

28.61

21 ×

==mk

fπ

1sec32.1 −=

Here the mass moves in a vertical directions and its position is specified by a coordinate xposition down ward. In minimum case it is convenient to fix the origin o at the position of static

equilibrium of the mass m . In this position the length of the string is 0 stL δ+ where stδ is the

static deflection i.e. the elongation of the spring due to the weight w .

stw kδ= (7)

To derive the equation of motion of the system we consider in figures the force acting on in atthis position are shown in fig. The spring force

0m x k x= =&& (8)

From Newton's second law the equation of motion is

s stm x F w k x k wδ+ + = − − +&& (9)

stkδ

sF

w

w

o

x

stL δ+0

m

29

VIBRATION OF A R.C. FRAME BUILDING

Vibration of a r.c. frame building as shown in Fig.4 can be idealized as Multi Degree of FreedomSystem (MDOFS). The slab of 5.0mx5.0m size and thickness of 0.1m is resting on four r.c.columns of equal size 0.3mx04m. The clear height of the columns is 3.0m. The columns areassumed fixed at the base.

Material properties

Z

X

Y

3

2

/56.23

2.0

/25000000

mkN

mkNE

===

ρν

Fig.4 R.C. frame structure

it reduces to

0m x k x+ =&& (10)

From the above explanation we conclude that when a mass moves in a vertical direction we canignore its weight provided that we choose the origin of the coordinate x at the position O of

static equilibrium. The weight w is balanced by the spring force due to the static deflection stδ. The spring constant.

st st

w mgk

δ δ= = (11)

Substituting we get,

2 stTg

δπ= (12a)

12 st

gf

π δ= (12b)

Thus when the mass moves in a vertical direction, measurement of the static deflection stδenable us to compute the period and frequency of vibration of the system. It is not necessary thatwe know the mass m or the spring constant k .

30

If the beams and slab are considered flexible, then the structure can be idealized as 2D portalframe in the direction of the vibration as shown in Fig.5.

Fig.5 2D idealized portal frame

The stiffness of the portal frame can be worked out as

+

+=

hILIhILI

hEI

k

b

c

b

c

c

23

66

3 (13)

where cI is the moment of inertia of column section in the direction of vibration, bI is the

moment of Inertia of the beam section, L is the span of the portal frame and h is the height ofthe portal frame.

If the slab is considered rigid then each column will undergo same amount of deformation. This

assumption leads to simplification which means the moment of inertia of beam bI can be taken

as infinity, therefore (7) reduces to

y

h

yg L

y

h

yg L

Fig.6 Rigid floor idealization

31

3

24hEI

k c= (14)

Therefore structure can further be idealized as a mass and spring system. The half floor weightand half the weight of the two columns will constitute the lumped mass at the center of the floorslab. The stiffness of each column can be added to get the total stiffness as given in (8). Thevarious forces acting on the free vibrating mass will be the forces due to the stiffness, dampingforces and inertia forces.

y

h

myk

yc &( )gyym &&&& +

k

m

yyg

yg

yg

Fig.7 Lump mass spring model idealization

If y is the deflection of the mass then the spring/ restoring force will be yk acting opposite tothe motion.

Restoring force = yk (15)

The damping force also acts opposite to the motion and is assumed to be proportional to velocityof the moving mass.

Damping force = yc & (16)

where c is the damping coefficient and y& is the velocity of the vibrating mass.

The inertia forces acting on the mass is the product of mass and absolute acceleration and actsopposite to the motion.

Inertia force = )()(2

2

yymyydtd

m gg &&&& +=+ (17)

where )( yyg &&&& + is the absolute acceleration of the mass and gy&& is the ground or support

acceleration and y&& is the acceleration of the mass relative to the support or ground.

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Equilibrium of forces gives the equation of motion of the system as follows:

0)( =+++ ykycyym g &&&&& (18)

which can be expressed as:

gymykycym &&&&& −=++

(19)This shows structures subjected to base or ground motion is subjected to a force equivalentto product of mass and ground or base acceleration. For undamped free vibration, the dampingand ground or base motion will vanish and the equation can be expressed as:

0=+ ykym && (20)

Assuming solution ptay sin= and substituting in the above equation, undamped frequency isobtained as:

mk

p = (21)

For mutidegree freedom system, the equation of motion can be expressed as:

yMyKyCyM &&&&& −=++ (22)

where KCM ,, are the mass, damping and stiffness matrices of the structure and y is thevector of independent displacements. The undamped free vibration of multi degree freedomsystem can be expressed as:

0=+ yKyM && (23)

Assuming a solution ptay sinφ= where φ is the vector normalized displacements. Substi-tuting y results in eigen value problem.

φλφ MK = (24)

The time periods of vibration for structural system shown in Fig.4 are compared with the ETABSsolution in Table 1. The time periods compare very well.

Table 1 – Time periods of vibration of structure (Fig.4) and a comparison with ETAB

Direction Mass (kN.sec2/m)

Stiffness (kN/m)

Time period(sec)

Time period (sec)

(ETABS) Translation in X direction (horizontal) Tortional motion about Z-axis Translation in X direction (horizontal) Translation in X direction (horizontal)

6.00 58.00 6.00 6.00

71112.0 597349.5 40000.0

4000000.0

0.0600 0.0620 0.0770 0.0077

0.0610 0.0690 0.0790 0.0077

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PERFORMANCE OF BUILDINGS IN PASTEARTHQUAKES: LESSONS LEARNT

D.K. PaulProfessor, Department of Earthquake Engineering, IIT Roorkee, Roorkee, 247667

INTRODUCTION

The observations of structural performance of buildings during earthquakes provide volumes ofinformation about the merits and demerits of the design and construction practices in a regionsince it is based on the actual test on prototype structures. The study helps in the evaluation ofstrengthening measures of buildings and modifying the provisions of the modern code of practicewith minimum additional expenditure.

Numerical techniques have made great stride in Earthquake Engineering and it is important tocritically evaluate the validity of these techniques by the experience of instrumented buildingsduring actual strong motion earthquakes which are generally carried out experimentally usingearthquake simulators.

The numerous buildings suffered severe damage in Caracas during the Venezuela earthquake(1967) which were designed according to modern methods as reported by Borges et al.(1969)and Degenkolb et al. (1969). Similar experiences were observed in many other earthquakes.This is the cause for great concern and there is a need for better understanding of the behaviourof buildings during some important earthquakes has been carried out. Finally, the important les-sons from the damage behaviour of buildings during earthquakes are summarised.

The indirect damages of buildings during earthquakes are some times far greater than the dam-ages due to earthquake itself, such as, out break of fire, rock fall, landslide, avalanche andtsunamis. However, these damages are not due to inadequacies in the design and planning andtherefore, not discussed here.

BEHAVIOUR OF BUILDINGS DURING PAST EARTHQUAKES

A description of behaviour of buildings during different earthquakes throughout the world aresummarised here for simple reason that they provide good engineering information about thebehaviour of structures and helps in evolving its strengthening measures. In many cases, illus-trates the effectiveness of earthquake resistant measures.

Chapter 4

34

Lisbon (Portugal) earthquake of Nov.1, 1755

It has the maximum intensity of X on Modified Mercalli (MM) scale of Lisbon. Nearly 15,000buildings in the city collapsed and some 60,000 people were killed. The large scale disaster waslargely aggravated by the narrow streets where it was practically impossible to prevent the rapidspread of fires and the piling up of debris. There were three shocks in all, the first was the mostsevere shock and there was not a single stone building remained intact, thirty-to monasteries and53 palaces were also destroyed; (Poliyakov, 1974).

Rann of Kutch earthquake of June 16th, 1819

This devastating earthquake occurred on 16th June 1819 between 6.45 and 6.50 pm resulting innearly 1543 deaths and huge loss of property. It was felt in Ahmedabad, Porbondar, Jaisalmer,Bhuj etc. In Bhuj alone more than 7000 houses were damaged. The houses built on low rockyridges suffered less damage whereas houses founded on a slope leading to plain of spring andswamps were completely ruined. The Anjar earthquake of 21st July 1956 of Magnitude 7 in thisregion also caused considerable property damage. There was total devastation for kutcha-puccaconstruction.

Bihar-Nepal earthquake of August 26, 1833

A violent earthquake of Magnitude 7.0 - 7.5 struck on August 26, 1833 between 5.30 and 6.00pm (IST) killing 414 people in Nepal and several hundred in India with severe damage atKathmandu, Bhatgaon, Khokha and Patan in Nepal, and Monghyer and Purnea district in India.At Bhatgaon a loss of 2000 houses (i.e. 42%) were reported. The maximum intensity reportedwas IX.

Assam (India) earthquake of June 12,1897

The magnitude was estimated to be greater than 8.5 and responsible for 1542 deaths. It occurredat 5.15 local time. The peak ground acceleration was estimated to have reached 50 of gravity. Itis one of the greatest earthquake of the world. All the stone and brick buildings were destroyedover an area of 370,000 sq kms. (Tandon and Srivastava,1974). Some of the buildings sank intothe ground upto their roofs due to liquefaction of soil. The traditional Ikra type of construction ofbuilding of Assam showed good performance.

Great Kangra earthquake of April 4, 1905

This earthquake of Magnitude greater than 8.0 occurred at 6.0 hrs 20.0m (IST) with its epicen-ter at 32.25N, 76.25E. The maximum MM intensity X was observed in the epicentral region hadtaken a toll o 20,000 lives. The buildings were built of sun dried bricks and some times with stonefoundations raised about 15 cm above ground. Roofs were normally of slates but thatch was alsoused. The damage were severe, the houses became a heap of sun dried bricks, slates and rafter.

San Francisco (California, USA) Earthquake of April 18, 1906

The earthquake had a magnitude of 8.3 and about 700 to 800 people died. Buildings on hardground received comparatively minor damage such as collapsed chimneys, shattered windows.

35

However, load bearing structural elements were not seriously damaged. Structures erected onsoft ground were severely damaged. Destruction of brick buildings was very severe with wallsand entire sections collapsing. Damage to structures on filled up ground was especially severedue to differential settlements. The tall buildings resting on piles withstood the earthquake welland it provided the first test of multistorey steel frame buildings. Extensive nonstructural damagewas common but none of these multistorey buildings were so heavily damaged so as to beunsafe. Wood frame construction performed very well. Unreinforced sand-lime mortar brickbearing walls performed poorly. During the earthquake, most of the fire station buildings in thecity were destroyed. The fires which were caused by the destruction of burning stoves and shortcircuits in electric wires lasted three days, (Wiegel, 1970).

Messina (Sicily) earthquake of Dec. 28, 1908

It has the maximum intensity of x on MM scale. Peak ground acceleration was 208 of gravity. Inthe past this city had been repeatedly subjected to severe earthquakes. During this earthquake,100,000 people (according to some data- 160,000) were killed, 98 percent of the buildings werecompletely destroyed (Polyakov,1974).

The reason for such disastrous consequences was primarily very poor quality of construction.The walls of the buildings were made of quarry stone laid in a weak lime mortar, no specialearthquake proof measures had been taken. The ground conditions were not also suitable. Thebuildings were erected on loose alluvium and highly weathered crystalline rock.

Kanto (Japan) earthquake of Sept. 1, 1923

The peak ground acceleration was about 50% of gravity. It destroyed the Tokyo and Yokohamacities. The earthquake and the fires that followed caused the death of over 140,000 people withjust as many injured. The number of buildings destroyed were 1,286,261 and 447,128 buildingswere destroyed by fire. Damage was specially severe in places where structures were built onloose alluvium and appreciably less on firm ground (Okamoto, 1973).

This earthquake illustrates the great influence of ground on the intensity of earthquake. theadvantages of structural frame systems and serious shortcoming of brick construction wereclearly established. Thus, for example, out of 710 reinforced concrete frame buildings, whichwere carefully investigated by Japanese specialists, 69 buildings (9.7%) was damaged and 16buildings (2.2%) were collapsed. Where as out of 485 brick buildings with load bearing brickwalls 47 buildings (9.7%) were completely destroyed and 383 buildings (79%) were severelydamaged. On the basis of these studies, the maximum height of brick buildings was limited to 9m in Japan.

Santa Barbara (California, USA) earthquake of June 29,1925

It had the magnitude of 6.3 on Richter scale. Substantial damage was observed in buildingsconstructed of unreinforced brick with lime mortar. In the residential area, most of the buildingswere of wood frame and found to have performed well. However, there was occasional failuredue to lack of bracing or rotten structural wood. (Inard 1925 and Wiegel 1970).

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Long beach (California, USA) earthquake of March 10, 1933

It had the magnitude of 6.3 on Richter scale. Earthquake bracing provisions in USA were incor-porated in the earthquake resistive design after this earthquake. Buildings with walls of brickmasonry having sand-lime mortar with wood roofs and floors suffered severe damage. After the1933 long beach earthquake the practice of unreinforced brick construction in USA ended,(Wiegel, 1970).

The larger wood frame dwellings had serious damages due to failure at or near the foundationlevel which was attributed to lack of bracing. However, the performance of wood frame build-ings was excellent as compared to other materials. Out of the 4575 wood frame residencesstudied in Compton, about 95% of them had less than 5% damage. Multistorey building damagewas common in Long Beach and in Los Angeles. The damage were greater in the lower stories.Pounding damage between multistorey buildings were frequently noted. Few buildings designedfor earthquake forces have performed well.

Great Bihar earthquake of Jan. 15, 1934

His disastrous earthquake of Magnitude 8.4 occurred at 2.0 pm with its epicenter at 26.5N,86.5E in which nearly 11,000 lives were lost. The areas affected have been found scatteredwithin a region of 48,60,000 sq km. There was complete damage to all the masonry buildings.Landslides have occurred in the mountain areas near Kathmandu, Udaipur, Garji and easternNepal. Large scale liquefaction was also reported in Purnea where houses have been tilted andsunk into the ground. At many places sand and water fountains erupted (Memoir GSI, 1934,Tandon and Srivastav, 1974).

Fukui (Japan) earthquake of June 28, 1948

The earthquake of magnitude 7.2 occurred at 4.0 a.m. The peak acceleration of 0.6 g wasobserved and the focal depth of 15 kms was estimated. During the earthquake 5268 people werekilled and 35,437 structures were destroyed. Forty six out of forty seven reinforced concreteframe (cast in-situ) buildings upto 9 stories high survived the earthquake well. One buildingwhich was completely destroyed was attributed to errors in calculations (Okamoto, 1973).

Ashkhabad (USSR) earthquake of October 6, 1948

It had the maximum intensity of IX on MM scale. The epicentral distance (D) to Ashkhabad wasabout 30 kms and the focal depth (h) was 40 kms. First there was a strong vertical shockfollowed by horizontal vibrations which lasted 10 seconds. The city suffered great destruction,especially buildings with sun dried and burnt brick walls. The main characteristic was the verypoor bond between the bricks and mortar for all type of buildings. The collapse of these buildingswas primarily due to the poor quality of the concrete in the frame elements (the grade was oftenlower than 100), the absence of stirrups at the joints and other structural defects. The Ashkhabadearthquake showed that high earthquake resistance of cast in situ reinforced concrete structuralelements was observed when high quality of construction was used. (Polyakov, 1974).

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Tajikistan (USSR9 earthquake of July 10, 1949

A enormous avalanche of rock crashed down and the entire village was buried under a pile ofstones reaching a height of about 12 meters. The lessons of the earthquake the danger shownthe dander to settlements on the slopes of mountains where landslides are anticipated, (Polyakov,1974).

Great Assam earthquake of 1950

The devastating earthquake of Magnitude 8.5 on Richter scale occurred at 14 hrs 09 m 30s(GMT) with epicenter 28.5n, 97.0E having a depth of focus of about 15 km. An area of nearly46,000 sq km suffered extensive damage. The epicenter of the shock was located on the unin-habited part just outside the north east boundary of India. It caused great destruction to propertyin north eastern Assam.

Kern Country (Southern California, USA) earthquake of July 21, 1952

The shock had a magnitude of 7.7. It was the first major earthquake to test the earthquakeresistant buildings in USA. Wood frame buildings withstood the earthquake well, however, minorplaster cracks and damage to unreinforced brick chimneys were observed, (Degenkolb, 1955and Wiegel, 1970).

All steel structures had almost negligible damage. In multistorey steel structures, the nonstructuraldamage was considerable. Reinforced concrete structures had minor damages. The damagewas in the form of cracking of nonstructural partitions etc. Poor quality of reinforced concreteconstruction showed significant damage. There was no cases of complete collapse or nearcollapse of a multistorey building even when poorly designed and built. Precast reinforcedconcrete structure failed due to failure of joints. One story reinforced concrete structures havingprecast walls with poured in reinforced concrete columns performed well. Severe damage wasseen in buildings constructed of sand lime mortar. Reinforced brick walls performed well.

Mexico city earthquake of July 28, 1957

The maximum intensity of VII was recorded at Mexico city. It is situated on highlywater-saturated clay with low bearing capacity. Under the layer of clay is a layer of compactsand 3-9 m thick. Before the earthquake, many of the buildings in Mexico city that were not builton piles, were damaged due to settlement which was aggravated by pumping out of water for thewater supply system. The above mentioned damages to buildings unquestionably increased thedamages due to the earthquake (Rosenblueth, 1960).

Thornley and Albin (1957) examined the damage of buildings on a small area of the city. On theanalysis of 46 buildings on a small area of the city subjected to the severest shock. Most of thebuildings were multistorey and had fairly similar conditions as regards their base and its vibrationduring the earthquake. Special attention was paid to the design of the foundations of the buildingswhich were divided into five groups. Of these five groups, these are built on piles driven to thelevel of compact sand (to a depth of 33.5 m). In the design of one of the types of pile foundations,the building did not rest on the piles but was suspended form them by means of bolts and cross-members. Among the 46 buildings considered, 7 buildings had 15 or more stories, one of which

38

was a 43 story building. this steel skeleton building (Tower Latino America) was built on pilefoundations. Of the five types of foundations considered, the foundation supported on concretepiles have performed well and is recommended for earthquake resistant buildings (includingmultistorey) even when ground conditions were as unfavourable as they were in Mexico city.

Among the various structures, buildings with steel or reinforced-concrete frames and withmonolithic reinforced concrete stiffening diaphrams showed good performance.

The earthquake in Mexico city showed that properly designed tall and high rise buildings have asufficiently high resistance to earthquakes of moderate size. None of the buildings with morethan 14 stories collapsed while 23 and 43 story buildings were not even damaged. On the otherhand some much lower buildings were completely destroyed.

Agadir (Morocco, Africa) earthquake of February 29, 1960

It had a magnitude of 5.5 and the maximum intensity recorded was XI. The focal depthestimated to 3 kms and epicentral distance to 1 km. It practically destroyed the whole citysituated in the coast of the Atlantic ocean in the north-western part of Africa. During theearthquake 12,000 people died and 12,000 injured. There was nearly total destruction of buildings(Polyakov, 1974).

The buildings erected in Agadir were not designed to resist earthquakes. Most of them were noteven designed to resist winds. The majority of the buildings had nonreinforced load bearing wallswhich had poor bond between mortar and brick (stone) and was the main reason for almost totaldestruction of buildings.

The energy release during this earthquake was primarily in a single pulse as in Eureka, Califor-nia, Earthquake of December 21, 1954. The full engineering implication of an earthquake releas-ing its energy in a single pulse are not well understood and the subject obviously needs morestudy. Although the size of earthquake was small, the damage to Agadir city lying over theepicentre, was devastating.

Chile (South America) earthquake of May 1960

The series of shocks began on May 21 with the largest shock of magnitude 7.5. This wasfollowed by several more shocks, four of the largest having magnitudes from 6.5 to 7.8. On May22, a larger shock occurred with magnitude 8.5. During the following month there were 50shocks with magnitudes from 5 to 7.

A total of 450,000 buildings were severely damaged of which 45,000 were completely destroyedand more than 1000 persons were killed. It was possible to study the performance of the modernbuildings which were designed according to country's earthquake resistant constructionregulations.

The severe damages were due to old buildings with plain brick walls which were apparentlyweakened by the previous earthquakes. Such wall construction in Chile is not permitted bycurrent regulations. Buildings with reinforced brick and concrete walls behaved much better.Better earthquake resistance of reinforced concrete frame walls with brick cladding and woodframe walls were observed.

39

The performance of a steel framed three-story building presented considerable interest. In lon-gitudinal and transverse directions provision was made for diagonal bracing (on the first story inboth directions). During the May 21 earthquake, the building was not damaged but the bracingwas damaged at the joints with the columns. Therefore, during the May 22, tremor, the buildingwas without bracing as a result of which its rigidity was sharply reduced (The fundamental timeperiod changed from 0.8 to 1.06 s). Despite the decrease in stiffness of the building in horizontaldirection it did not receive any damage during another stronger earthquake. It was apparentlythe reduced rigidity of the building, which attracted less inertia forces, and consequently survivedthe earthquake.

Skopje (Yugoslavia) earthquake of July 26, 1963

A total of 1700 people lost their lives and 3300 injured. During the earthquake (M=6.0, h=10 km,D=10 km) 8.5 percent of the buildings were completely destroyed, 33.7 percent of the buildingswere so seriously damaged that they could not be restored, [BERG et al., (1964) and Polyakov,1974].

One story old small buildings with bearing walls of sun dried or burnt brick and of rough naturalstone laid in lime or clay mortar. The roofs were made of heavy tiles. In many cases the wallswere reinforced with wood frames having diagonal struts. Most of these structures were de-stroyed or badly damaged that the restoration was inadvisable. Buildings with wood frames anddiagonal struts behaved some-what better.

Two to four story old buildings had brick walls 25 and 38 cm thick laid in lime mortar. Most ofthese buildings were collapsed and other collapsed during aftershocks. Just as in the Agadirearthquake (and others), the collapse of exterior walls that were poorly connected with theinterior walls was observed. Brick buildings, weakened by large openings on the ground floor,suffered heavy damage. Brick buildings with large halls in which the rigidity of structural ele-ment differed sharply from the rigidity of elements in other parts of the buildings collapsed.

Modern brick buildings upto 6 stories high built according to standard designs were severelydamaged and many of them completely collapsed. The building upto 14 stories high with cast insitu reinforced concrete frames and brick cladding, such buildings sometimes had reinforcedconcrete diaphrams at staircases and elevators shafts. All the buildings withstood the earthquakefairly well specially when none of these buildings were designed to resist seismic forces.

Niigata (Japan) earthquake of 1964

The earthquake (M=7.5, h=40 km, D=50 km) has caused considerable destruction in the city ofNiigata which was primarily due to very poor ground conditions. The predominant time period othe soil layers of city of Niigata varied from 0.25s to 0.5s. It was observed that the damages tothe buildings were heavy on soil having predominant time periods close to 0.5s and less other-wise, (Mawasumi, 1968).

The main cause of damage was the liquefaction of soil underneath. The rigid reinforced con-crete buildings under gone large settlement and tilting. One such building completely toppledover. Among the 1500 reinforced concrete buildings in Niigata, 310 suffered damage, with twothirds of them settling or tilting without noticeable damage to above ground structural elements.

40

Serious damage occurred to closely spaced building due to mutual pounding during seismic shocks.this should be taken into account in designing the expansion joints. In areas of well consolidatedground, there was no damage.

Examination of foundations showed destruction in many cases of reinforced concrete piles.Buildings erected on short piles drived to poorly compacted soils underwent considerable tiltingand settlement. Above ground structural elements of buildings erected on piles driven on hardsoils were not damaged. Buildings with basements suffered considerably less tilting thenbuildings on shallow strip-footing foundations.

Anchorage (Alaska) earthquake of March 27, 1964

It was one of the greatest earthquake (M=8.4, h=20 km, D=130 km at Anchorage) in the history.The damage to the structures were heaviest, and many of the buildings were completelydemolished. The predominant period of the soil layer was estimated to be near 0.5s. This waspossibly the reason that the tall buildings in the city with natural periods close to the predominantperiods suffered more damage than lower buildings. Residential wood frame buildings exhibitedfairly good earthquake resistance except in some cases when their foundations were destroyed.Least damage was sustained by wood structures built on firm ground, (Kunze et al., 1965;Steinbrugge, 1965 and Wiegel, 1970).

The Anchorage earthquake also provided a number of examples of the behaviour of precast,prestressed reinforced concrete structural elements. The precast elements were jointed by welding.A large number of the buildings collapsed. Other precast reinforced concrete buildings alsosuffered serious damage. It was observed, that in all cases destruction and damage to precast,prestressed structural elements were caused by poor behaviour of joints of supports. Theprecast, prestressed elements as a rule were not destroyed.

Tashkent (USSR) earthquake of April 26, 1966

The earthquake (M=5.4, h=8 km, D=0) though small caused severe damages. The location ofthe epicentre was right under the city that accounted for the large vertical component of groundmovement which was the reason for devastation. The predominant period of ground wasestimated to 0.1s. (Polyakov, 1974).

Nearly all the brick buildings were damaged to some degree. But many old sun dried brickbuildings in the centre of the city were damaged so badly that they had to be demolished.

Hindukush (India) earthquake of June 6, 1966

No accelerograph was located in the area, however, few response recorders were actuated,which have indicated a maximum acceleration of about 0.055 g, (Krishna and Arya, 1966). Theold building construction of timber encased in masonry walls showed vertical cracks at thecorners. In some cases separation of walls, cracking of jack arches over door opening, tilting ofwalls etc. were also observed. The timber joints were found to be deteriorated.

Six storied r.c. frame building, showed some shear cracks in the roof beams and longitudinalcracks in the slab between the beams. The main reason for these shear cracks in beams appears

41

to be the earthquake forces applied at roof level on the mass of the roof as well as on the massof some non-structural elements standing on the roof for architectural regions.

The two storeyed hospital building constructed in 1:1:1 lime sand and surkhi mortar. The buildinghas performed well except the crack where it widens in section. These cracks may be attributedto significant change in stiffness of the building. A similar two story Medical college building inlime sand surkhi survived with very minor cracks in the walls.

Anantnag earthquake of February 20, 1967

This earthquake of Magnitude 5.3 ~ 5.7 with a depth of focus of 24 km struck at nearly 8.49 pm(IST). A total of 786 houses were totally damaged and nearly 25,000 houses were partiallydamaged [Gosain and Arya (1960)].

Kashmir valley has been shaked by many severe earthquakes in the past. The earthquakes of22.6.1969; June 6, 1828, May 30, 1885 and September 2, 1963 were the severest. The earth-quake of 30th May 1885 was one of the most disastrous earthquake in Kashmir valley. Duringthis earthquake about 6000 persons were killed.

Koyna earthquake of Dec. 11, 1967

The Magnitude of the earthquake was recorded as 6.5 and the depth of focus was about 8 kmwith its epicenter at 17 22.4N, 73 44.8E. It occurred at 22 hrs 51m 19s (GMT). The maximumMM intensity of VIII+ was observed. The area was considered seismically inactive. Earthquakehas damaged 40,000 houses and 177 persons lost their lives. The peak acceleration recordedwas 0.67 g [Arya, Chandrasekaran and Srivastava (1968)].

The traditional construction in the area was non seismic and had little resistance against lateralforces. Most of the building structures in the area were single storeyed built in masonry. TheKoynanagar experienced very heavy shocks resulting in severe damages. The cladding walltimber framework buildings failed, whereas, modern random rubble masonry buildings sufferedheavy damage. Stone masonry was also heavily damaged than the brick masonry. At Koynahundreds of failures was due to bulging out of wall which caused the fall of stone on one facewhile on the other face standing intact. The outside face many not be able to withstand thetension with the result that the stones would get loosened and fall down. The buildings weremostly founded on murum and there were hardly any failure of foundations.

The epicenter of the earthquake was very close to the Koyna dam. The accelerograph installedwithin the dam provided the most valuable instrumental data.

Off Tokachi (Japan) earthquake of May 16, 1968

The earthquake of magnitude 7.9 occurred under sea 170 kms east of the city of Hachinobe.The damage of reinforced concrete buildings were severe which consists of destruction of cityHan, public library, technical high school at Hachinobe and Hakodate University. These were allbuilt by modern techniques, caused great concern among engineering circles. (Okamoto, 1973).

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Broach earthquake of March 23, 1970

A shallow earthquake of Magnitude 6.0 occurred at Broach in the early hours of March 23,1970. The epicenter was at 21.7N, 72.9E. Twenty three persons were reported to have died andabout 250 persons were injured. About 115 houses badly damaged or collapsed while 2500houses were partially damaged [Bulsari and Thakkar (1970)].

Kinnaur earthquake of Jan. 19, 1975

The magnitude of the earthquake was estimated as 6.7 and the maximum observed intensity inthe region was IX on MM scale. The earthquake caused death of sixty people and severalhundred severely injured. The traditional construction in the area was non seismic and had littleresistance against the lateral forces. Nearly 2,000 dwellings were heavily damaged, (Singh et. al.1975). The random rubble masonry and dressed stone masonry construction with heavy flatroofs suffered extensive damage. Buildings constructed in hollow concrete blocks or dressedstone masonry in cement-mortar developed small cracks in walls. Light structures made ofcorrugated iron sheets nailed to timber frames and arches did not suffer any damage. Thetemples, monasteries and monuments also suffered badly.

Indo-Nepal earthquake of May 21, 1979

The magnitude of earthquake was 6.0 on Richter scale and the maximum intensity was VI onthe MM scale, (Ashwani et al., 1981).

The quality of construction in the region was poor. The maximum damage occurred to thehouses of random rubble stone masonry (rrsm) in mud mortar having foundation on loose soil.Partial or complete collapse of mud walls have been noticed. Dressed stone masonry buildingwith cement mortar developed wall cracks.

Western Nepal-India earthquake of July 29, 1980

The main shock with estimated magnitude ranging from 6.2 to 6.5 caused considerable damageto buildings and loss of life. The maximum intensity estimated was VIII on MM scale,(Satyendraand Ashok, 1981).

Due to remoteness of the region, almost all the village buildings are constructed of stacks ofrandom rocks pieces (without any mortar) wet mud plaster on their interior sides and coveredwith a sloping roof of slabs resting on timber beams and rafters. The majority of newconstruction use mud mortar, however, few use cement mortar. The traditional construction asdescribed offers little or no resistance to lateral forces during earthquakes and thus sufferedsevere damage.

Random rubble stone masonry showed complete collapse. The gable end walls collapsedresulting in partial collapse of the adjacent structure. Failure of timber posts and rafters alsoresulted in collapse of some roofs. Dressed stone masonry in the absence of any mortardeveloped cracked in the cement plaster. Poor bonding at the junctions resulted in loss of contactbetween the cross walls. Reinforced concrete construction did not suffer any damage. Drypacked stone masonry walls with continuous lintel band over openings and cross walls did notsuffer any damage.

43

Jammu and Kashmir (India) earthquake of August 24, 1980

The earthquake has been assigned magnitude 5.2 on the Richter scale and the maximumintensity was recorded VIII on MM intensity scale. Eighty percent of the houses were eitherdamaged or totally collapsed. The traditional construction is predominantly random rubble stonemasonry with mud mortar. Mud houses in the Bhaddo area suffered heavy damage and so as therandom rubble masonry. A large size bounding stone, known as Dasalu in local dialect, is used atsome places particularly at corners made of two walls. Where Dasalu is not used properly, thecorners of the walls opened out resulting in the collapse of building. Light weight structure madeof corrugated iron sheets mailed in limber trusses did not suffer any damage, [(Prakash andMam, 1981)].

The bonding stone Dasalu is found to be effective in the walls constructed of random rubblemasonry. For its effectiveness the spacing of these should be about 1.0 to 1.5 meters bothhorizontally and vertically.

Great Nicobar (India) earthquake of January 20, 1982

The earthquake of Richter magnitude 6.3 occurred at the east coast of Great Nicobar island.The focal depth was estimated to 28 kms, (Agrawal, 1982).

The houses of Nicobars founded on multiple deep piles of 10 to 15 cm dia separated fromground, have not damaged. The timber cum hollow block masonry construction also faired wellwith minor damages. Buildings on fills have shown damage.

Dhamar earthquake (Yemen) of December 13, 1982

The earthquake (M=5.9, h=10 km) which caused great damage in Dhamar province and adjoin-ing areas. The maximum modified Mercalli (MM) intensity in the area was estimated as VIII,[(Arya et al., 1982)].

The random rubble stone masonry and mud brick houses were subjected to severe damageresulting in partial and complete collapse responsible for nearly 2500 lives and injury to the 3000people. It was estimated that about 70,000 houses have been damaged.

The failure was mainly due to separation of walls at the corners and T-junctions. this points outto the inherent weakness in the stone masonry construction used, namely, very weak mortar aswell as lack of proper bond between any two walls at right angles to each other. The bulging ofthe wall masonry outward or inward and falling away of half the wall thickness either way wasa common feature. Overturning of the walls occurred due to severe shaking after the walls hadseparated from the cross walls. The lateral load action was further accentuated on those wallswhich were carrying the roof load through the wooden beams and one or both of them collapsedalongwith the roof crashing down with them.

The mud and adobe houses are weak in tension, shear and compressive strength. Thus theseparation of wall at corners and junctions takes place easily under ground shaking, the crackspassing through the blocks themselves. After the walls fail either due to bending or shearingcombined with the compressive loads, the whole house crashes down.

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Cachchar earthquake of December 30, 1984

The earthquake of Magnitude 5.6 occurred on December 30, 1984 with its epicenter approxi-mately 24.641N, 92.891E.

Dharamshala earthquake of April 26, 1986

The earthquake of Magnitude 5.7 occurred at 13.0 hrs 5m 17s (IST) on April 26, 1986 withitsepicenter at 32.1N, 76.3E Dharamshala-Kangra, Himachal Pradesh in North India. The focaldepth was estimated about 10 km. The maximum intensity was VIII close to Naddi villagewhere all the kutchcha houses were severely damaged and many of them collapsed. Only sixhuman lives were lost.

Assam Earthquake of August 6, 1988

The earthquake of Magnitude 7.2 occurred at 6.36 hrs (IST) on August 6, 1988 with its epicenterat . The focal depth was estimated to 96 km. Guwahati, Jorhat, Sibsagar and Silchar wereshaken. No deaths were reported because the epicenter of the earthquake was in a remote areaand possibly Assam houses (Ikra and bamboo houses) are able to resist earthquake much better.

Bihar-Nepal earthquake of August 21, 1988

The earthquake of Magnitude 6.6 struck at 4 hrs 39 m 11.25 sec (IST) with its epicenter in Nepalnear the Bihar-Nepal border (Lat 26.775 and long. 86.609) in close proximity to 1934 earthquakeepicenter. The focal depth is estimated to be 71 km. The maximum intensity of VIII+ wasobserved at Darbhanga and Munghyer in Bihar and Dharan in Nepal. This earthquake has taken281 lives in Bihar and nearly 650 lives in Nepal. The total number of injured persons in Bihar are3767. It damaged/ collapsed 1.5 lacks houses/buildings in Bihar alone [Paul, Thakkar et al.1988].

At Darbhanga the high intensity was mainly attributed to the soft alluvial soil and liquefactionresulting in large scale subsidence of soil while in Dharan the high intensity is attributed toamplification of ground acceleration due to hill and hill slope. The recent r.c.c. constructions withcodal; provision have shown better performance while old and poorly built load bearing unreinforcedmasonry brick buildings performed badly. Large scale liquefaction of ground was observed in theGangetic plane resulting in ground subsidence. Mud houses and brick houses laid in mud mortarwere affected most in the villages. Severe damage to old masonry buildings having jack archconstruction were observed. Framed construction have shown better performance.

Uttarkashi Earthquake of October 20, 1991

The earthquake of Magnitude 6.6 rocked the Uttarkashi region at 2.53hrs(IST) with its epicenterat Village Agora ( ) and focal depth 12 km. The maximum intensity in epicentral track wasobserved IX on Modified Mercalli scale. The earthquake caused enormous destruction of housesand loss of life, killing nearly 770 people and injured nearly 5000, mostly all due to collapse ofrandom rubble residential houses. The affected region lies between seismic zone IV and Vaccording to seismic zoning map of India. The maximum affected area were Uttarkashi, Tehriand Chamoli districts. Telecommunication and power supply were badly effected due to dam-

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aged telephone and electric poles. Rubble stone masonry houses in mud mortar close to theseverely effected area were totally collapsed and others got severe damage. Many school andhealth buildings were also damaged.

Damage to stone masonry houses Collapse of the steel lattice bridge at Gawana

Many bridges were severely damaged/collapsed. Gawana steel lattice bridge located about 6 kmfrom Uttarkashi on road to Gangotri collapsed, severely affecting the relief and rescue opera-tions immediately after the earthquake. Widespread rock falls landslides/rock slides were ob-served mostly along the road causing heavy damage to hilly roads and blocking it.

The Latur (killari) Earthquake of Sept. 30, 1993

The moderate shallow focus earthquake of Magnitude 6.4 occurred in Peninsular India with itsepicenter near Killari created havoc. The Peninsular India has been considered seismically stable.The earthquake caused strong ground shaking in the region of Latur, Osmanabad, Sholapur,Gulberga and Bidar. There was heavy damage in a localised area of 15 km close to Killari whichis on the Northern side of river Terna. The maximum intensity in the epicentral track was VIII+on Modified Mercalli scale. It destroyed more than 28700 houses, damaging about 170,000houses and killing about 9000 people [Arya(1996)]. The random rubble stone houses in mudmortar totally destroyed. The heavy roofs and thick walls with little shear and no tensile strengthwere the main reasons for the failure.

The most common construction of random rubble stone walls laid in mud mortar are made thick(70 to 180 cm) with small openings for the doors and windows. The foundations of these housesare taken to a depth varying from 60 to 250 cm below the top cover of black cotton soil. The roofconsists of timber rafters in two perpendicular directions over which wooden planks and a thicklayer of mud is laid. The mud layer on roof varies between 30 to 80 cm making very heavy. Thewalls did not have the interlocking stones and the houses did not have any earthquake resistantfeatures[Sinvhal et al.(1994), Iyengar el al.(1994)]

The Jabalpur Earthquake of May 21, 1997

The earthquake of Magnitude 6.1 occurred on May 21, 1997 at 04 hrs 22 s in Southern Indiawith its epicenter near Jabalpur with its focus at 33.0 km. The earthquake lasted 20 secs. Themaximum intensity on MM intensity scale is estimated to be VIII. The latitude and longitudewere 23.18N 80.02E. The Southern India has been considered seismically stable. The earth-

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quake caused strong ground shaking in the region of Jabalpur, Seoni, Mandla and other towns inthe Narmada belt of Madhya Pradesh. About 25 people were killed and more than 100 injured.Most deaths were due to collapse of houses. There was widespread damage in Ragchi, Garhaand Sarafa areas on the city's outskirts. In Jabal, some buildings in Khumeria cantonment, whichhas the contry's oldest factory, developed cracks. Water supply was disrupted at many places inthe city as pipelines burst. Telephone lines and electricity supply were also affected.

Bhuj Earthquake of January 26, 2001

The earthquake of Magnitude 6.9 occurred on January 26, 2001 and has caused widespreaddamage to variety of buildings and many of them have collapsed. Total deaths reported were19500. For the first time in India large number of urban buildings including the multi-storeybuildings at Bhuj, Ahmedabad, Gandhidham and other places have damaged/ collapsed. Themushrooming of multi-storey buildings without any consideration of earthquake resistant designand construction practices has generated a countrywide debate about its seismic safety. It hascaused damage to the common type of load bearing buildings and r.c.c. framed buildings.

Most of the rural construction of mud, adobe, burnt brick and stone masonry either in mud orcement mortar have shown severe damage or collapsed. The stone masonry buildings undergosevere damage resulting in complete collapse and pileup in a heap of stones. The inertia forcesdue to roof/floor is transmitted to the top of the walls and where the roofing material is improp-erly tied to the wall, it will be dislodged. The weak roof support connection is the cause ofseparation of roof from the support and lead to complete collapse. At many places the height ofthe random rubble stone masonry walls in mud mortar/ poor cement mortar was about 5.0m.

Damage to stone masonsry houses Collapse of five storey Bachau bus station

These were provided with earthquake band at only lintel level and therefore, damage was ob-served in the high walls between the lintel and the roof level. The failure of bottom cord of rooftruss may also cause complete collapse of truss as well as the whole building. The Bhuj earth-quake has again showed that stone houses are most vulnerable to earthquakes as it was ob-served in Uttarkashi, Killari and Chamoli earthquakes.

As the prosperity of Gujarat state flourished, multi-storey buildings started mushrooming. In thelast ten years many four storey and ten storey multi-storey buildings were constructed. Themulti-storey buildings without a lift were constructed upto four storeys and buildings with liftwere constructed upto ten storeys. Unscrupulous builders and architects unaware of any earth-

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quake resistant provisions have been constructing buildings. The collapse of newly built apart-ments and office blocks prove this point. The modern r.c.c. frame construction consists of barer.c.c. beam-column frame and the masonry infill. The masonry infill varies from dressed stone inmud mortar, clay brick masonry in cement mortar, cement concrete block masonry in mud/cement mortar. Most of the multi-storey buildings in Ahmedabad and Ghandhinagar were ofr.c.c. frame construction with brick/ cement concrete block masonry in cement mortar as infillmaterial. Most of these type of construction was of stilt type i.e. soft storey construction. In thistype of construction either very few or no infill walls are provided in the ground floor and is leftopen for parking the vehicles of the residents.

The damage to multi-storey buildings in Bhuj is found to be wide spread. It is interesting to notethat multi-storey buildings have also damaged as far distances as Ahmedabad, Gandhidham andSurat. Whereas well designed and well constructed r.c.c. framed buildings following the IndianStandard Code of practice have performed very well during the earthquake. Most of thebuildings constructed by CPWD, Post and Telegraph and other government agencies haveperformed well

The damage in r.c. framed buildings is mostly due to failure of infill, or failure of columns orbeams. The column may have damaged by cracking or buckling due to excessive bending com-bined with dead load. The buckling of columns is significant when the columns are slender andthe spacing of the stirrup in the column is large. Severe crack occurs near the rigid joints offrame due to shearing action which may lead to complete collapse. Most of the damageoccurred at the beam column junction. Widespread damage was also observed at the interfaceof stone or brick masonry infill and r.c.c frame. In most of the cases diagonal cracks appeared inthe stone or brick infill. The buildings resting on soft ground storey columns without or with veryfew infill walls have undergone severe damage and many have collapsed.

Great Tsunamigenic Sumatra Earthquake of Dec. 26, 2004

A great Tsunamigenic earthquake measuring 9.3 on Richter Scale (MW = 8.2) having a focaldepth of 10 km struck Northern Sumatra, Indonesia at 00:58:50 UTC on Dec. 26, 2004accompanied by several strong aftershocks having magnitude ranging from 5.0 to 7.3 and withepicentral locations ranging from west coast northern Sumatra to Andaman-Nicobar islands,Indian region. The main shock near Sumatra generated tsunami that hit the Andaman and NicoboarIslands and caused extensive damage to lives and property. The official death toll in India hasrisen to more than 15500.

The earthquake intensity estimated in Port Blair, Andaman Nicobar Island is VI and in coastalegion is about VII.

- The water level in the sea at Port-Blair has been raised by about 1.0 m and many land areascame under water suggesting the land mass has gone down due to the major earthquakeupheaval.

- The main earthquake shock also generated tsunamis which hit the islands and east coast ofmainland India at different intervals of time and with different wave heights. The height oftsunami waves at Port Blair was about 1-2 m where as in Car-Nicobar the height was about

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10m. The tsunami waves affected about 0.5m to 1.5 km inside the coast line as shown in Fig. 4.At Car-Nicobar Island tsunamis swept the shore and caused severe damage to the buildingslocated near the coast. Andaman Island was also affected by earthquake and tsunami.

Important coastal structures and human habitat should therefore be away from the coast and thefoundation should be above the maximum tide level as far as possible.

- There was large scale ground failure such as ground cracking, large scale subsidence andliquefaction was observed which resulted damage/ failure to many buildings and port structures. At Car Nicobar Air Force station damage to the concrete runway at the joints wereobserved due to concrete blocks hitting each other.

- Significant damage was observed in port and harbour structures, and bridges (their lifelinestructures) due to earthquake vibration mainly since its foundations rested on loose marinesaturated deposits or filled upland. Many damages occurred due to earthquake vibrationsleading to settlement/ liquefaction in many cases and later subjected to tsunami waves.

- Wharf and jetties have damaged mostly due to collapse and submergence of part of Jetty,pounding of deck blocks and the blocks have undergone relative horizontal displacementwhich has misaligned the crane rails.

- In Port Blair RCC frame buildings have performed well and undergone minor damage. Theconstruction of structures on piles has shown better performance.

Collapse of buildings due to tsunami waves

No damage in the water tank structure

Damage to asymmetric structure on stilts Collapse of part of Fisheries Jetty

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Kashmir (Muzaffarad) Earthquake of October 8, 2005

An earthquake of Magnitude, 7.4 ocurred on October 8, 2005 Saturday at 9:20.38 (IST) withepicenter at Muzaffarabad (PoK, latitude 34.432o N, longitude 73.737o ). The earthquake wasfollowed manty after shocks. It is estimated that about 83,000 lives were lost in PoK andPakistan and about 1300 lives lost in India including 72 army personnel, 7510 people sustainedinjuries. About 35,000 houses collapsed and equal number partially damaged, 80% of schoolsdamaged in Uri and Tangdhar

Typical wooden frame Dhajji-dewari Typical damage to stone masonry wall construction

- Most prevalent construction is of mud wall, stone and brick masonry. The roof is woodentruss with GI sheet covering.

- Mixed construction consisting of random rubble masonry, dressed stone masonry, burnt brick/dried clay brick masonry and wood have performed badly

- Building construction practice in general does not comply with Earthquake Resistant Practice as outlined in Building Codes

- Most of the stone masonry construction have collapsed due failure of wall, however, roofsystem has behaved very well

- Dhajji-Dewari construction have performed well. They have not collapsed undergone minorcracks

- Seismic safety of houses mainly depends on the stability of the random rubble masonry wall

- The light roof consisting of timber framing system covered with GI sheet have performedwell during earthquake

- Large scale Capacity building program of Engineers and Masons have to be undertaken

- Rehabilitation & Reconstruction should comply earthquake resistant features in the building

- Communinity Awareness about earthquake resistant practice should be undertaken in largescale

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Kabi Monastery Cracks in the corners

Sikkim Earthquake of February 14, 2006

A moderate earthquake of Magnitude 5.3 occurred at 6.25 am on February 14, 2006 in Sikkim.The epicenter was located 26 km WNW away from the capital city Gangtok. The Gangtok citywas was subjected to an intensity of V/VI. Two Indian Army soldiers were killed when thevehicle they were traveling in was struck by a rock fall at Sherathang near the border outpost atNathula in Sikkim.

The earthquake damagewas quite high for this size of earthquake. A large number of Govern-ment as well as private buildings were damaged to various grades by the earthquake. Some ofthe worst affected are the Raj Bhawan, State Secretariat, Enchey Monastery, Police Headquar-ter, Press Building, Lall Bazar, S.T.N.M. Hospital etc. Many reasons can be attributed to this.The buildings in general have been designed for dead and live loads only. These have not beendesigned for earthquake forces as per the IS codal practices. Many monasteries such as theEnchy Monastery located in Gangtok, Kabi Monastery about 20 km North of Gangtok andLabrang Monestry about 45 km towards North of Gangtok got damaged. Figure shows thecracks on the monastry walls.

PERFORMANCE OF VARIOUS TYPE OF BUILDINGS

Different types of buildings suffer different degrees of damage during earthquakes and the samehas been studied here.

Mud and adobe houses

Unburnt sun dried bricks laid in mud mortar are called adobe construction. Mud houses are thetraditional construction, for poor and most suitable in view of their initial cost, easy availability,low level skill for construction and excellent insulation against heat and cold. More than 100million people in India live in these type of houses. There are numerous examples of completecollapse of such buildings in 1906 Assam, 1948 Ashkhabad, 1960 Agadir, 1966 Tashkant, 1967

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Koyna, 1975 Kinnaur, 1979 Indo-Nepal, 1980 Jammu and Kashmir and 1982 Dhamarearthquakes. It is very weak in shear, tension and compression. Separation of walls at cornersand junctions takes place easily under ground shaking. The cracks pass through the poor joints.After the walls fail either due to bending or shearing in combination with the compressive loads,the whole house crashes down. Extensive damage was observed during earthquake specially ifit occur after a* rainfall, (Krishna and Chandra,1983).

Better performance is obtained by mixing the mud with clay to provide the cohesive strength.The mixing of straw improves the tensile strength. Coating the outer wall with waterproofsubstance such as bitumen improves against weathering. The strength of mud walls can beimproved significantly by split bamboo or timber reinforcement. Timber frame or horizontaltimber runners at lintel level with vertical members at corners further improves its resistance tolateral forces which has been observed during the earthquakes.

Masonry Buildings

Masonry buildings of brick and stone are superior with respect to durability, fire resistance, heatresistance and formative effects. Masonry buildings consist of various material and sizes (i)large block (block size > 50 cms) - concrete blocks, rock blocks or lime stones; (ii) concretebrick-solid and hollow; (iii) natural stone masonry. Because of its easy availability, economicreasons and the merits mentioned above this type of construction is widely used. In very remoteareas in Himalayas buildings are constructed of stacks of random rock pieces without any mor-tar. The majority of new construction use mud mortar, however, few use cement mortar also.

Causes of failure of masonry buildings

These buildings are very heavy and attract large inertia forces. Unreinforced masonry walls areweak against tension (horizontal forces) and shear, and therefore, perform rather poor duringearthquakes. These buildings have large in plane rigidity and therefore have low time periods ofvibration which results in large seismic force. These buildings fall apart and collapse because oflack of integrity. The lack of structural integrity could be due to lack of 'through' stones, absenceof bonding between cross walls, absence of diaphragm action of roofs and lack of box likeaction.

Common type of damage in masonry buildings

All of them undergo severe damage resulting in complete collapse and pileup in a heap of stones.The inertia forces due to roof/floor is transmitted to the top of the walls and if the roofingmaterial is improperly tied to the wall, it will be dislodged. The weak roof support connection isthe cause of separation of roof from the support and lead to complete collapse. The failure ofbottom cord of roof truss may also cause complete collapse of truss as well as the wholebuilding.

If the roof/floor material is properly tied to the top walls causing it to shear off diagonally in thedirection motion through the bedding joints. the cracks usually initiate at the corner of the open-ings. The failure of pier occurs due to combined action of flexure and shear. Near vertical cracksnear corner wall joints occur indicating separation of walls.

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For motion perpendicular to the walls, the bending moment at the ends result in cracking andseparation of the walls due to poor bonding. Generally gable end wall collapses. Due to largeinertia forces acting on the walls, the wythe of masonry is either bulges outward or inward. Thefalling away of half the wall thickness on the bulged side is a common feature. The bonding stoneis found to be effective as in Jammu-Kashmir earthquake of August 24, 1980. Unreinforceddressed rubble masonry (DRM) have shown slightly better performance than random rubblemasonry. The most common damage is due to cracks in the walls. The masonry with lower unitmass and greater bond strength shows better performance. The unreinforced masonry as a ruleshould be avoided as a construction material as far as possible in seismic area.

Reinforced masonry buildings

Reinforced masonry (random rubble or dressed) buildings have withstood the earthquakes well,without appreciable damage. For horizontal bending, a tough member (reinforced concrete band)capable of taking bending is found to performs better during earthquakes. If the corner sectionsor openings are reinforced with steel bars even greater strength is attained. Even dry packedstone masonry wall with continuous lintel band over openings and cross walls did not undergoany damage.

Brick-R.C. frame Buildings

This type of building consists of r.c. frame structure and brick laid in cement mortar as infill. Thistype of construction is suitable in seismic areas.

Causes of failure of r.c. frame buildings

The failures are due to mainly lack of good design of beams/columns frame action and founda-tion. Poor quality of construction. Inadequate detailing or laying of reinforcement in variouscomponents particularly at joints and in columns/beams for ductility. Inadequate diaphragm ac-tion of roofs/floors. Indequate treatment of infill masonry walls.

Common type of damage in r.c. frame buildings

The damage is mostly due to failure of infill, or failure of columns or beams. Spalling of concretein columns. The column may be damaged by cracking or buckling due to excessive bendingcombined with dead load. The buckling of columns are significant when the columns are slenderand the spacing of the stirrup in the column is large.

Severe crack occurs near the rigid joints of frame due to shearing action which may lead tocomplete collapse. The differential settlement also causes excessive moments in the frame andmay lead to failure. Design of frame should be such that the plastic hinge is confined to beamonly, because beam failure is less damaging than the column failure.

Wooden Buildings

This is also most common type of construction in areas of high seismicity. It is also most suitablematerial for earthquake resistant construction due to its light weight and shear strength acrossthe grains as observed in 1933 Long Beach, 1952 Kern County, 1963 Skopje, and 1964 Anchor-

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age earthquakes. However, during off-Tokachi earthquake (1968), more than 4000 woodenbuildings were either totally or partially damaged. In addition there were failure due to sliding andcaving in due to softness of ground. the main reason of failure was its low rigidity at the jointswhich acts as a hinge. Failure is also due to deterioration of wood with passage of time. Woodframes without walls have almost no resistance against horizontal forces. Resistance is highestfor diagonal braced wall. Buildings with diagonal bracing in both vertical and horizontal planeperform much better. The traditional wood frame Ikra construction of Assam and houses ofNicobars founded on wooden piles separated from ground have performed very well duringearthquakes. Wood houses are generally suitable upto two storeys.

Reinforced Concrete Buildings

This type of construction consists of shear walls and frames of concrete. Substantial damage toreinforced concrete buildings were seen in the Kanto (1923) earthquake. Later in Niigata (1964),Off-Tokachi (1968) and Venezuela (1967) earthquakes it suffered heavy damages. The

damage to reinforced concrete buildings may be divided broadly into vibratory failure and tiltingor uneven settlement. When a reinforced concrete building is constructed on comparatively hardground vibratory failure is seen, while on soft ground tilting, uneven settlement or sinking isobserved.

In case of vibratory failure the causes of damage may be considered to be different for eachcase, but basically, the seismic forces which acted on a building during the earthquake exceededthe loads considered in the design, and the buildings did not have adequate resistance and ductil-ity to withstand them. In general these buildings performed well as observed in Skopje 1963 andKern county 1952 earthquakes.

The shear walls are found to be effective to provide adequate strength to the buildings. Severedamage to spandrel wall between the vertical openings are observed.

Tilting and sinking of reinforced concrete buildings during earthquakes were seen in the Kantoand Niigata earthquakes. Most failed because the dead weights could not be supported after thesettling of the ground. Such damage is peculiar to buildings on soft ground. the damage becomeshigher in the following order: pile foundation, mat foundation, continuous foundation andindependent foundation.

The hollow concrete block buildings with steel reinforcement in selected grout filled cells haveshown good performance. The precast and prestressed reinforced concrete buildings also suf-fered severe damage mostly because of poor behaviour of joints or supports. The precast andprestressed element as a rule were not destroyed as observed in 1952 Kern country and 1964Anchorage earthquakes.

Steel Skeleton Buildings

Buildings with steel skeleton construction differ greatly according to shapes of cross sectionsand methods of connection. They many be broadly divided into two varieties, those employingbraces as earthquake resistant elements and those which are rigid frame structures. The formeris used in low buildings while the later is used in high rise buildings.

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When braces are used as earthquake resistant elements, it is normal to design so that all horizon-tal forces will be borne by the braces. This type of building is generally light and influence ofwind loads are dominant in most cases. However, there are many cases in which the braceshave shown breaking or bucking in which joints have failed (Wiegel, 1970).

Steel skeleton construction, particularly the structural type in which frames are comprised ofbeams and columns consisting of single member H-beams, is often used in high rise buildings.The non-structural damage is common but none of these building severely damages as observedin 1906 San Francisco earthquake.

Steel and Reinforced Concrete composite Structures

Steel and reinforced concrete composite structures are composed of steel skeleton and rein-forced concrete and have the dynamic characteristics of both. It is better with respect to fireresistance and safety against buckling as compared to steel skeleton. Whereas compared toreinforced concrete structure it has better ductility after yielding. As these features are theproperties which are effective for making a building earthquake resistant and are found toperform better during earthquakes (Wiegel, 1970).

ANALYSIS OF DAMAGE BEHAVIOUR OF BUILDINGS

A review of damages caused by an earthquake helps in the improvement of aseismic design andconstruction practices, i.e. learning by mistakes. It also provides an excellent test of the state ofthe art of earthquake resistant construction.

Based on the above study of the behaviour of buildings during earthquakes the various factorscontributing for various degree of damages are grouped as follows:

Ground Motion Characteristics

Earthquake originate at a depth below the earth surface and causes random vibratory motion ofthe ground with variable amplitudes and periods. The duration of the main part of the vibrationlast couple of seconds. As a result of ground shaking, buildings founded over it also starts vibrat-ing, causing inertia forces to act on the masses of all the components of a building, the magnitudeof which will be a function of the ground motion intensity and building characteristics (mass,stiffness and damping) of the building. the intensity of an earthquake at a site depends on:

(i) Size or magnitude of earthquake;(ii) magnitude and number of force and after shocks;(iii) distance from focus or epicentre. For Tashkent (USSR and Agadir earthquake the

epicentres were located in the centre of the city. Although earthquake magnitude M=5.5was quite small even then there were severe damages;

(iv) duration of earthquake;(v) type of under lying soil (predominant period of soil layers i.e. frequency and amplitude

(wave form) of ground motion;(vi) damping characteristics of the underlying soil and;(vii) depth of water table.

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The damages to the buildings will be higher, for higher magnitude and long duration earthquakes,less epicentral distance, poor underlying soil and higher water table. Earthquakes with anintensity of VI (on modified Mercalli scale) or less do not result in serious damage. Earthquakeintensity of x and more are so strong that it destroys most of the buildings. Damages alsodepends on the intensity and the number of strong fore and after shocks. the

damage effect is cumulative and, therefore, the main shock plus all aftershocks usually aretreated as a single event from an engineering point of view. Steep slopes, inclined rock layers,landslides, faults etc. also increase the seismic intensity.

Building features contributing damage

The various building features contributing to damage are listed below:

Poor Planning

- Unsymmetric section in plan causes eccentricity which causes high shearing stresses due totorsion.

- Provisions of large openings (doors and windows) which causes common failure at thecorners of openings due to the action of shear forces.

- Proximity of buildings damages occur as a result of collision between two adjacent buildings.- Large Spans of walls.

Poor building materials

- There are several building materials which are commonly used such as mud, sun dried brickor adobe, stone masonry, brick, wood, bamboo, steel, reinforced concrete or a combinationof any of these. Following are some of the reasons of damages due to failure of buildingmaterial.

- Buildings made of heavy material such as stone masonry generally fail because it attractsinertia forces proportional to the mass of the structure. Lighter building material such aswood and bamboo are most suitable for earthquake resistance construction which haveshown good performance during earthquakes.

- Poor quality of material having less tensile, shear, compressive stresses and low modulus ofelasticity cause heavy damage to buildings such as buildings made of sun dried brick oradobe, and unreinforced construction. Poor quality of reinforced concrete, M100 or lessshow poor performance. Richer concrete with stood the earthquakes better.

- Poor quality of mortar results in weak plane and is the reason for many damages in the pastearthquakes.

Poor design

- Buildings designed without seismic considerations have suffered extensive damages. However, the buildings designed for low seismic coefficients of code have performed satisfactorily under going plastic deformations.

- Lack of lateral strength in the structure. Buildings with insufficient framing and inadequatenumber of shear walls have suffered severe damage.

- Structures having less ductility have performed badly during earthquakes. Steel and rein

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forced concrete buildings have performed well due to its high ductility. Large ductility is alsonot suitable because it permits large deformation of structure and fails due to excessivedeformation.

- Unequal distribution of mass or stiffness or both causes damage. In the multistorey building,the concentration of shear walls in the form of rigid core with very flexible columns. Construction of water tanks etc. at the roof level causes damage due to its heavy mass.

Poor detailing and unsatisfactory construction

- Lack of adequate structural connections have caused severe damage such as beam-columnconnection, roof bearing wall connection and wall to wall connections at the corners. Theroof and bearing wall connection should be able to transfer the inertia forces. Improper roofsupport connection is the cause of many building failure.

- Lack of adequate joint connections between precast and prestressed members have causeddamage however, the members itself did not fail.

- Inadequate skill in laying the brick lacking proper bond (poor workmanship).- Concrete column with large stirrup spacing causes failure due to lack of concrete confine

ment.

Other reasons

- Failure due to deterioration of strength with passage of time.- Buildings with less damping contribute to high damage.- Failure of nonstructural members like parapets, chimneys and window panes.- Narrow streets are blocked by the falling of debris and it is impossible to prevent the

rapid spread of fire which causes indirect damages, and also hinders relief operations.

Building-foundation-soil interaction

The quasi resonance during an earthquake, i.e. coincidence of the predominant period of vibra-tions of soil layers and the fundamental period of structure causes severe damage to the build-ings. The predominant period of ground vibration is the fundamental period of the soil layers atthat site. Smaller periods are noted for firmer or rocky soil and lager for soft soil. Generally, firmsoils are more suitable for earthquake resistance for all types of buildings. Sometimes structur-ally strong buildings fail due to inadequate foundation design. Following are the reasons fordamages due to interaction.

- Tall buildings resting on soft soil undergo severe damage due to quasi resonance, if thefoundations are not properly designed.

- Short buildings resting on firm soil undergo severe damage due to quasi resonance unlessfoundation isolation systems are used.

- Buildings constructed on poor soil such as fills suffer severe damage due to its settlement.- Excessive settlement of foundation soil causes cracking and failure of superstructure- Structure resting on loose sand with high water table may lead to liquefaction and

building may sink, tilt or both.- Isolated footings are likely to be subjected to differential settlement.- Shallow foundations deteriorate because of weathering.

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LESSONS FROM THE DAMAGE BEHAVIOUR OF BUILDINGS

Learning by mistakes Yes, each earthquake damage points out the inadequacies of the prevalentdesign and construction practices in that region. The buildings constructed by taking properearthquake resistant measures based on the lessons from various earthquakes have certainlyhelped in minimising the degree of damages. The observation of damage behaviour of buildingsduring earthquakes confirm the direction of modern code provisions.

Following are the summary of the lessons learnt

- It is observed that the poor man's house has been most vulnerable to damages and most ofthe loss of lives are due to the collapse of these houses constructed in traditional materialslike adobe, unreinforced bricks, stone and the like without adequate earthquake resistantmeasures. Therefore, effort should be made for creating general awareness about the tech-nology of earthquake resistant design and construction among the masses.

- The pattern of damage reveals that if the earthquake resistant measures as specified inbuilding codes are adopted buildings are quite safe from seismic viewpoint.

- The layout of buildings should be as simple as possible and there should not be any suddenchange in the distribution of mass or stiffness.

- Use as far as possible light weight building material such as bamboo, timber and PVC inhighly seismic areas.

- Avoid construction of heavy structures at the roof such as water tank etc.- Adequate strength in longitudinal and transverse direction should be provided. Additional

vertical load is subjected on beams and columns due to vertical component of ground motionwhich should be catered for in the design.

- The frame of the building should have adequate ductility so as to permit energy dissipationthrough plastic deformations.

- Proper detailing of joints (wall to roof, wall to wall, beam to column) for all type of construc-tion should be made. In precast and wood buildings joints are the vulnerable locations offailure.

- Site selection should be based on local geology and the subsoil properties which modify theearthquake ground motion. A seismic microzoning survey in high seismic area will be helpfulin this decision making.

- Avoid quasi resonance i.e. the fundamental natural frequency of structure should be awayfrom the predominant period of the ground.

- Hard foundation is found to be suitable for all types of building. Construction of buildings onloose soil such as fill should be avoided unless proper care is taken in the foundation design.

- Loose sand with high water table subjected to violent ground shaking which may lead toliquefaction. The liquefaction causes differential settlement, tilting or sinking of buildings.

- Shallow foundation deteriorates due to weathering. Isolated footing undergo differential settle-ment. Tall buildings resting on piles withstood the earthquakes well.

- Settlement on hill slopes were landslide is expected should be avoided.- For important and tall buildings proper dynamic analysis should be carried out.- Strong columns and weak beam design concept should be aimed so as to prevent total

collapse. Close ties should be provided in columns were large moment is expected.

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- Narrow streets get blocked by failing debris during earthquakes and hinder relief operationsafter the earthquake. In fact the narrow streets become the death trap for many. It alsoprevents in controlling the spread of fire.

- Buildings such as hospitals, fire stations, communication (telegraph and telephone exchange)buildings etc. should be designed and constructed for earthquake resistant, so as to remainfunctional after the earthquake for quick relief operations.

- More earthquake damage of buildings should be studied and continuous updating of buildingcodes and construction techniques should be carried out.

REFERENCES

Agrawal, P.N. (1982), Damage due to Great Nicobar India Earthquake of January 20, 1982,Proc. 7th Sym. on Earthquake Engg., University of Roorkee, Nov. 10 12, pp.3-12.Arya, A.S., A.R. Chandrasekaran and L.S. Srivastava (1968), Koyna Earthquake Investiga-tions, Vol. V. No. 182, pp 83-86, March, June.Arya, A.S., L.S. Srivastava and Satyendra P. Gupta (1983), Report on Dhamar Earthquake ofDecember 13, 1982, Earthquake Engineering Deptt., University of Roorkee.Arya, A.S. and S.K. Thakkar (1984), Tall Building-Seismic Design Seminar on Tall Structuresand use of Prestressed Concrete in Hydraulic Structures, Srinagar, May 24-26.Ashwani kumar, P.N. Agrawal, and A.R. Chandrasekaran (1981), A Study of Indo-Nepal Earth-quake of May 21, 1979, Proc. Earthquake Disaster Mitigation, Univ. of Roorkee, pp 43-52,March.Berg, G.V. and J.L. Stratt, (1964), The 1963 Skopje Earthquake, Proceedings of the Third WorldConference on Earthquake Engineering, Auckland and Wellington, New Zealand, Vol. 3.Borges, J.F., J. Grases and A. Ravara (1969), Behaviour of Tall Buildings During the CaracasEarthquake of 1967, Proc. Fourth World Conf. on Earthquake Engg. Vol. 3, Chile.Brijesh Chandra and M. Lal (1970), Behaviour of Load Bearing Brick Shear Walls with Open-ings, Bull. Indian Soc. of Earth. Tech., Vol. 7, No.3.Bulsari, B.S. and M.C. Thakkar (1970), Response of structures in Broach Earthquake, Bull.ISET Vol. VII, 4, pp 197-206.Brijesh Chandra and Krishen Kumar (1974), Earthquake Resistant Construction of Brick Build-ings, Earthquake Engineering, Jai Krishna Sixtieth Birth Anniversary Commemorative Volume,Sarita Prakashan, pp 123-138.Degenkolb, H.J. (1955), Structural Observations of the Kern Country Earthquake, trans. Am.Soc. Civil Engineer, 120, pp 1280-1294.Degenkolb, H.J. and R.D. Hanson (1969), The July 29, 1967, Venezuela earthquake Lessons forEarthquake Engineers, Proc.IV World Conf. on Earthquake Engg., Vol. 3, Chile.Despeyroux, J. (1984), some Lessons to be Drawn from the El-Asnam Earthquake of October10, 1980, Proc. 8WCEE, San Francisco, USA.Dewell, H.D. and B. Willis (1925), Earthquake Damage to Buildings, Bull. Seism. Soc. Am., 15,pp 282-301.Engle, H.M. (1936), The Montana Earthquakes of October 1935, Structural Lessons, Bull. Seis.Soc. Am., 26, 99-109.

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Housner, G.W. (1959),Behaviour of Structures During Earthquakes, J. Engg. Mech. Div. ASCE,Vol. 85, EM4.Housner, G.W. et al (1971), Engineering Features of the San Fernando Earthquake, Bull. ISET,Vol. 8, 2, pp 75-84).Housner, G.W. and D.E. Hudson (1958), the Port Hueneme Earthquake of March 18, 1957, Bull.Seism. Soc. Am., 48, pp 163-168.Inard, C.D. (1925), report of Engineering Committee on the Santa Barbara Earthquake, Bull.Seism, Soc. Am., 15, pp 302-304.Jai Krishna (1967), Earthquake Resistant Design of Building-Lessons From Recent Earthquakes,Bull. Indian Soc.of Earthquake tech., vol. 54,No. 1, pp 242-261.Jai Krishna and A.S. Arya (1966), Damage to Buildings at Srinagar During Hindukush Earth-quake of June 6, 1966, Earthquake Engg. Studies, University of Roorkee, roorkee.Jai Krishna and Brijesh Chandra (1965), Strengthening of Brick Buildings Against EarthquakeForces, proc. III WCEE, New Zealand.Jai Krishna and Brijesh Chandra (1969), Strengthening of Brick Buildings in Seismic Zones,Proc. IV WCEE, Chile.Jai Krishna and Brijesh Chandra (1972), Earthquake Engineering in Asia, J. Build. International,pp 35-37.Jai Krishna and Brijesh Chandra (1983), Earthen Buildings in Seismic Area of India, DisasterManagement, Vol.3, No. 1,Jan-March, pp 52-55.Jai Krishna and Brijesh Chandra and S.B. Kanungo (1966), Behaviour of Load Bearing WallsDuring Earthquakes, Proc. Third Symp. on Earthquake Engg., university of roorkee, Roorkee.Joshi, R.N. (1968), Study of Damages and Throws in Koyna Earthquake, Bull. ISET, Vol. V. No.1,2, pp 11-24, March-June.Kawasumi, Hirost (1968), edited), General Report on the Nigata Earthquake of 1964, tokyoElectrical Engineering College Press.Kunze, F.J., et al (1965), The March 27, 1964, Alaskan Earthquake, Portland Cement Associa-tion.Mallick, D.V. (1964), Engineering Studies of Building Response During El-Asam Earthquake ofOctober 10, 1980, Proc. 8WCEE, San, Francisco, USA.Mallick, D.V. and Y.A. Meriami (1982), Lessons from El-Asnam Earthquake of 10th October1980, 7th Sym. on Earthquake Engg., University of Roorkee, Roorkee, Nov. pp 13-18.Okamoto, S. (1973), Introduction to Earthquake Engineering, Chapter 18, pp 527-538, Universityof Tokyo Press.Polyakov, S. (1974), Design of Earthquake Resistant Structures, Chapter III, Translated in En-glish by A. SCHWARTE, Mir Publishers, Moscow.Ramachandran, B., S.R. Pradhan and A.S. Dhancta (1976), A Review of Seismicity of EasternHimalaya, Proc. Himalayan Geology Seminar, GSI Publication, pp 41-50.Ravindra Prakash and S.K. Mam (1981), Behaviour of Buildings in Bhaddu-Bilawar Area Dur-ing August 24, 1980 Earthquake,Proc. Earthquake Disaster Mitigation, Univ. of roorkee, pp 63-70, March.

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Rosenblueth, E.(1960), the Earthquake of July 28, 1957 in Mexico City, Proc. Sec. World Conf.on Earthquake Engg., Vol. 3, tokyo and Kyoto, Japan.Satyendra P. Gupta, Ashok Kumar and R.C. Agrawal (1981), Preliminary Report on WesternNepal-India Border Region Earthquake of 29 July, 1980, Proc. Earth. Disaster Mitigation, Uni-versity of Roorkee, pp 53-62, March.Singh, S., P. Sinha, A.K. Jain, V.N. Singh and L.S. Srivastava (1975), Preliminary Report onJanuary 19, 1975 Kinnaur Earthquake in Himachal Pradesh, Earthquake Engg. Studies, EQ-75-4, University of Roorkee.Steinbrugge, K.V. (1965), structural Engineering Aspects of the Alaskan Earthquake of March27, 1964, Report at the Third World Conference on Earthquake Engineering, New Zealand.Steinbrugge, Karl V. (1970), Earthquake Damage and Structural Performance in the UnitedStates, Chapter 9, in Earthquake Engineering Edited Report L. Wiegel, Prentice-Hall, Inc.Englewood Cliffs, N.J.Steinbrugge, K.V., and V.R. Bush (1960), Earthquake Experience in North America, 1950-1959,proc. of the Second World Conf. on Earthquake Engg., vol. I, pp 381-396.Steinbrugge, K.V. and W.K. Cloud (1962), Epicentral Intensities and Damage in the HebgenLake, Montana Earthquake of August 17, 1959, Bull. Seis. Soc. Am. 52, pp 181-234.Tandon A.N. and H.N. Srivastava (1974), Earthquake Occurrence in India, Earthquake Engi-neering, Chapter I, Jai Krishna Sixtieth Birth Anniversary Commemorative Volume, SaritaPrakashan Meerut, India, pp 1-48.Thornley, I.H and Pedro Albin (1957), Earthquake Resistant Construction in Mexico City, CivilEngineering, October.Ulrich, F.P. (1941), the Imperial Valley Earthquakes of 1940, Bull. Seis. Soc. Am. 31 pp 13-31.Wiegel, R.L. (1970), Earthquake Engineering, Chapter 9, Prentice-Hall, Inc., Englewood, Cliffs,N.J.---(1907), American Society or Civil Engineers The Effects of the San Francisco Earthquake ofApril 18, 1906 on Engineering Constructions, Trans. Am. Soc. Civil Engr., 59, pp 208-329.---(1963), An Engineering Report on the Chilean Earthquake of May 1960, Bull. of the seismo-logical Society of America, No. 2, Vol. 53.---(1965), Japan National Committee on Earthquake Engineering, Niigata Earthquake of 1964,proc. Third World Conf. on Earthquake Engg., Auckland and Wellington, Vol. 3, New Zealand.---(1977), Code of Practice for Earthquake Resistant Design and Construction of Buildings,Indian Standard Institution IS:4326-1976, Manak Bhawan, New Delhi.---(1977), Influence of Natural Disasters (Earthquakes) on Educational Facilities, Part II- Strength-ening of Buildings Against Earthquakes, Earthquake Engineering Studies, EQ 77-14, Universityof Roorkee, Roorkee, December.---(1980), Basic Concepts for Seismic Codes Non Engineered Construction, Report of Commit-tee 11, International Association for Earthquake Engineering.---(1981), A Manual of Earthquake Resistant Non-Engineered Construction, Indian Society ofEarthquake Technology, Publication Fine Press, New Delhi

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LESSONS ON DETAILING FROM PASTEARTHQUAKES

Pankaj AgarwalAsstt. Professor, Department of Earthquake Engineering, IIT Roorkee, 247 667

INTRODUCTION

Conventional earthquake resistant design of a reinforced concrete building depends on its basicelement called ductility, which enables redistribution and reduction of internal actions, and dissi-pation of earthquake energy. Observations of past earthquakes have shown that there is a needto pay attention to proportioning, to ensure that inelastic action occurs at appropriate location,and detailing, to ensure adequate ductility in the location that yield of these elements. In thispaper, some of the more prominent observations and failure modes of individual structural ele-ments are summarized.

REINFORCED CONCRETE COLUMNS

The columns have damaged mainly due to lack of confinement, large tie spacing, insufficientsplices length, inadequate splicing at the same section, hook configurations, poor concrete qual-ity, less than full height masonry infill partitions, and a combinations of many of the above,compounded with vertical and geometrical irregularities. Failure of column has catastrophicconsequences for a structure. The most common modes of failure of column are as follows.

Mode 1: Formation of plastic hinge at the base of ground level columns

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Mechanism

In older reinforced concrete building frames, column failures were more frequent since thestrength of beams in such constructions was kept higher than that of the columns. This shearfailure brings forth loss of axial load carrying capacity of the column. As the axial capacitydiminishes, the gravity loads carried by the column are transferred to neighbouring elementsresulting in massive internal redistribution of forces, which is also amplified by dynamic effectscausing spectacular collapse of building (Moehle and Sezen, 2000).

Reason

Wide spacing of transverse reinforcement

Design Considerations

To improve an understanding of column shear strength, as well as to understand how the gravityloads will be supported after a column fails in shear. The clause 7.3 of IS 13920: 1993 is a step tomeet this requirement.

Mechanism

When a column is subjected to seismic motion, its concrete begins to disintegrate and the loadcarried by the concrete shifts to its longitudinal reinforcement. This additional load causes buck-ling of longitudinal reinforcement. As a result, the column shortens and looses its ability to carryeven the gravity load (Kono and Watanabe, 2000).

Reasons

Insufficient confinement length and improper confinement in plastic hinge region due to smallernumber of ties

Design Consideration

This type of damage is sensitive to the cyclic moments generated during the earthquake andaxial load intensity. Consideration of plastic hinge length or length of confinement is needed. Theclause 7.4 of IS 13920: 1993 is a step to meet this requirement.

Mode 2: Diagonal shear cracking in mid span of columns

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Mechanism

Splices of column longitudinal reinforcement in older buildings were commonly designed forcompression only with relatively light transverse reinforcement enclosing the lap. For example,as per IS: 456 - 1978, a lap splice length of 20 or 24 longitudinal bar diameters with transversereinforcement should be equal to the least column dimension or 16 longitudinal bar diameter.Under earthquake motion, the longitudinal reinforcement may be subjected to significant tensilestresses, which require lap lengths for tension substantially exceeding those for compression. Asa result slip occurs along the splice length with spalling of concrete (Wallace and Melek, 2000).

Reasons

Deficient lap splice length of column longitudinal reinforcement with lightly spaced transversereinforcement, particularly if the splices just above the floor slab, which is very common in olderconstruction.

Design Consideration

Lap splices should be provided only in the center half of the member length and should beproportionate to tension splice. The clause 7.2 of IS 13920: 1993 is a step to meet thisrequirement.

Mode 4: Shear failures in captive columns and short columns

Captive Column

Column whose deforming ability is restricted and only a fraction of its height can deform later-ally. It is due to presence of adjoining non-structural elements, columns at slopping ground,partially buried basements etc.

Short Column

Column is made shorter than neighbouring column by horizontal structural elements such asbeams, girder, stair way landing slabs, use of grade beams, and ramps.

Mode 3: Shear and splice failure of longitudinal reinforcement

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Mechanism

A reduction in the clear height of captive or short columns increases the lateral stiffness. There-fore, these columns are subjected to larger shear force during the earthquake since the storeyshear is distributed in proportion to lateral stiffness of the same floor. If these columns,reinforced with conventional longitudinal and transverse reinforcement, and subjected torelatively high axial loading, fail by splitting of concrete along their diagonals, if the axial loadinglevel is low, the most probable mode of failure is by shear sliding along full depth cracks at themember ends. Moreover, in the case of captive column by adjoining non-structural walls, theconfinement provided to the lower part of the column is so effective that usually damage isshifted to the short non- confined upper section of the column.

Reasons

Large shear stresses, when the structure is subjected to lateral forces are not accounted for inthe standard frame design procedure

Design Consideration

The best solution for captive column or short column is to avoid the situation otherwise useseparation gap in between the non-structural elements and vertical structural element with ap-propriate measures against out-of-plane stability of the masonry wall. The clause 7.4 of IS13920: 1993 is a step to meet this requirement.

REINFORCED CONCRETE BEAMS

There is little evidence that the buildings have collapsed due to beam failure. Only a fewexamples exist in which buildings have exhibited plastic hinging in the beam. The probableregions of hinging are at and near their intersections with supporting columns. An exception maybe where a heavy concentrated load is carried at some intermediate point on the span. Thecauses of hinging are lack of confinement of concrete core and support for the longitudinal

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compressive reinforcement against inelastic buckling. The shear- flexure mode of failure is mostcommonly observed during the earthquakes, which may be described as follows.

Mode: Shear- flexure failure

Mechanism:

Two types of plastic hinges may form in the beams of multi-storeyed framed construction de-pending upon the span of beams. In case of short beams or where gravity load supported by thebeam is low, plastic hinges are formed at the column ends and damage occurs in the form ofopening of a crack at the end of beam otherwise there is the formation of plastic hinges at andnear end region of beam in the form of diagonal shear cracking.

Reasons:

lack of longitudinal compressive reinforcement, infrequent transverse reinforcement in plastichinge zone, bad anchorage of the bottom reinforcement in to the support or slip of the longitudinalbeam reinforcement, bottom steel termination at the face of column.

Design Consideration

Adequate flexural and shear strength must be provided and verification by design calculation isessential. The beams should not be too stiff with respect to adjacent columns so that the plastichinging will occur in beam rather than column. To ensure that the plastic hinge zones in beamshave adequate ductility, the following must be considered (Booth, 1994)

- Lower and upper limits on the amount of longitudinal flexural tension steel (clause 6.2.1 of IS13920: 1993).

- A limit on the ration of the steel on one side of the beam to that of on the other side (clause6.2. 2 to 6.2.4 of IS 13920: 1993).

- Minimum requirements for the spacing and size of stirrups to restrain buckling of the longitudinal reinforcement (clause 6.3.2 of IS 13920: 1993).

REINFORCED CONCRETE BEAM - COLUMN JOINTS

Beam-column joints are critical element in frame structures and are subjected to high shear andbond-slip deformations under earthquake loading. Account for cross-sectional properties of the

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joint region, amount and distribution of column vertical steel, inadequate or absence of reinforce-ment in beam-column joints, absence of confinement of hoop reinforcement, inappropriate loca-tion of bar splices in column are the common causes of failure of beam-column joints. The mostcommon modes of failure in beam -column joint are as follows.

Mode: Shear failure in beam column joint

Mechanism

The most common failures observed in exterior joints are due to either high shear or bond(anchorage) under severe earthquakes. Plastic hinges are formed in the beams at the columnfaces. As a result, cracks develop throughout the overall beam depth. Bond deterioration nearthe face of the column causes propagation of beam reinforcement yielding in the joint and ashortening of the bar length available for force transfer by bond causing horizontal bar slippagein the joint. In the interior joint, the beam reinforcement at both the column faces undergoesdifferent stress conditions (compression and tension) because of opposite sighs of seismic bend-ing moments resulting in failure of joint core (UNDP, 1983).Reasons: Inadequate anchorage of flexural steel in beams, lack of transverse reinforcement

Design Considerations

Exterior Joint

The provision on anchorage stub for the beam reinforcement improves the performance ofexternal joints by preventing spalling of concrete cover on the outside face resulting in loss offlexural strength of the column. This increases diagonal strut action as well as reduces steelcongestion as the beam bars can be anchored clear of the column bars. The clause 6.2.5 of IS13920: 1993 is a step to meet this requirement.

Interior Joint: Reliable anchorage of the beam reinforcement in the joints.

REINFORCED CONCRETE SLABS

Generally slab on beams performed well during earthquakes and are not dangerous but cracks inslab creates serious aesthetic and functional problems. It reduces the available strength, stiff-

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Mechanism

Damage to slab oftenly occurs due to irregularities such as large openings at concentration ofearthquake forces, close to widely spaced shear walls, at the staircase flight landings.

Reasons: Existing micro cracks which widen due to shaking, differential settlement

Design Consideration

- Use secondary reinforcement in the bottom of the slab- Avoid the use of flat slab in high seismic zones, provided this is done in conjunction with a

stiff lateral load resisting system

REINFORCED CONCRETE SHEAR WALLS

Shear walls generally performed well during the earthquakes. Four types of failure mode aregenerally observed (Penelis and Kappos, 1997).

Modes: (i) Diagonal tension - compression failure in the form of cross- shaped shear cracking (ii)sliding shear failure cracking at interface of new and old concrete (iii) flexure and compressionin bottom end region of wall and finally (iv) Diagonal tension in the form X shaped cracking incoupling beams

ness and energy dissipation capacity of building for future earthquake. In flat slab construction,punching shear is the primary cause of failure. The common modes of failure are;

Mode: Shear cracking in slabs

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Mechanism

Shear walls are subjected to shear and flexural deformation depending upon the slendernessratio. Therefore, the damage in shear walls may generally occurs due to inadequate shear andflexure capacity of wall. Slender walls are governed by their flexural strength and crackingoccurs in the form of yielding of main flexure reinforcement in the plastic hinge region, normallyat the base of the wall. Squat walls are governed by their shear strength and failure takes placedue to diagonal tension or diagonal compression in the form of inclined cracking. Coupling beamsbetween shear walls or piers may also damage due to inadequate shear and flexure capacity.Sometimes damage occurs at the construction joints in the form of slippage and related drift.

Reasons

- Flexural/ boundary compression failure- inadequate transverse confining reinforcement tothe main flexural reinforcement near the outer edge of wall and in boundary elements

- Flexure /Diagonal tension - inadequate horizontal shear reinforcement (clause 9.4 of IS13920: 1993).

Flexural/ Diagonaltension

Flexural/ Diagonaltension

Flexure Shear cracks

Diagonal tension-compression failure

Sliding shear failure Flexure and compression

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Mechanism:

Frame with infill possesses much more lateral stiffness than the bare frame, and hence initiallyattracts most of the lateral force during an earthquake. Being brittle, the infill starts to disinte-grate as soon as its strength is reached. Infills that were not adequately tied to the surroundingframes, sometimes dislodges by out-of-plane seismic excitations.

Reasons

Infill causes asymmetry of load application, resulting in increased torsional forces and changes inthe distribution of shear forces between lateral load resisting systems.

Design Considerations

Two strategies are possible either complete separation between infill walls and frame by provid-ing separation joint so that the two systems do not interact or complete anchoring between frameand infill to act as an integral unit. Horizontal and vertical reinforcement may also be used toimprove the strength, stiffness, and deformability of masonry infill walls.

Design Considerations

- The concrete shear walls must have boundary elements or columns thicker than walls,which will carry the vertical load after shear failure of wall (clause 9.4 of IS 13920: 1993).

- A proper connection between wall vs. diaphragm as well as wall vs. foundation to completethe load path (clause 9.1 of IS 13920: 1993).

- Proper bonding at construction joint in the form of shear friction reinforcement (clause 9.8 ofIS 13920: 1993).

- Provision of diagonal steel in the coupling beam (clause 9.5 of IS 13920: 1993).

INFILL WALLS

Infill panels in reinforced concrete frames are the cause of unequal distribution of lateral forcesin the different frames of a building, producing vertical and horizontal irregularities etc. Thecommon modes of failure of infill masonry are in plane or shear failure.

Mode: Shear failure of masonry infill

- Sliding shear - absence of diagonal reinforcement across the potential sliding planes of theplastic hinge zone

- Coupling beams - inadequate stirrup reinforcement and no diagonal reinforcement

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Mode:

Mechanism

Parapet walls are acceleration sensitive in the out-of-plane direction; the result is that they maybecome disengaged and topple

Reasons

Not properly braced

Design Considerations

Analyzed for acceleration forces and braced and connected with roof diaphragm

REFERENCES

Booth, E (1994). "Concrete Structures in Earthquake Regions", Longman Scientific and Techni-cal, Longman Group UK Limited.Guevara, L.T. and Garcia, L.E. (2005). "The Captive and Short Column Effect," EarthquakeSpectra 21(1), 141-160.Kono, S, and Watanabe, F. (2000). "Damage Evaluation of Reinforced Concrete Columns underMulti-axial Cyclic Loadings," The Second U.S. - Japan Workshop on Performance Based Earth-quake Engineering Methodology for Reinforced Concrete Building Structures, PEER 2000/10.

Moehle, J.P., Wood, K.J. and Sezen (2000). "Shear failure and Axial Load Collapse of ExistingReinforced Concrete Columns," The Second U.S. - Japan Workshop on Performance BasedEarthquake Engineering Methodology for Reinforced Concrete Building Structures, PEER2000/10.

PARAPETS

Un-reinforced concrete parapets with large height-to-thickness ratio and improper anchoringto the roof diaphragm may also constitute a hazard. The hazard posed by a parapet increasesin direct proportion to its height above building base, which has been generally observed.The common mode of failure of parapet wall is against out-of-plane forces, which isdescribed as follows.

Brittle flexure out-of-plane failure

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Penelis, G G. and Kappos, A. J. (1997). "Earthquake-Resistant Concrete Structures" E & FNSPON an Imprint of Chapman & Hall.UNDP/UNIDO Project RER/79/015 (1983). "Repair and Strengthening of Reinforced Con-crete, Stone and Brick Masonry Buildings," Building construction under seismic conditions in theBalkan Regions, Vol. 5. United Nations Industrial Development Programme., Austria.

Wallace, J.W. and Melek, M (2000). "Column Splices: Observed Earthquake Damage, ModelingApproaches, and the PEER/ UCLA Research Program," The Second U.S. - Japan Workshopon Performance Based Earthquake Engineering Methodology for Reinforced Concrete BuildingStructures, PEER 2000/10.

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73

GEOTECHNICAL CONSIDERATIONS IN EARTHQUAKERESISTANT DESIGN

B.K. MaheshwariAsstt. Professor, Dept. of Earthquake Engineering, IIT Roorkee, 247 667

INTRODUCTION

Effects of earthquakes on ground and foundations have been described briefly. Design offoundations from earthquake consideration is discussed for practicing engineers. Codal provi-sions for the design are elaborated in a simple way. Description has been divided into threedistinct parts:

i. Earthquake Effects and Earthquake Damageii. General Foundation Design (without Earthquake)iii. Codal Provisions (IS: 1893–Part1: 2002) for the ERD of Foundations

EARTHQUAKE EFFECTS AND EARTHQUAKE DAMAGE:

In this section, geotechnical aspects of earthquake damage are described. It includes effects ofground cracks (i.e. Surface Rupture), liquefaction, and landslides.

SURFACE RUPTURE

Most earthquakes do not create ground surface fault rupture. However, large earthquakes attransform boundaries are usually accompanied by ground surface fault rupture. The length ofthe fault rupture can be quite significant e.g. the estimated length of surface faulting in the 1964Alaskan earthquake varied from 600 to 720 km. Surface fault rupture associated with earth-quakes is important because it has caused severe damage to buildings, bridges, dams, tunnels,canals and underground utilities. There were disastrous examples of surface rupture associ-ated with the Chi-Chi (Taiwan) earthquake (M = 7.6) on Sept. 21, 1999. Fig. 1 and 2 showdamage to civil engineering structures associated with this earthquake.

January 26, 2001, Bhuj (India) earthquake was also a large magnitude (Mw =7.7) earthquakebut no primary surface fault rupture was identified. Many ground failures reported are due toliquefaction related ground deformation.

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LIQUEFACTION

Most critical damage (due to earthquakes) on ground is when soil deposits have lost theirstrength and appeared to flow as liquids. This phenomenon is termed as liquefaction in whichstrength of soil is reduced, usually to the point where it is unable to support structures.

Fig. 1 View of a damaged dam by surface fault rupture (Chi-chi earthquake)

Fig. 2 Close-up view of pier of Wu-Shi Bridge damaged by surface fault rupture Chi-chi EQ.

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Liquefaction occurs only in saturated sandy soils, therefore most commonly observed near wa-ter bodies. It is typically occurs in soil with a high groundwater table, its effects are most

commonly observed in low-lying area or area adjacent to rivers, lakes, bays and oceans. Lique-faction phenomena can affect buildings, bridges, buried pipelines, and other constructed facilitiesin many different ways. In general the effects of liquefaction involves several related phenom-ena e.g., flow failures, lateral spreading, and sand boils, which are discussed in following para-graphs.

Flow failures

occur when the strength of the soil drops below the level needed to maintain the stability understatic conditions. Flow failures have caused the collapse of earth dams and other slopes, and thefailure of foundations. Figs. 3-4 show examples of settlement and bearing capacity failures dueto liquefaction (Niigata, Japan earthquake on June 16, 1964).

Lateral spreading

is a liquefaction related phenomenon characterized by incremental displacements during earth-quake shaking. Lateral spreading is quite common near bridges and the displacements it pro-duces can damage the abutments, foundations, and superstructure of bridges as shown in Fig. 5.

Sand boils

produced by ground water rushing to the surface are present in the level-ground liquefaction thatdoes not involve large lateral displacements. Sand boils are not damaging by themselves butindicates the presence of high ground water pressures, whose eventual dissipation can producesubsidence and differential settlements (Fig. 6).

Fig. 3 Kawagichi-cho apartment buildings suffered liquefaction-induced bearing capacityfailure during the Niigata (Japan) earthquake on June 16, 1964.

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Fig. 4 Liquefaction-induced settlement and tilting of an apartmentbuilding - Niigata earthquake

Fig. 5. The Showa Bridge following the 1964 Niigata earthquake. Lateral spreading causedbridge pier foundation to move and rotate sufficiently for simply supported bridge span to fall

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Fig. 6 Sand boil in rice field following the 1964 Niigata earthquake.

Liquefaction during Bhuj earthquake (January 26, 2001):

The earthquake induced liquefaction and related ground failures over an area of greater than15,000 square km. Surface manifestations of liquefaction include sand blows, sand blow craters,and lateral spreading. Areas where widespread liquefaction occurred include the Great Rann ofKachchh, Little Rann, Banni Plain, Kandla River and Gulf of Kachchh (Fig. 7). These areascontain low-lying salt flats, estuaries, intertidal zones, and young alluvial deposits (meizoseismalarea), which are typically considered to have a very high susceptibility to liquefaction.

Fig. 7 Map showing general distribution of liquefaction resulting from theBhuj earthquake

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According to many residents in the meizoseismal area, fountains of water ranging from 1 to 2 min height formed during and immediately following the Bhuj earthquake. So much water ventedto the surface in the Banni Plain and Great Rann that temporary streams flowed in previouslydry channels. The surface water was so extensive that the media proclaimed the return of amythical river, possibly the Sarasvati. Satellite imagery suggests that liquefaction may have oc-curred near Naliya and Lakhpat along the coast about 180 km west of the epicenter. In additionthere are reports of ground failure indicative of liquefaction as far away as the Sabaramati Riversouth of Ahmedabad, about 240 km east of the earthquake epicenter.

Significant settlement of the backfill above a natural gas pipeline was observed over many km ina stretch of desert between the Little Rann and Great Rann. A four span, two-lane reinforcedconcrete bridge on National Highway 8A was under construction at the time of earthquake andwas severely damaged. Significant damage occurred at the east abutment to the support bentand wing walls. This could be attributed to liquefaction resulting in lateral spreading near theabutment and causing a rotational failure of the abutment and first pier. The Surajbadi Bridges;a railway bridge and two highway bridges suffered damages due to liquefaction.

LANDSLIDES

Strong earthquakes may cause landslides. In majority of the cases landslides are small but earth-quakes have also caused very large slides. In a number of cases, earthquake-induced landslideshave buried entire towns and villages (Fig. 8). Earthquake induced landslides cause damage bydestroying buildings or disrupting bridges and other facilities. Many earthquakes landslides resultfrom liquefaction phenomena, but many other simply represent the failures of slopes that weremarginally stable under static conditions.

The Bhuj earthquake also produced numerous rockfalls from steep slopes and road-cuts. Rockfallsincluded topple failures and surfacial raveling. Blocks up to 2 m across were displaced on thenorth side of the Island Belt near Khadir Island. Failures of embankments and cut-slopes werealso widespread. Slope failures were most highly concentrated in the area near Bhuj and Bhachau.No large-scale rotational failures were observed on native slopes.

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Fig. 8. Village of Yungay, Peru, (a) before and (b) after being buried by a giant landslide in the1970 Peruvian earthquake. The same palm trees are visible at the left side of both photographs.The landslide involved 50 million cubic meters of material that eventually covered an area ofsome 8000 square kilometers. About 25,000 people were killed by this landslide, over 18,000 inthe villages of Yungay and Ranrahirca.

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In Chamoli (Himalaya, India) earthquake on March 29, 1999, ground cracks at several placesdeveloped as part of slope failure and these pose threat to the down-slope settlements. Crackswere seen in asphalt roads at some locations, indicating the possibility of failure due to

ground slippage. At several sites, large-scale earthquake-induced landslide/rock falls wereobserved as shown in Fig. 9.

Fig. 9 Chamoli (India), earthquake: A major landslide about 1 km north of Gopeshwar. Itblocked the road traffic to Okimath for a considerable period. (Source: NICEE, IITK,India, website)

GENERAL FOUNDATION DESIGN (WITHOUT EARTHQUAKE):

The various types of structural foundations can be grouped into two categories, namely:

1. Shallow Foundations2. Deep Foundations

Usually foundations are considered shallow if depth of foundation (Df) is less than or equal towidth of foundation (Bf). A shallow foundation transmits structural loads to the soil strata at arelatively small depth. In deep foundations, the load is supported partly by frictional resistancearound the foundation and the rest by bearing at the base of the foundation.

The choice of a particular type of foundation depends on the magnitude of loads, thenature of the subsoil strata, the nature of the superstructure and its specific require-ments . For reasons of economy, shallow foundations should be the first choice of afoundation engineer unless they are considered inadequate.

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Common types of shallow foundations are shown in Fig. 10 and briefly described as:

(a) Strip footing or continuous footing: Commonly used below walls (length is much greater thanwidth L>>B), Fig. 10a.

(b) Spread footing: Square or circular in section, commonly used below a column (isolated – Fig.10b). Or below more than one column (combined – Fig. 10c and 10d) when the shape iscommonly rectangular or trapezoidal in plan.

(c) Raft or mat foundation which covers the entire area of a structure, transmitting the entirestructural load or load from several columns (Fig. 10e)

Fig. 10. Common types of footings: (a) Continuous footing (L>>B), (b) Spread footing (square,

circular or rectangular), (c) combined footing (trapezoidal) (d) strap footing (e) mat or raftfoundation

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General Requirements of Foundations

For a satisfactory performance, a foundation must satisfy the following three basic criteria:

(a) Location and Depth Criterion: A foundation must be properly located at such a depth that itsperformance is not adversely affected by factors such as lateral expulsion of soil frombeneath the foundation, seasonal volume changes causes by freezing and thawing and pres-ence of adjoining structures. Indian Standard Code (IS: 1904-1986) makes the recommen-dation that a foundation should be located at a minimum depth of 50 cm below natural groundsurface.

(b) Bearing Capacity Criteria: A foundation must be safe against shear strength failure or soilrupture. An adequate factor of safety is provided to avoid bearing capacity failure. Threedifferent types of failure mechanism are shown in Fig. 11. Fig. 12 indicates the type offailure mode that can be expected for a footing in sands. Shallow foundations in very densesand can be expected to fail in general shear failure mode. Shallow foundations in loose sandand deep foundations are likely to fail in punching shear.

Fig. 11. Typical modes of failure: (a)general shear failure (b) punching shear

failure (c) local shear failure

Fig. 12. Regions of three different modes offailure

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According to Terzaghi, ultimate bearing capacity (qu) for a strip footing is given by:

γγBNqNcNq qcu 5.0++=

where Nc, Nq and Ny are dimensionless bearing capacity factors which depends on the angle ofshearing resistance of the soil. c, q and y are cohesion, surcharge, unit weight of soil respectivelywhile B is the width of foundation.

Above equation for bearing capacity is based on general shear failure and it is modified for othermodes of failure e.g. for local shear failure. Also it is for strip footings and modified for squareand circular footings. Further, if water table is at a depth less than width of footing (B) below thebase of the footing, reduction in bearing capacity is considered.

(c) Settlement Criteria: The settlement of a foundation, especially the differential settlementmust be within permissible limit. Excessive settlement may affect the utility of the structure,may even cause damage to the structure. The total settlement S consists of immediatesettlement, primary consolidation and secondary compression.

sci SSSS ++=

Bearing Capacity from Building Codes

Safe bearing capacity (SBC) varies widely according to type of soil, approximate values can beestimated according to IS: 1904-1961 as shown in Table 1.

Table 1: Values of safe bearing capacity for various soils according to IS: 1904-1961.

Cohesionless Soils1. Gravel, sand and gravel, compact and offering high resistance to penetration when excavated by tools2. Coarse sand, compact and dry3. Medium sand, compact and dry4. Fine sand, silt (dry lumps easily pulverized by the fingers)5. Loose gravel or sand gravel mixture: loose coarse to medium sand6. Fine sand, loose and dryCohesive Soils1. Soft, shale, hard or stiff clay in deep bed, dry2. Medium clay readily indented with a thumb nail3. Moist clay and sand clay mixture which can be indented with strong thumb pressure4. Soft clay indented with moderate thumb pressure5. Very soft clay which can be penetrated several inches with the thumb6. Black cotton soil or other shrinkable or expensive clay in dry condition (50 % saturation)

450

450250150250100

450250150

10050150

Description SBC (kN/m2)

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Permissible Total and Differential Settlements

The effect of settlement upon the structure depends on its magnitude, its uniformity, the length ofthe time over which it takes place and nature of the structure itself.

According to National Building Code of India (SP: 7-1970), for simple spread footing allowablebearing pressure should be such that differential settlement does not exceed 1/300. This condi-tion is generally satisfied if total settlement is limited to 50 mm for sands and 75 mm for clayeysoils.

Types of Foundations to Suit Subsoil Conditions

A foundation has to transfer the structural load to the supporting soil in such a way that the soilneither fails in shear nor settles excessively. Before selecting a proper type of foundation, suchas shallow foundation (strip footing or combined footing or raft foundation) or deep foundation(piles or piers), it is essential to know the subsoil conditions and the soil properties at the site.Design of foundation is worked out only after an appropriate foundation type has been chosen.

Selection of foundation type for different soil profiles (Fig. 13) are illustrated in followingparagraphs:

Case (a): Dense sand provides a good bearing stratum for spread footings with their depthgoverned by consideration of possible erosion or scour. Deep foundations such as piles may berequired only if uplift or any other unusual forces are present.

Case (b): Subsoil being stiff or firm clay, spread footings are satisfactory for conventionalneeds. Piles are required only for unusual situations mentioned in case (a).

Case (c): The upper firm clay stratum provides a satisfactory bearing stratum for a spreadfooting only if the loads are low to medium and the footings are not placed close to the underlyingsoft clay stratum. Otherwise, deep foundations such as piles or piers are to be provided upto adepth where sufficient load bearing capacity is forthcoming.

Case (d): This is a classic example of the subsoil condition where a deep foundation such aspiles or piers can be provided, bearing directly on or socketed inside the rock stratum.

Case (e): Another typical example of the choice of deep foundation. Case in situ piles such asbulb piles into the sand stratum are most suitable.

Case (f): Spread foundation is ruled out. Raft foundation can, however be considered as apossibility. If the loads are not heavy, the possibility of first densifying the loose sand by vibro-floatation and then using spread foundation can be considered as another alternative. If thesealternatives are not satisfactory, the driven piles are the best choice, as they would help densifythe sand soil.

Case (g): Spread foundation is not suitable. If practically feasible, a partially or a fully compen-sated raft foundation may be provided. If not, friction piles would be the best choice. The lengthof piles can be increased so as to bring the settlement within limit.

85

Fig. 13. Some typical subsoil conditions

86

Case (h): The upper compact sand layer, a good bearing stratum, is too thin to place a spreadfoundation in it because of the likelihood of excessive settlement in the underlying soft layer.Drilled piers with an enlarged base formed in the hard clay layer or bored and cast in situ pileswith a bulb formed in the hard clay layer are suitable.

Case (i): The poor fill layer is too thick to consider replacing it with a better material. Deepfoundation like driven and cast in situ piles or drilled piers extending into the medium dense sandlayer, or better still, going into the compact glacial till stratum would prove to be satisfactory. It isunnecessary and uneconomical to go further deep into the rock.

Case (j): Pile foundation, bearing in the upper portion of the dense sand layer, would be satisfac-tory. This would prevent stresses reaching the clay layer and causing significant settlement.Another alternative is to remove the 2 m thick poor fill layer and replace it with a compacted filland provide spread foundation in the new fill material.

Case (k): For loads which are not very large, piles or piers bearing on the upper zone of thedense sand layer may be considered, ensuring the resulting settlement in the clay layer will bewithin limit. Compensated raft is another possibility that can be considered. For really heavyloads, driven steel piles or caissons bearing on rock stratum will be suitable.

Case (l): Since rock is available at only 4.5 m depth, piles or piers bearing on rock wouldobviously be most satisfactory. But if basement floors are going to be useful, excavating the soilup to rock level and providing two basement floors with the base slab resting on rock, would beideal.

Design of Deep Foundations

Deep foundations include pile foundations, well foundations, caisson foundations. Deign of thesefoundations may be found in standard text books for example Rao and Ranjan (2002), Bowles(1997).

CODAL PROVISIONS (IS: 1893-PART1: 2002) FOR THE ERD OF FOUNDATIONS

For the Earthquake Resistant Design (ERD) of structures, IS: 1893-Part 1: 2002, provides gen-eral provisions. Though not much detail is given in the present code about foundation design butsimple guideline is provided. Clause 6.3.5.2 of the code suggests the increase in allowable pres-sure in soils while considering earthquake forces.

Accordingly when earthquake forces are included, the allowable bearing pressure in soils shallbe increased as given in Table 1 of the code. Also in soil deposits consisting of submerged loosesands and soils falling under classification SP, to avoid liquefaction or to avoid excessive total anddifferential settlements, code suggests following minimum N-values (Corrected SPT value ac-cording to IS 2131):

1. In Zones III, IV and V: N =15

2. In Zone II: N = 10

87

Further, Note 3 and Note 4 in the Table 1 shall be considered for desirable N values. Codesuggests that for locating new settlements and important projects such sites should be avoided.Otherwise, this aspect of the problem needs to be investigated and appropriate methods ofcompaction or stabilization adopted to achieve suitable N values. Alternatively, deep pile founda-tion may be provided and taken to depths well into the layer which is not likely to liquefy. Marineclays and sensitive clays are also known to liquefy and will need special treatment according tosite condition.

Few of the geotechnical provisions mentioned in the code need a review. These are related to

(i) Soil Classification: The group symbols given in Table 1 are not consistent with the soil clas-sification according to IS: 1498-1970.

(ii) Increase in Allowable Bearing Pressure in Soil: According to international practice increasein bearing pressure is only up to one third (33 %) instead of 25 to 50% suggested in code.Further no increase in bearing pressure may be recommended for soft soils.

(iii) Determining the N values for layered site for identifying the response spectrum: it may beweighted average.

All these three issues are discussed in detail in the document IITK-GSDMA-EQ13-V1.0 whichcan be downloaded from NICEE wbsite.

REFERENCES:

Day R.W. (2002). Geotechnical Earthquake Engineering Handbook, McGraw-Hill Handbooks,New York.

EERI (2002). Bhuj India Earthquake of January 26, 2001 Reconnaissance Report, EarthquakeSpectra, Supplement to Vol. 18, Earthquake Engineering Research Institute, USA

Kramer S.L. (1996). Geotechnical Earthquake Engineering, Prentice-Hall, Englewood Ciffs,NJ, USA.

Prakash S. (1981), Soil Dynamics, McGraw-Hill Book Company, New York, USARanjan G. and Rao A.S.R. (2000), Basic and Applied Soil Mechanics, second edition, New AgeInternational (P) Ltd., Publishers, New Delhi.

88

89

PHILOSOPHY AND PRINCIPLESOF EARTHQUAKE RESISTANT DESIGN

Yogendra SinghAsstt. Professor, Department of Earthquake Engineering, IIT Roorkee, 247 667

INTRODUCTION

Man has been building shelters for time immemorial. He has observed his buildings beingwashed away by floods and landslides and razed down by earthquakes and fire. He has learntlessons from these calamities and developed methods to safeguard his construction.

V

Vdes Linear ElasticBuilding Response

Lack of Knowledge on EarthquakeDemand and BuildingCapacity

?

Inelastic ResponseVdes

V Elastic Forces Reduced for Design by R

maxyield

S

des

Performance Point

Demand Reduced Based on Inelastic Capacityof building

Sd

(a) (b)

(c) (d)

Fig. 1 Evolution of Earthquake Resistant Design

Chapter 7

90

The first significant point regarding the earthquakes, which was learnt by mankind was thatearthquakes cause lateral loads on buildings. Today, it may appear very obvious and easy tounderstand, but it was a great leap of understanding when somebody presented this idea firsttime. Since then, our understanding of earthquakes and their effects on buildings has increased alot.

Initially there was no understanding of origin and occurrence of earthquakes. Now we havesignificant information about origin of earthquakes and their recurrence periods in different partsof the world. Further, we have a fair idea of the expected characteristics of earthquakes likely tooccur in different parts of the country and world. Although, this information, by no means, isadequate for predicting the characteristics of expected ground shaking at a given location, wecan estimate the average probable values on regional basis for design purposes.

The first concept of earthquake resistant design was (Fig.1) to design the buildings for a lateralload which was 5% to 10% of gravity load. Later, it was discovered that earthquake force on astructure depends on its time period of vibrations. Further, it was seen that a structure canwithstand much higher force during earthquake, than for which it was designed. This is due toductility of structures. Ductility is the property of the structure by which it can deform plasticallywithout loosing its vertical load carrying capacity. The current practice of designing earthquakeresistant structures, takes into account both strength and ductility of structures, under earth-quake loads.

CHARACTERISTICS OF EARTHQUAKE GROUND MOTION

Earthquakes occur due to rupture of earth's crustal rock along the planes of weakness calledfaults. Magnitude of an earthquake is an indicator of the total energy released during therupture, while Intensity is the severity of shaking of ground at a given location. Among theseveral prevalent scales, Richter scale is the most commonly used scale for magnitude ofearthquake and MMI and MSK scales are the most popular scales for measuring the Intensity ofscale. The damage at a site is indicated by the intensity of ground shaking at the site. Thedamage potential of ground shaking at a location depends on the following parameters:

(i) Amplitude of ground motion, i.e. Peak Ground Acceleration (PGA), Peak Ground Velocity(PGV), and Peak Ground Displacement (PGV),

(ii) Frequency content of ground motion, and(iii) Duration of earthquake.

It is customary to consider the effect of first two parameters in the design, which are repre-sented in the form of response spectrum. The effect of duration of earthquake is generally notconsidered by the current codes of practice, as it is difficult to be estimated and modelled. Theseparameters in tern depend on the magnitude of earthquake, rupture characteristics, path charac-teristics, local topology and local geotechnical conditions at site. All these characteristics to-gether cause large variation in ground motion within a city or a region.

91

This variation can be considered inSeismic Microzonation of the city,which is a costly and involved pro-cess.

In India efforts have been initiatedtowards Microzonation of someimportant cities. However, thepresent code of practice is basedon the concept of SeismicMacrozonation, in which an aver-age ground shaking level is assignedto a considerably large zone. Atpresent the country is divided intofour zones as shown in Fig. 2. Fig.3shows the normalized shape of theresponse spectra as per IS:1893-2002. Three different shapes

Fig. 2 Seismic Macrozonation of India

Fig. 3. Normalised spectral shape as per IS:1893-2002

92

have been suggested for three different types of soil conditions. These shapes take into accountthe local site conditions to some extent. The code specifies zone factors, which may be inter-preted as the Effective Peak Ground Accelerations, for each zone. The normalized spectralshape multiplied by the zone factor results in the design response spectrum for the given zoneand the given soil type.

Another important parameter of the earthquakes is their probability of occurrence, which isnormally interpreted in terms of the average return period of the earthquake of a given severity.It is obvious that a larger magnitude earthquake will have lower probability of occurrence orlarger average return period. Our code (IS:1893-2002) has defined two levels of earthquakes -Maximum Considered Earthquake (MCE) and Design Basis Earthquake (DBE). The code issilent about the probability of occurrence of these earthquakes, but, as per the prevailing defini-tions world over, the MCE corresponds to 2% probability of exceedance in 50 years and anaverage return period of 2,500 years, while the DBE corresponds to 10% probability of exceedancein 50 years and an average return period of 475 years. The severity of ground shaking in MCEis about double of that in DBE and it is considered as a theoretical limit on the maximum groundshaking, which can occur at a site.

PHILOSOPHY AND PRINCIPLES OF EARTHQUAKE RESISTANT DESIGN

It should be clear that earthquakes result in very high lateral forces on structures. it will beuneconomical to design all the buildings for such high earthquake forces. As earthquakes arerare events, the IS: 1893 outlines the philosophy of earthquake resistant design that the buildingshould not have any significant structural damage under moderate earthquakes, which arerelatively frequent. On the other hand, under a major earthquake, which is rare (average returnperiod being 1000 years or more), the building may undergo severe damage, but it should notcollapse in any case, as collapse results in large scale loss of life.

To avoid collapse of buildings during earthquakes there are four basic principles: (i) Reducedmass; (ii) Symmetry and Continuity of construction, (iii) Strength and overstrength and (iv)Ductility of structure. There are functional limits on reduction of mass, but it is obvious that alight weight structure will attract less force compared to a heavy structure. Seismic performanceof a symmetric and regular structure has been observed to be much better than the asymmetricand irregular structures. The common types of irregularities found in buildings are described inthe following sections. Role of strength and overstrength in resisting the inertia forces due toearthquakes is obvious. Overstrength is that part of the strength of the structure, which is notexplicitly estimated in the design process and considered as a reserve strength. It arises due tohigher material strength, strain hardening, strength increase due to strain rate effect, memberoversize, provided reinforcement more than required, codal minimum requirements, effect ofnon-structural elements and redundancy etc.

The role of ductility in resisting the earthquakes is not that obvious to common sense. It can bevisualized by considering the earthquake ground motion as an energy imparted to the

93

structure, which is to be dissipated by the structure. Ductility is the property of the structurewhich helps in energy dissipation without excessive damage or collapse of the structure. This isbeing dealt with in detail in the following sections.

COMMON IRREGULARITIES TO BE AVOIDED

Regularity of stiffness and strength, and symmetry of configuration are the most important fac-tors governing the seismic performance of a building. The current code identifies this fact, andthe following irregularities have been described in the code:

Torsion Irregularity

To be considered when floor diaphragms are rigid in their own plan in relation to the verticalstructural elements that resist the lateral forces. Torsional irregularity to be considered to existwhen the maximum storey drift, computed with design eccentricity, at one end of the structurestransverse to an axis is more than 1.2 times the average of the storey drifts at the two ends of thestructure.

Re-entrant Corner

Plan configurations of a structure and its lateral force resisting system contain re-entrant cor-ners, where both projections of the structure beyond the re-entrant corner are greater than 15percent of its plan dimension in the given direction.

Diaphragm Discontinuity

Diaphragms with abrupt discontinuities or variationsin stiffness, including those having cut-out oropen areas greater than 50 percent of the gross enclosed diaphragm area, or changes in effec-tive diaphragm stiffness of more than 50 percent from one storey to the next.

Out-of-Plane Offsets

Discontinuities in a lateral force resistance path, such as out-of-plane offsets of vertical ele-ments.

Non-parallel Systems

The vertical elements resisting the lateral force are not parallel to or symmetric about the majororthogonal axes or the lateral force resisting elements.

Soft Storey

A soft storey is one in which the lateral stiffness is less than 70 percent of that in the storeyabove or less than 80 percent of the average lateral stiffness of the three storeys above.

Extreme Soft Storey: A extreme soft storey is one in which the lateral stiffness is less than 60percent of that in the storey above or less than 70 percent of the average stiffness of the threestoreys above. For example, buildings on STILTS (Fig. 4) will fall under this category.

94

Fig. 5 In- plane irregularity Fig. 6 Out of plane irregularity

In-Plane Discontinuity in Vertical Elements Resisting Lateral Force: An in-plane offset of thelateral force resisting elements greater than the length of those elements (Fig. 5).

Out-of-Plane Discontinuity in Vertical Elements Resisting Lateral Force: An out-of-plane offsetof the lateral force resisting elements (Fig. 6).

Fig. 4 Failure of an extreme soft storey building

Mass Irregularity: Mass irregularity shall be considered to exist where the seismic weight ofany storey is more than 200 percent of that of its adjacent storeys. The irregularity need not beconsidered in case of roofs.

Vertical Geometric Irregularity: Vertical geometric irregularity shall be considered to existwhere the horizontal dimension of the lateral force resisting system in any storey is more than150 percent of that in its adjacent storey.

95

Weak Storey: A weak storey is one in which the storey lateral strength is less than 80 percent ofthat in the storey above. The storey lateral strength is the total strength of all seismic forceresisting elements sharing the storey shear in the considered direction.

IMPORTANCE OF DUCTILITY IN EARTHQUAKE RESISTANT DESIGN

Fig. 6 shows the resistance vs. displacement curve for a typical building. The ductility is definedas the ratio of the maximum displacement um to the yield displacement uy . Larger is the capacityof the building to deform after yielding, without collapse, larger is the ductility of the building. Thisductility is very important in loss of energy under cyclic loading, such as earthquake loading. Theeffective damping ratio depends on the ratio of the energy dissipated due to hysteresis, ED ineach cycle and the total strain energy Eso. Fig. 7 shows the two energies as the areas under loaddeformation curves. The effective damping result is reduction of effective earthquake forces onthe building.

Displacement

um= uy

umuyO

ry

Resis

tanc

e

Actual Yield Point Effective Yield

LevelUseful Limit of Displacement

Actual Resistance

Effective Elastic Limit

ED

Keffective

Bilinear representation of capacity spectrum

Capacity spectrum

Spec

tral

Acc

eler

atio

n

Kinitial

Spectral Displacement

Eso

dpdY

ay

ap

Fig. 6 Typical load-displacement curvefor a building

Fig. 7 Energy dissipation due to ductilityresulting in effective damping

Fig. 8 shows the reduction in effective earthquake force on the building due to its ductility. Asshown in the Fig., it has been observed that total displacement of a yielding long period (i.e. tall)building remains almost same, under a given earthquake, irrespective of its ductility. This is called"Equal Displacement Principle". This means that we can design a structure in a number ofcombinations of strength and ductility. If the building has no ductility, we have to design it for avery high lateral force. As shown in the Fig., the reduction in force R is equal to ductility µ insuch a case.

For a short period (i.e. short) building the total displacement is not the same but the total energyabsorbed by the building remains same. The total energy absorbed is shown by the area of the

force-displacement curve and as shown in the Fig. 8, 12 −= µR , in this case.

Ductility of building depends on the material of construction, and proportion and detailing of thecomponents of the building. Based upon the ductility of different of building IS: 1893 gives thereduction factors for the buildings (Table-1).

96

HOW CAN WE MAKE RC BUILDINGS DUCTILE

Concrete is known to be brittle material, i.e. it fails suddenly when subjected to load. But con-crete can be made ductile when confined by reinforcement. Fig. 9 shows the behaviour ofunconfined and confined concrete. It can be seen that confinement not only increases the strengthof concrete, but it tremendously increases the ductility of concrete. The confinement of concreteis obtained by providing stirrups. Here, it is very important, that stirrups should be hooked at 1350

into the core concrete, otherwise these stirrups open up under force due to earthquake and theconfining action is not available.

Ecu

Escc

Ec

f i

Et Eco 2Eco Esp

Compressive Strain, Ec

Ecc

f'c

f'cc

Com

pre

ssiv

e S

tress

, fc

Assumed for cover concrete

Unconfined concrete

First hoopFracture

Confined concrete

Fig. 9 Behaviour of Confined and Unconfined Concrete

FE

Displacement

Ductile

FY

Sei

smic

Forc

eElastic

RY

m

mY

FE

FY

2

2

Displacement

Sei

smic F

orce

ElasticDuctile

Y m

m

Y(R+1)

Fig. 8 Equal Displacement and Equal Energy Principle

97

Table 1. Response Reduction Factors as per IS:1893-2002

S. No.

Lateral Load Resisting System R

1. Ordinary RC moment-resisting frame ( OMRF ) 3.0 2. Special RC moment-resisting frame ( SMRF ) 5.0 3. Steel frame with

a) Concentric braces b) with Eccentric braces

4.0 5.0

6. Steel moment resisting frame designed as per SP 6 5.0 7. Load bearing masonry wall buildings

a) Unreinforced b) Reinforced with horizontal RC bands c) Reinforced with horizontal RC bands and vertical bars at

corners of rooms and jambs of openings

1.5 2.5 3.0

8. Ordinary reinforced concrete shear walls 3.0 9. Ductile shear walls 4.0

10. Buildings with Dual Systems Ordinary shear wall with SMRF Ordinary shear wall with SMRF Ductile shear wall with OMRF Ductile shear wall with SMRF

3.0 4.0 4.5 5.0

Further, even with confinement, RC members are sufficiently ductile in bending action only, butnot in axial and shear action. Therefore, we have to ensure that RC members should yield inflexure and not in axial or shear action. This can be ensured by designing the RC members insuch a way that their shear and axial load capacity is higher than their capacity in flexure. Thisconcept is called "Capacity Design" and it can be understood by the following analogy.

>PE/Pi

PiSPiPiS

P1

(c)(b)(a)

Pi

2

(n+1)

(n+ )

lun + 2

+n 2'

1

''

+

Brittle Chain LinksDuctile Links Brittle Links

l

221

2

+

Ductile Chain LinksDuctile Linksn Brittle Links

PE

PO

PiS

Pi

PO

PO = Pi = PE

l

1

'

2

POP IS

l1

'

1

Fig. 10 Analogy for Capacity Design

98

Fig. 11 Local and Global failure mechanisms

Fig. 10, shows a chain, which has one ductile link, while all other links are brittle. This chain issubjected to load P at the ends, as shows in the Fig. Now, the question is, whether the failure ofchain will be brittle or ductile? This can be answered, if we know whether the ductile link is goingto fail first or a brittle link. If the capacity of all brittle links is higher than the ductile link, thefailure of the chain will be ductile, otherwise it will be brittle. This concept is used in making abuilding to behave in a ductile manner by designing all the brittle modes to have higher strengththan the ductile modes.

In a building two modes of failure are possible (Fig. 11). In the first mode of failure columns ofone storey yield and building fails in a local mechanism. On the other hand, in the second modeof failure, all the beams yield first than the columns. This type of failure mechanism is calledglobal mechanism. It is obvious that the second mode of failure provides much larger ductilitythan the first mode. This can be achieved by designing the beams of the building weaker than thecolumns. "Weak beam and strong column design" is the most important concept of buildingdesign.

REFERENCES

IS 1893-2002, Criteria for Earthquake Resistant Design of Structures, Part 1 General Provisionsand Buildings, Bureau of Indian Standards, New Delhi.IS 456-2000, Plain and Reinforce Concrete - Code of Practice, Bureau of Indian Standards,New Delhi.IS 13920-1993, Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces- Code of Practice, Bureau of Indian Standards, New Delhi.Key, David, 1988, Earthquake Design Practice for Buildings, Thomas Telford, London.IS 4326-1993, Earthquake Resistant Design and Construction of buildings - Code of practice,Bureau of India Standards, New Delhi.Penelis, George G., and Kappos, Andreas J., 1997, Earthquake Resistant Concrete Structures, E& FN Spon.Paulay T., and Priestley, M.J.N., 1992, Seismic Design of Reinforced Concrete and MasonryBuildings," John Wiley & sons, Inc., New York.ATC 40, 1996, Seismic Evaluation and Retrofit of Concrete Buildings, Applied Technology Council,California.

99

EARTHQUAKE RESISTANT DESIGN, IS:1893-2002 CODE

D.K. PaulDepartment of Earthquake Engg., IIT Roorkee, Roorkee, 247667

INTRODUCTION

If an elastic design is based on the most severe earthquake, the structure is expected to experi-ence, then it would result in a very uneconomical design. Unless a structure is designed toundergo limited damage without collapse for the most severe earthquake, an economically ac-ceptable design cannot possibly be achieved. It is observed that structures designed for lowseismic coefficient have withstood much severe earthquakes successfully with some minor dam-age. This is attributed to reserve capacity of structure, which is available while the structureundergoes limited damage. If this reserve capacity of the structure is utilized, then the designallowing limited damage by limiting the ductility will be economically acceptable. The acceptabledamageability is different for different structures. The most acceptable approach would be todesign structures to resist most frequent moderate earthquake elastically and then check theresistance for infrequent most severe earthquake allowing limited damage without collapse,which may occur in useful lifetime of a structure. To account for ductility as above, the elasticaverage spectra were reduced by a factor of nearly 5, which was adopted by the earlier Codes.

HISTORICAL BACKGROUND

To safeguard the structures from the devastating earthquakes in the country, the Indian Stan-dards Institution initiated action nearly 45 years ago for the formation of unified recommenda-tions for earthquake resistant design of structures which resulted, for the first time, in the publi-cation of IS:1893-1962 "Recommendation for Earthquake Resistant Design of Structures". Itwas subsequently revised in 1966. Since the publication of the first revision of this standard, itwas felt to revise the standard again incorporating many changes adding a more rational ap-proach for design of buildings. These were incorporated in the second revision of IS:1893 broughtout in 1970. As a result of increased use of the Standard, considerable amount of suggestionswere received for modifying some of the provisions of the Standard and therefore, third revisionof the Standard was brought out in 1975. Additional data, knowledge and experience made itpossible to bring out fourth revision of Indian Standard IS:1893-1984, "Criteria for EarthquakeResistant Design of Structures". In the fifth revision of IS 1893 (2002), IS 1893 has been splitinto the following, five parts and only Parts 1 and 4 have been brought out.

Chapter 8

100

Part 1: General provision and buildingsPart 2: Liquid retaining tanks-elevated and ground supportedPart 3: Bridges and Retaining wallsPart 4: Industrial structures including stack like structuresPart 5: Dams and embankments

The Code of Practice for Earthquake Resistant Design and Construction of Buildings (IS: 4326-1976) has been revised and subdivided into five Codes.

IS: 4326-1993 Code of Practice for Earthquake Resistant Design and Construction of BuildingsIS: 13827-1993 Guidelines for Improving Earthquake Resistance of Earthen

BuildingsIS: 13828-1993 Guidelines for Improving Earthquake Resistance of Low Strength MasonryIS: 13920-1993 Code of Practice for Ductile Detailing of Reinforced Concrete Structures subjected to Seismic ForcesIS: 13935-1993 Guidelines for Repair and Seismic Strengthening of Buildings

A COMPARISION OF IS:1893-2002 AND IS:1893:1984 CODES

The Indian Standard "Criteria for Earthquake Resistant Design of Structures (IS: 1893-1984)"has been revised. The seismic provision for the multi-storey-framed building as per IS: 1893-1984 and the revised Code are compared. Basic difference between the IS: 1893-1984 andrevised Code is that in earlier Code horizontal seismic coefficient for design is calculated from alower value of basic seismic coefficient, and upgraded it to design basis earthquake by multiply-ing it with many factors like importance factor and soil-foundation factor. While in revised Code,horizontal seismic coefficient are calculated for Design Basis Earthquake (DBE) from Maxi-mum Credible Earthquake (MCE) by dividing it a factor of 2 and other factor like responsereduction factor.

Code recommends mainly two methods, seismic coefficient and the response spectrum method.In both the methods due considerations are given to the seismic Zone where the structure islocated as well as to the importance of the structure, soil foundation system, ductility of construc-tion, flexibility of the structure, and weight of the building.

Important modifications made in the Revised Code

i) The seismic Zone map is revised with only four Zones, instead of five. Zone I has beenmerged with Zone II. Hence, Zone I do not appear in the new zoning only Zones II, III, IV,and V do.

ii) The values of seismic Zone factor have been changed; these now reflect more realisticvalues of effective peak ground acceleration under Maximum Credible Earthquake (MCE)in each seismic Zone.

iii) Response spectra are now specified for three types of founding strata, namely rock, me-dium and soft soil, separately.

101

iv) Empirical expression for estimating the fundamental natural period Ta of multi-storeyedbuildings with regular moment resisting frames has been revised.

v) This revision adopts the procedure of first calculating the actual force that may be experi-enced by the structure during the probable maximum earthquake, if it were to remain elas-tic. The, concept of response reduction factor is introduced in place of the earlier perfor-mance factor.

vi) The soil-foundation system factor is dropped. Instead, a clause is introduced to restrict theuse of foundations vulnerable to differential settlements in severe seismic Zones.

vii) Torsional eccentricity values have been revised upwards in view of serious damages ob-served in buildings with irregular plans.

viii) Modal combination rule in dynamic analysis of buildings has been revised.

LOAD COMBINATION

Most multi-storey building construction in India is done in reinforced concrete. Steel is usuallyused only for industrial structures because of high cost. The limit state design method is com-monly used for design of buildings. In the limit state design method, for both the reinforcedconcrete and pre-stressed concrete, the material strength partial safety factor is prescribed at1.5 on concrete strength and at 1.15 on the yield stress of steel. The following load combinationsshall be accounted for reinforced and pre-stressed concrete structures.

where DL is the dead load; IL is the imposed load and EL is earthquake load.

In the design of steel structures, IS 800 (1984) allows the use of the working stress or the plasticmethod of design. In the plastic design of steel structure, the following load combinations shall beaccounted for:

The responses due to each component may be combined using the assumption that when themaximum response from one component occurs, the responses from the other two componentsare 30% of their maximum. The response due to earthquake force (EL) is the maximum of thefollowing three cases:

a) 1.5(DL + IL) b) 1.2(DL + IL ± ELX) c) 1.2(DL + IL ± ELY) d) 1.5(DL ± ELX) e) 1.5(DL ± ELY) f) 0.9DL ± 1.5ELX g) 0.9DL ± 1.5ELY

a) 1.7(DL + IL) b) 1.7(DL ± ELX) c) 1.7(DL ± ELY) d) 1.3(DL + IL ± ELX)e) 1.3(DL + IL ± ELY)

1) ± ELX ± 0.3ELY ± 0.3ELZ 2) ± ELY ± 0.3ELY ± 0.3ELZ 3) ± ELZ ± 0.3ELY ± 0.3ELZ

102

The three components can also be combined on the basis of square root of sum of the square(SRSS)

DESIGN LATERAL FORCE

The design lateral force shall first be computed for the building as a whole. This design lateralforce shall then be distributed to the various floor levels. The overall design seismic force thusobtained at each floor level shall then be distributed to individual lateral load resisting elementsdepending on the floor diaphragm action.

Design Seismic Base Shear, BV

The design seismic base shear is given by

WAV hB = (1)

where hA = horizontal seismic coefficient

W = Seismic weight (total dead load + appropriate amount of imposed load)

Seismic Weight of Building, W

The seismic weight of each floor is its full dead load plus appropriate amount of imposed load.The fraction of live load up to 3 kN/m2 of distributed floor load is 25 percent of imposed load andabove 3 kN/m2 is 50% of imposed load. While computing the seismic weight of each floor theweight of columns and walls in any storey shall be equally distributed to the floors above andbelow the storey. The seismic weight of the whole building is the sum of the seismic weight ofall the floors.

Horizontal Seismic Coefficient, hA

The value of horizontal seismic coefficient hA is given by

hA = )/()/)(2/(

IRgSZ a

(2)

Zone factor, Z

The seismic Zone map for the country was developed based on the epicentral distribution ofsignificant past earthquakes, the isoseismal distribution of significant past earthquakes, the geo-logical and tectonic data. The zoning map is based on expected maximum seismic intensity in aregion and does not divide the country into areas of equal risk. In the Code there are only fourZones. The modified Mercelli Intensity (MMI) of VI, VII, VIII and IX (and above) are associ-ated with the four Zones, II, III, IV and V. The Zone I has been upgraded to Zone II.

Zone factor (Z) refers to the zero period acceleration value for the Maximum CredibleEarthquake (MCE) in a Zone. In determining the base seismic coefficient a factor 2 in thedenominator of Z is used, so as to reduce the Maximum Credible Earthquake (MCE) to theDesign Basis Earthquake (DBE) value. Zone factor (Z) is given for Maximum Credible

103

Earthquake (MCE) in Table 1..

Table 1. : Values of seismic Zone factor

Serial no. Zone no. Z1 V 0.362 IV 0.243 III 0.164 II 0.105 I 0.10

Average response acceleration coefficient, )( gSa

Average response acceleration coefficient is obtained from the plot )( gSa vs. T (time period

of vibration), for different soil types.The design acceleration spectrum for vertical motions, when required, may be taken as two-thirds of the design horizontal acceleration spectrum. Figure 1 shows the proposed 5 percentspectra for rocky and soils sites and Table 2 gives the multiplying factors for obtaining spectralvalues for various other dampings.

Table 2 Multiplying Factors for Obtaining Values for Other Damping

Damping Percent

0 2 5 7 10 15 20 25 30

Factors 3.20 1.40 1.00 0.90 0.80 0.70 0.60 0.55 0.50

Fig. 1 Response Spectra for Rock and Soil Sites for 5 Percent Damping

Fundamental Natural Period

The fundamental period may either be established by experimental observations on similar build-ings or calculated by any rational method of analysis. In the absence of such data the approxi-mate fundamental natural period of vibration (T) in seconds is calculated as follows:

104

i) for moment resisting frame building without brick infill panels75.0075.0 hTa = for RC building

75.0085.0 h= for steel frame building (3)ii) for all other including moment-resistant frame buildings with brick infill panels

dh

Ta09.0= (4)

where h is the height of building, in m, and d is the base dimension of the building at the plinthlevel, in m, along the considered direction of the lateral force.

Importance Factor

Value of importance factor is taken as 1.5 for all important service and community structures,and 1 for all other buildings. The designer can choose a suitable value depending upon theimportance of the structure.

Response Reduction Factor

In view of the energy absorbing capacity available in inelastic range, ductile structures will beable to resist shocks without much damage. Depending on the perceived seismic damage perfor-mance of the structure, characterized by ductile or brittle deformations a factor R is introducedin the Code. However in calculating the horizontal seismic coefficient the ratio (R/I) shall not beless than 1.0. Table 2 gives the values of R for various types of buildings.

Table 2 Response Reduction Factor, R, for Building SystemsSI. No. Lateral Load Resisting System R

(1) (2) (3) Building Frame Systems

(i) Ordinary RC moment-resisting frame (OMRF) 2) 3.0 (ii) Special RC moment-resisting frame (SMRF)3) 5.0 (iii) Steel frame with

a) Concentric braces b) Eccentric braces

4.0 5.0

(iv) Steel moment resisting frame designed as per SP 6 (6) Building with Shear Walls4)

5.0

(v) Load bearing masonry wall buildings5) a) Unreinforced b) Reinforced with horizontal RC bands c) Reinforced with horizontal RC bands and vertical bars at

comers of rooms and

1.5 2.5 3.0

(vi) Ordinary reinforced concrete shear walls6) 3.0 (vii) Ductile shear walls7)

Buildings with Dual System8 ) 4.0

(viii) Ordinary shear wall with OMRF 3.0 (ix) Ordinary shear wall with SMRF 4.0 (x) Ductile shear wall with OMRF 4.5 (xi) Ductile shear wall with SMRF 5.0

105

Distribution of Design Force

The design base shear VB shall be distributed to different floor levels of the building as per thefollowing expression:

∑=

= n

jjj

iBi

hW

hWVQ

1

2

21

(5)

where iQ = Design lateral force at floor I; iW = Seismic weight of floor; ih = Height of floor

i measured from base, and n = Number of storeys in the building is the number of levels at whichthe masses are located.

DYNAMIC ANALYSIS

Dynamic analysis shall be performed to obtain the design seismic force, and its distribution todifferent levels along the height of the building and to the various lateral load-resisting elements,for the following buildings:

Regular Buildings- Those greater than 40m in height in Zone IV, and V, and those greater than90m in height in Zone II and III.

Irregular Buildings- All framed buildings higher than 12.0 m in Zone IV and V, and thosegreater than 40 meters in height in Zone II and III. There are two types of irregularity in buildings

Dynamic analysis may be performed either by the Time History Method or by the Response Spectrum Method. However, in either method, the design base shear (VB) shall be compared with a base shear ( BV ) calculated using a fundamental period Ta. Where VB. is less than BV , all the response quantities (for example member forces, displacements, storey forces, storey shears and base reactions) shall be multiplied by BB VV .

Time History Method

Time history method of analysis, when used, shall be based on an appropriate ground motion andshall be performed using accepted principles of dynamics.

Response Spectrum Method

Response spectrum method of analysis shall be performed using the design spectrum or by asite-specific design spectrum.

Free Vibration Analysis

Undamped free vibration analysis of the entire building shall be performed as per establishedmethods of mechanics using the appropriate masses and elastic stiffness of the structural sys-tem, to obtain natural periods (T) and mode shapes of those of its modes of vibration that needto be considered.

106

Modes to be considered

The number of modes to be used in the analysis should be such that the sum total of modalmasses of all modes considered is at least 90 percent of the total seismic mass and missing masscorrection beyond 33 percent. If modes with natural frequency beyond 33 Hz are to be consid-ered, modal combination shall be carried out only for modes upto 33 Hz. The effect of highermodes shall be included by considering missing mass correction following well established pro-cedures.

Analysis of building subjected to design forces

The building may be analyzed by accepted principles of mechanics for the design forcesconsidered as static forces.

Modal combination

The peak response quantities (for example, member forces, displacements, storey forces, storeyshears and base reactions) shall be combined as per Complete Quadratic Combination (CQC)method.

∑∑= =

=r

i

r

jjiji

1 1

λρλλ (6)

where, r = Number of modes being considered; ijρ = Cross-modal coefficient; iλ =

Response quantity in mode i (including sign); jλ = Response quantity in mode j (including sign),

2222

5.12

)1(4)1()1(8

ββζβββζρ

++−+=ij (7)

where ζ = Modal damping ratio (in fraction); β = Frequency ratio = ij ωω ; iω = Circular

frequency in ith mode, and jω = Circular frequency in jth mode,

Alternatively, the peak response quantities may be combined as follows:

a) If the building does not have closely-spaced modes, then' the peak response quantity )(λ due

to all modes considered shall be obtained as

∑=

=r

kk

1

2)(λλ (8)

where

kλ = Absolute value of quantity in mode k , and

r = Number of modes being considered.

107

b) If the building has a few closely-spaced modes, then the peak response quantity *λ due tothese modes shall be obtained as

∑=r

ccλλ*

(9)

where the summation-is for the closely spaced modes only. This peak response quantity due tothe closely spaced modes ( *λ ) is then combined with those of the remaining well-separatedmodes by the method described in(a) above.

Buildings with regular, or nominally irregular. plan configurations may be modelled as a system ofmasses lumped at the floor levels with each mass having one degree of freedom, that of lateraldisplacement in the direction under consideration. In such a case, (lie following expressions shallhold-in the computation of the various quantities:

a) Modal Mass- The modal mass (Mk) of mode k is given by

∑

∑

=

=

= n

iiki

n

iiki

k

Wg

WM

1

2

2

1

)(φ

φ

(10)

where g = Acceleration due to gravity; ikφ = Mode shape coefficient at floor i in mode k , and

iW = Seismic weight of floor i

b) Modal Participation Factors - The modal participation factor (Pk) of mode k is given by:

∑

∑

=

== n

iiki

n

iiki

k

W

WP

1

2

1

)(φ

φ

(11)

c) Design Lateral Force at Each Floor in Each Mode - The peak lateral force (Qik) at floori in mode k is given by

ikikkik WPAQ φ= (12)

where kA = Design horizontal acceleration spectrum value corresponding to natural period of

vibration of mode k .

108

Modal Analysis

This method of analysis is based on the dynamic response of the building idealized as having alumped mass and stiffness in various storeys with each mass having one degree of freedom, thatof lateral displacement in the direction under consideration. Response in each mode isdetermined by using the following relationship

Design lateral force at each floor is obtained by (12) and storey shear forces are obtained by(13). The peak response quantities (e.g., storey forces, storey shears, and base reactions) shallbe combined as per Complete Quadratic Combination (CQC) method or SRSS. Lateral forces ateach storey due to all modes are obtained by (14).

Dynamic analysis can be performed either by time history method or by the response spectrum

method. In either method the design base shear (VB) shall be compared with base shear ( BV )

calculated using fundamental period Ta Where VB is less than BV , all the response shall be

multiplied by ( BV /VB).

Determination of mode shape coefficient (φir):

A popular method for determination of the fundamental mode is the iterative Stodola Method.The equation of motion for a free vibrating motion of a multi-storeyed lumped mass can bewritten as:

[M] [ X&& ] + [K] [X] = 0 (15)

in which [M] is the diagonal matrix, [K] the stiffness matrix in relation to lateral displacementand, [X] and [ X&& ] are displacement vector corresponding to storey displacement and accelera-tion vector corresponding to storey acceleration matrices, respectively. Assuming the free vibra-tion is simple harmonic,

[X] = [φ ] sin wt (16)

d) Storey shear Forces in Each Mode - The peak shear force )( ikV acting in storey i in mode kis given by

∑+=

=n

ijikik QV

1(13)

e) Storey Shear Forces due to All Modes Considered - The peak storey shear force )( iV instorey I due to all modes considered is obtained by combining those due to each mode.

f) Lateral Forces at Each Storey Due to All Modes Considered - The design lateral forces

roofroof VF = and

1+−= iii VVF (14)

109

φ represents the shape of vibrating system, which does not change with time but varies only with amplitude, ω represents circular frequency of the system. Equation (15) can be written as,

-ω2[M] [φ] + [K] [φ] = 0 (17)

which can be solved to

[G] [M] [φ] = 2

1ω

[φ], where [G] = [K]-1 (18)

this equation is of the form

[A][X] = λ[X] (19)

which represents an eigen value problem whose solution leads to evaluation of natural frequency and corresponding mode shape. Knowing ω, the fundamental period for mode can be computed as:

1

2ωπ

=T (20)

TORSION

Building frames are unsymmetrical in plan as well as elevation. This leads to horizontal twistingof frames when subjected to wind or earthquake forces. This occurs when in building centre ofmass and centre of rigidity do not coincide. The design forces are to be applied al the centre ofmass appropriately displaced so as to cause design eccentricity between the displaced centre ofmass and centre of rigidity. However, negative torsional shear shall be neglected. The designeccentricity, edi lo be used at floor i shall be taken as:

−+

=isi

isidi beor

bee

05.0

05.05.1 (21)

whichever of these gives the more severe effect in the shear of any frame where die = Static eccentricity at floor i defined as the distance between centre of mass and centre of rigidity, and ib = Floor plan dimension of floor i perpendicular lo the direction of force. The factor 1.5 represents dynamic amplification factor, while the (actor 0.05 represent the extent of accidental eccentricity.

The accidental storey shear due to horizontal torsional moment may be calculated approximately by assuming the vertical elements at each storey to be fixed at the ends to parallel rigid plates. The torsional shear force acting on each element may then be taken as proportional to its lateral stiffness and its distance from the centre of rigidity of the storey under consideration.

In order to understand the method of determining the additional shear due to torsion, the building plan given in figure is examined. If xk and yk are lateral stiffness of a particular

element along the X and Y axes, then coordinates of the centre of rigidity, rX and rY with respect to an origin o are given in figure.

110

BUILDINGS WITH SOFT STOREY

In case buildings with a flexible storey, such as the ground storey consisting of open spaces forparking that is stilt buildings, special arrangement needs to be made to increase the lateral strengthand stiffness of the soft/open storey.

Dynamic analysis of building is carried out including the strength and stiffness effects of in fillsand inelastic deformations in the members, particularly, those in the soft storey, and the membersdesigned accordingly

Alternatively, the following design criteria arc lo be adopted after carrying out the earthquakeanalysis, neglecting the effect of infill walls in other storeys:

a) the columns and beams of the soft storey arc to be designed for 2.5 limes the storey shearsand moments calculated under seismic loads specified in the oilier relevant clauses: or.

b) besides the columns designed and detailed for the calculated storey shears and moments.Shear walls placed symmetrically in both directions of the building as far away from the centreof the building as feasible: to be designed exclusively for 1 5 times the lateral storey shear forcecalculated is before

DEFORMATIONS

Storey Drift Limitation

Code specified that the maximum horizontal relative displacement due to earthquake forcesbetween two successive floors shall not exceed 0.004 times the difference in levels betweenthese floors.

=rX ∑∑

y

y

k

xk , and =rY ∑

∑x

x

kyk

In which yx, are the coordinates and yx kk , are stiffness of the various elements in the twodirections, respectively. The summation is taken over all the vertical elements in the storey.The

rotational stiffness xyI of the structure about centre of rotation rC is given by,,

∑ += ][ 22 xkykI yxxy

In which x and y are the distances of elements from the centre of rigidity rC . if the

torsional moment is T, the torsional shears xV and yV on any column line can be computed as:

=xV xxp

kyIT

. , and

=yVpI

Tx yyk

In which xxk and yyk are the total stiffness of the column line under consideration in the x andy directions respectively..

111

Deformation Compatibility of Non-Seismic Members

For building located in seismic Zones IV and V, it shall be ensured that the structural compo-nents, that are not a part of the seismic force resisting system in the direction under consider-ation, do not lose their vertical carrying capacity under the induced moments resulting fromstorey deformations equal to R times the storey displacements.

Separation Between Adjacent Units

Two adjacent buildings, or two adjacent units of the same building with separation joint in be-tween shall be separated by a distance equal to the amount R times the sum of the calculatedstorey displacements of each of them, to avoid damaging contact when the two units deflecttowards each other. When floor levels of two similar adjacent units or buildings are at the sameelevation levels, factor R in this requirement may be replaced by R/2.

MISCELLANEOUS

Foundations

The use of foundations vulnerable to significant differential settlement due to ground shakingshall be avoided for structures in seismic Zones III, IV and V. In seismic Zones IV and V,individual spread footings or pile caps shall be interconnected with ties except when individualspread footings are directly supported on rock. All ties shall be capable of carrying, in tensionand in compression, an axial force equal to Ah /4 times the larger of the column or pile cap load,in addition to the otherwise computed forces.

Cantilever Projections

Vertical projections

Tower, tanks, parapets, smoke stacks (chimneys) and other vertical cantilever projections at-tached to buildngs and projecting above the roof, shall be designed and checked for stability forfive times the design horizontal seismic coefficient Ah . In the analysis of the building, the weightof these projecting elements will be lumped with the roof weight.

Horizontal projections

All horizontal projections like cornices and balconies shall be designed and checked for stabilityfor five times the design vertical coefficient (that is = 10/3 Ah).

The increased design forces as above are only for designing the projecting parts and their con-nections with the main structures. For the design of the main structure, such increase need notbe considered.

Compound Walls

Composed walls shall be designed for the design horizontal coefficient Ah with important factorI = 1.0.

112

Connections Between Parts

All parts of the building, expect between the separation sections, shall be tied together to act asintegrated single as beams to columns and columns to their footings, should be made capable oftransmitting a force, in all possible directions, of magnitude (Qi/Wi) times but not less than 0.05times the weight of the smaller part of the total of dead and imposed load reaction. Frictionalresistance shall not be relied upon for fulfilling these requirements.

SOIL FOUNDATION FACTOR

The soil-foundation system has several important effects on the seismic behaviour of astructure. First, the expected ground motion varies for different soil profiles. Second, theflexibility due to soil and foundation deformation leads to a higher natural period and increaseddamping, and thus in most cases a reduced seismic force.

EXAMPLE 1:

An eight storeyed RC framed building with live load of 3 kN/m2 (see Fig.1) is to be constructedin Agra (seismic Zone III). Work out seismic forces on the structure. All beams and columnsmay be assumed to be of 250 x 400 mm and 400 x 500 mm respectively. The roof and floor slabsmay be assumed as 150 mm thick. The wall is all round 120 mm thick.Solve the problem using both 1984 and 2002 code versions.

Table 3 - Dead weights

Item Size (L x B x H) (m) Number d.l.@( 3/ mkN ) Dead weight ( kN Beam

Column

Slab

Wall

0.4 x 0.25 x 7.5

0.4 x 0.5 x 3

22.5 x 22.5 x 0.15

22.5 x 3 x 0.12

24

16

1

4

24

24

24

20

0432.0

0230.4

1822.5

0648.0

Imposed load at all floors except roof floor (is taken as 25% of imposed load for 3 kN /m2 )

= 22.5 x 22.5 x 3 x 0.25 = 379.7 kN

Lumped mass at floor level 1

W1 = 432.0 + 230.4 + 1822.5 + 648.0 + 397.7 = 3512.6 kN

similarly , W1 = W2 = W3 = W4 = W5 = W6 = W7 = 3512.6 kN

Lumped mass at roof floor,

W8 = 432 + 115.2 + 1822.5 + 324 = 2693.7 kN

Design base shear

According to IS: 1893-1984

WKCV hB α=

113

where K = performance factor = 1.6 (for problem); =C a coefficient depending the flexibility of structure with the increase in the number of storeys depending upon fundamental period T ; =hα design seismic coefficient, =W total dead load + appropriate amount of live load; T = fundamental time period

sdhT 445.0/09.0 == Design seismic coefficient ( hα ) is calculated as:

Seismic coefficient method

oh Iαβα =

where, β = a coefficient depending upon the soil foundation system; I = factor depending upon importance of the structure; oα = basic horizontal seismic coefficient; For the problem β = 1, I = 1, oα = 0.04

hα = 0.04

Response spectrum method:

hα )/( gSIF aoβ=

where, oF = seismic Zone factor for average acceleration spectra = 0.20 for Zone III; gSa / = average acceleration coefficient for calculated time period, T = 0.445; gSa / = 0.17 from the graph.

hα = 0.034 Base shear calculation:

(i) Seismic coefficient method

WKCV hB α=

K = 1.6, C = 0.62, hα = 0.04, W = 27281.9 kN

BV = 1082.5 kN

(ii) Response spectrum method

W = 27281.9, hα = 0.034, K = 1.6, C = 0.85

BV = 1261.5 kN

According to revised Code 2002:

BV = hA W

W = Total gravity load of the building, = 27281.9 kN

114

ELEVATIONFig. 1

115

Table 5: Nodal forces and seismic shear forces at various levels

iQ Vi(shear force) kN Floor ih

(meters) 2

iWh

IS:1893 2002

IS: 1893-1984

IS:1893 2002 IS: 18931984

1

2

3

4

5

6

7

8

3

6

9

12

15

18

21

24

31613.4

126453.6

284520.6

505814.4

790335.0

1138082.4

1549056.8

1551571.7

9.7

38.7

87.0

154.7

241.6

348.0

473.7

474.5

6.7

26.7

60.0

106.7

166.8

240.2

326.9

327.5

1827.9

1818.2

1779.5

1692.5

1537.8

1296.2

948.2

474.5

1261.5

1254.8

1228.1

1168.1

1061.4

894.6

654.4

327.5

Σ=5977447.4

Time period for building without bracing or shear walls is calculated as

dhTa /09.0= = 0.445 s, from plot∴ 5.2/ =gSa

Z = 0.16, I = 1, R = 3 ; hA = 0.067

BV = 0.067 x 27721.1 = 1857.3 kN

Table 4 – Comparison of 1984 and 2002 codes

IS: 1893-1984 Revised IS: 1893-2002 Parameter Formula Value Formula Value

Time period dhTa /09.0= 0.445 sec dhTa /09.0= 0.445 sec

Spectral acceleration gSa / 0.17 gSa / 2.5

Seismic coefficient )/( gSIF aoh βα =

0.034 hA =

)/()/)(2/(

IRgSZ a

0.067

Base shear (kN) WKCV hB α= 1261.5 BV = hA W 1827.8

Distribution of lateral seismic shear force induced along the height of the building is given by the formula,

Qi = VB

∑=

=

ni

ii

i

Wih

Wih

1

2

2

In which ih is the height of ith floor measured from the base of the building.

116

EXAMPLE 2:

Analyze a 15 storeyed RC building as shown in Fig.2. The live load on all the floors is 2 kN /m2 and soil below the building is hard. The site lies in Zone V. All the beams are of size 400 x 500 mm and slabs are 150 mm thick. The sizes of columns are 600 x 600 mm in all the storeys and the wall all round is 120 mm thick. Also analyze if the building is on soft soil site.

Analysis of the Building

Calculations of dead load, live load and storey stiffness: As in case of seismic coefficient method, dead loads and live loads at each floor are computed and lumped. Stiffness in a storey is lumped assuming all the columns to be acting in parallel with each column contributing stiffness corresponding to 3/12 LEIK c = , where I is the moment of inertia about bending axis, L the column height and E the elastic modulus of column material. The total stiffness of a storey is thus ∑ cK . The value of cKI ,

and ∑ cK for all the floors/storeys is 1.08 x 1010 m m4 , 90240 mkN / and 1894800 mkN / respectively.

4 @ 7.5 m = 30 m

3 @ 7.5 m = 22.5 m

Elevation

15 @

3.0

m =

Fig. 2ELEVATION

Fig. 2

117

Table 6 - Calculation of Dead loads

Item Size (m x m x m) Number d.l.@ )/( 3mkN Dead weight ( kN )

Beam

Column

Slab

Wall

7.5 x 0.4 x 0.5

3.0 x 0.6 x 0.6

22.5 x 30 x 0.15

(22.5 + 30) x 3 x 0.12

31

20

1

2

24

24

24

20

1116.0

0518.4

2420.0

0756.0

Σ = 4810.4

Imposed load at all floors except roof floor (taken as 25% for live load 30 2/ mkN )

= 22.5 x 30 x 2.0 x 0.25 = 337.5 kN

Total dead load on all floors except roof = 4810.4 + 337.5 = 5147.9 kN

Dead load on roof floor = 1116 + 259.2 + 2420 + 378 = 4173.2kN

The first three natural frequencies and the corresponding mode shapes are determined and are given below.

Table 7 - Mode shape coefficients ( riφ ) and time period at various floor levels

Floor Mode 1 Mode 2 Mode 3 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1

0.356 0.353 0.347 0.336 0.323 0.305 0.285 0.261 0.235 0.206 0.175 0.143 0.108 0.073 0.037

-0.355 -0.33

-0.273 -0.190 -0.089 0.019 0.127 0.222 0.296 0.342 0.356 0.336 0.285 0.206 0.108

0.353 0.283 0.140 -0.039 -0.208 -0.324 -0.355 -0.296 -0.158 0.019 0.192 0.315 0.356 0.305 0.175

Period in seconds 1.042 0.348 0.210

Horizontal seismic coefficient for design

(i) According to the revised Code, hA =)/(

)/)(2/(IR

gSZ a

Assuming 5% damping in all the 3 modes, I =1.0, R =5, Z =0.36 (for Zone V)

1st mode g

Sa)1(

= 0.95; )1(hA = 0.034 (rock site) and

gS a

)1(

= 1.6 ; )1(hA = 0.0576 (soil site)

118

2nd mode g

S a)2(

= 2.50; )2(hA = 0.09 (same for both); 3rd mode

gS a

)3(

= 2.50; )3(hA = 0.09

(i) According to the IS: 1893-1984, g

SIF

rar

h

)(

0)( βα =

assuming 5% damping in all three modes, I = 1.0, β = 1.0, oF = 0.40 (in Zone V)

1st mode g

S a)1(

= 0.105; )1(hα = 0.042; 2nd mode

gSa

)2(

= 0.184; )2(hα = 0.0737

3rd mode g

Sa)3(

= 0.200; )3(hα = 0.080

The next step is to obtain seismic forces at each floor level in each individual mode. Modeparticipation factors in each mode are to be obtained. For this, Table 4 would be found convenientwhere in the method is explained for computation of P1 (mode participation factor for firstmode).

Table 8: Computation of mode participation factor P1

Floor No. Weight, iW Mode iiWφ 2iiWφ

coefficient, iφ1 5143.4 0.037 190.3 7.02 5143.4 0.073 375.5 27.43 5143.4 0.108 555.5 60.04 5143.4 0.143 735.5 105.25 5143.4 0.175 900.1 157.56 5143.4 0.206 1059.5 218.37 5143.4 0.235 1208.7 284.08 5143.4 0.261 1342.4 350.39 5143.4 0.285 1465.9 417.810 5143.4 0.305 1568.7 478.511 5143.4 0.323 1661.3 536.612 5143.4 0.336 1728.2 580.713 5143.4 0.347 1784.7 619.314 5143.4 0.353 1815.6 640.915 3924.0 0.356 1396.9 497.3

Σ17788.8 Σ4980.8

P1 = 8.49808.17788

= 3.571

Having obtained P1 = 3.57, P2 and P3 are obtained similarly as 1.18 and 0.698, respectively.

Seismic forces is then calculated as per equation (12) and is given in tabular form for modes 1,2 and 3 respectively.

119

Table 9- Computation of lateral forces and shears (first mode)

IS:1893-2002 IS: 1893-1984 )1(

iQ ∑= ii QV )1( Floor No.

iW iφ

Rock site

Soil site Rock site

Soil site

)1(iQ ) ∑= ii QV )1(

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

5143.4 5143.4 5143.4 5143.4 5143.4 5143.4 5143.4 5143.4 5143.4 5143.4 5143.4 5143.4 5143.4 5143.4 3924.0

0.037 0.073 0.108 0.143 0.175 0.206 0.235 0.261 0.285 0.305 0.323 0.336 0.347 0.353 0.356

23.2 45.8 67.8 89.8

109.9 129.4 147.6 163.9 179.0 191.5 202.9 211.1 218.0 221.7 170.6

39.2 77.2

114.3 151.3 185.2 218

248.6 276.1 301.5 322.7 341.7 355.5 367.1 373.5 287.3

2172.2 2149.0 2103.2 2035.4 1945.6 1835.7 1706.3 1558.7 1394.8 1215.8 1024.3 821.4 610.3 392.3 170.6

3659.2 3620

3542.8 3428.5 3277.2 3092 2874

2625.4 2349.3 2047.8 1725.1 1383.4 1027.9 660.8 287.3

28.1 55.8 83.0

110.28 134.5 158.2 180.3 200.4 218.4 234.2 247.4 258.0 265.9 271.0 208.4

2652.9 2629.8 2569.0 2486.0 2376.7 2242.2 2084.0 1903.7 1703.3 1484.9 1250.7 1003.3 745.3 479.4 208.4

Table 10 Computation of lateral forces and shears (second mode)

IS:1893-2002 IS: 1893-1984 Floor No.

iW iφ )2(

iQ ∑= ii QV )2( )2(iQ ) ∑= ii QV )2(

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

5143.4 5143.4 5143.4 5143.4 5143.4 5143.4 5143.4 5143.4 5143.4 5143.4 5143.4 5143.4 5143.4 5143.4 3924.0

0.108 0.206 0.285 0.336 0.356 0.342 0.296 0.222 0.127 0.019 -0.089 -0.190 -0.275 -0.330 -0.355

59.0 112.5 155.6 183.5 194.0 186.8 161.6 121.2 69.3 10.3 -48.6 -103.7 -150.2 -180.2 -147.9

623.6 564.6 452.1 296.5 113.0 -81.4

-268.2 -429.8 -551.0 -620.3 -630.6 -582.0 -478.3 -328.0 -147.9

48.7 92.8

128.2 151.4 160.3 154.0 133.2 99.8 57.0 8.7

-40.3 -85.6

-122.7 -148.3 -121.9

515.3 466.6 373.8 245.6 94.2 -66.1

-220.1 -353.3 -453.1 -510.1 -518.8 -478.5 -392.9 -270.2 -121.9

120

Table 11: Computation of lateral forces and shears (third mode)

IS:1893-2002 IS: 1893-1984 Floor No.

iW iφ )3(

iQ ∑= ii QV )3( )3(iQ ∑= ii QV )3(

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

5143.4 5143.4 5143.4 5143.4 5143.4 5143.4 5143.4 5143.4 5143.4 5143.4 5143.4 5143.4 5143.4 5143.4 3924.0

0.175 0.305 0.356 0.315 0.192 0.019 -0.158 -0.295 -0.355 -0.324 -0.208 -0.039 0.140 0.283 0.353

56.5 28.5

115.0 101.8 62.0 6.14 -51.0 -95.3

-114.7 -104.7 -67.2 -12.6 45.2 91.4 87.3

218.3 161.8 63.3 -51.7

-153.4 -215.4 -221.6 -170.6 -75.3 39.4

144.1 211.3 223.9 178.7 87.3

50.1 87.3

101.8 90.0 54.9 5.6

-45.2 -084.2 -101.5 -92.5 -59.5 -11.2 40.1 81.0 77.0

193.7 143.6 56.3 -45.5

-135.5 -190.4 -196.0 -150.8 -66.6 34.9 127.4 186.9 198.1 158.0 77.0

Combination of shears for the three modes: According to the IS: 1893-1984 by superposition of first three modes as follows

Vi = (1- γ) 2)(3

1

)(3

1

}{ ri

r

ri

r

VV ∑∑==

+ γ

where, V i(r) = absolute value of maximum shear at the ith storey in the rth mode; =γ 0.65 for building of height 45m

According to the revised Code: by Complete Quadratic Combination (CQC) method.

λ= jiji

r

j

r

i

λρλ∑∑== 11

where, λi and λ j are response quantity in mode i and j respectively

ρij = 2222

5.12

)1(4)1()1(8

ββζβββζ

++−+

β = Frequency ratio =i

j

ωω

Above quadratic combination of λ in matrix form can be written as

[λ11 λ12 λ13] ρ11 ρ12 ρ13 λ11

ρ21 ρ22 ρ23 λ21

ρ31 ρ32 ρ33 λ31

121

Drift (Lateral displacement or sway)

Table 12: Comparative values of shear forces and drift or maximum inter storey displacement (Stiffness iK = 1804.80 mmkN / )

Shear forces )(kNVi Relative displacement , max )/( ii KV mm IS:1983-2002 IS:1983-2002

Storey

IS: 1893-1984

Rock site Soil site

IS: 1893-1984

Rock site Soil site

1 2 3 4 5 6 7 8 9

10 11 12 13 14 15

2937.7 2872.5 2737.5 2596.0 2460.8 2337.8 2243.1 2105.0 1924.5 1731.2 1547.9 1316.7 1030.3 689.8 307.3

2266.0 2233.2 2157.7 2058.0 1946.0 1851.6 1743.1 1628.2 1503.5 1367.7 1206.0 1032.8 810.0 545.0 244.7

3718.4 3667.3 3572 3442 3283

3100.5 2895

2665.8 2415.7 2140

1842.3 1515.6 1155.6

759 334.7

1.63 1.59 1.51 1.43 1.36 1.29 1.24 1.16 1.06 0.96 0.85 0.73 0.57 0.38 0.17

1.26 1.24 1.20 1.14 1.07 1.02 0.96 0.90 0.83 0.75 0.67 0.57 0.45 0.30 0.13

2.06 2.03 1.98 1.90 1.82 1.71 1.60 1.47 1.34 1.18 1.02 0.84 0.64 0.42 0.19

122

123

EARTHQUAKE RESISTANT LOW STRENGTH MASONRY BUILDINGS

Pankaj AgarwalAsstt. Professor, Department of Earthquake Engineering, IIT Roorkee, 247 667

INTRODUCTION

Masonry is one of the most traditional, oldest materials and widely accepted medium for housingconstruction in India. This construction system is usually made spontaneously and informallywith the help of local masons without any or only a little intervention by professional experts,therefore it is termed as non-engineered construction. Non- engineered construction in India isgenerally made with fieldstone, fired brick, concrete blocks, adobe or rammed earth, wood or acombination of locally available traditional materials. The long history of earthquakes and age-old tradition of construction should have lead to the reasoning, logic and assumption that suffi-cient precautionary measures are to be incorporated in these constructions to withstand theearthquake forces. But, on the contrary, this is not the case. Past experience has shown thatcollapse of non-engineered construction is the single largest factor contributing to the huge lossesand casualties during earthquakes till now. Unfortunately, however, the subject of earthquakeresistant construction of such buildings has not received the attention it deserves and the con-struction practices continue to ignore the warning issued by nature time and again. There maybe two possibilities for this situation, either people are unaware and do not know about theearthquake resistant measure of masonry construction or they doubt the efficiency, proficiencyand efficacy of these measures. The present chapter will deal with the earthquake resistantprovisions in non-engineered construction in general and brick and stone masonry buildings inparticular along with experimental verification to built confidence among the people.

FAILURE MODE OF MASONRY BUILDINGS

An appropriate selection of suitable retrofitting schemes depends entirely upon the failure modeof individual masonry construction. There are innumerable modes of failure of walls as observedby the reconnaissance team and documented in various published papers and reports. Althoughthe type of construction, site of construction, structural typology of masonry buildings varies indifferent regions yet there damage caused by seismic activity may be identified uniformly. Thetwo most common modes of masonry failure may be called out-of-plane failure and in-planefailure. The structural walls perpendicular to seismic motion are subjected to out-of-plane bend-ing results in out-of-plane failure featuring vertical cracks at the corners and in the middle of thewalls. The structural walls parallel to seismic motion are subjected to in-plane forces i.e. bendingand shear cause horizontal and diagonal cracks in the walls respectively. The other types of

Chapter 9

124

masonry failure are diaphragm failure, pounding, connection failure and failure of non-structuralcomponents. A brief discussion of each mode of masonry failure is described as under.

OUT-OF-PLANE FAILURE

Inadequate anchorage of the wall into the roof diaphragm and limited tensile strength of masonryand mortar unitedly cause out-of-plane failure of wall in un-reinforced masonry buildings, whichare the most vulnerable. The resulting flexural stress apparently exceeds the tensile strength ofmasonry leading to rupture followed by collapse. Moreover long span diaphragms cause exces-sive horizontal flexure. Out-of-plane wall movement has been characterized as shown in Figure1 (Zuccaro and Papa 1999).

Figure 1: Out-of-plane failure characterization

IN-PLANE FAILURE

In-plane failures of walls in un-reinforced masonry structures due to excessive bending or shearare most common as is evident from double diagonal (X) shear cracking. This crack patternfrequently found in cyclic loading indicates that the planes of principal tensile stress in the wallsremain incapable of withstanding repeated load reversals leading to total collapse. As the groundmotion takes place for a short duration the walls are subjected to only one or two significantloading reversals and do not collapse totally. Fortunately by the time the shear cracks becomeunduly severe, the gravity load carrying capacity of the wall is not jeopardized. Diagonal tensioni.e. "X" cracks occurs mainly in short piers, rocking (top and bottom) in slender piers. Thesecracks happen to be worse at lower storey. In-plane failures are characterized as in Figure 2,(Pasquale and Orsmi, 1999).

1. Vertical cracks in the corner and/or T walls

2. Horizontal cracks along the facade

3. Partial collapse of an exterior wall4. Wythe separation5. Cracks at lintel and top of slen

der piers6. Cracks at the level of the roof7. Masonry ejection

Figure 2: In-plane failure characterization

1. Vertical cracks on openings2. Diagonal shear cracks on parapets and in doors and window lintels3. Diagonal shear cracks in the masonry piers between openings4. Crushing of corners of walls due to excess of compression stress5. Horizontal flexure cracks on top and/ or base of masonry piers6. Vertical cracks at wall intersections7. Passing through vertical cracks at wall intersections8. Spalling of material at the location of floor beam due to pounding9. Separation and expulsion of the intersection zone of two corner walls

125

DIAPHRAGM FAILURE

The failure of the diaphragm is a rare phenomenon in the event of seismic motion. Damage tothe diaphragm never impairs its gravity load carrying capacity. Lack of tension anchoring pro-duces a non-bending cantilever action at the base of the wall resulting from the push of dia-phragm against the wall. The in-plane rotation of the diaphragms ends and the absence of a goodshear transfer between diaphragms and reaction walls account for damage at the corners ofwalls. Figure 3 illustrates a wall failure resulting from excessive diaphragm flexibility. This prob-lem remains non-existent in strengthened buildings and is very rare in anchored buildings. Instrengthened buildings, separation remains worse at or near the centerline of the diaphragm.

a bFigure 3: Failure of diaphragms (a) shear failure, FEMA 306, 1999 (b) failure resulting from

diaphragm flexibility in Loma Prieta earthquake, 1989

FAILURE OF CONNECTION

Seismic inertial forces that originate in all elements of buildings are delivered to horizontal dia-phragms through structural connections. The diaphragms distribute these forces among verticalelements, which in turn transfer the forces to the foundation. Hence, an adequate connectioncapable to transfer the in-plane shear stress from the diaphragms to the vertical elements and toprovide support to out-of-plane forces on these elements is essential between the diaphragmsand the vertical elements. This type of failure is characterized by diagonal cracks disposed onboth the walls' edges causing separation and collapse of corner zones, Figure 4. This phenom-enon magnifies due to inadequately strengthened openings near the walls' edges and by floorsinsufficiently connected to the external walls.

a bFigure 4: Failure of connection of walls (a) characterization of failure, FEMA 306, 1999 (b)

collapse of corner zone (Dolce, Masi and Goretti, 1999)

126

NON-STRUCTURAL COMPONENTS

The non-structural components in masonry buildings are parapet walls, partition walls, mumty,water tanks, canopies, projections, staircase etc. These non-structural elements behave likecantilevers if they remain unstrained and are subjected to greater amplification as compared toground motion becoming prone to failures, Figure 5.

a bFigure 5: Failure of non-structural components (a) parapet failure, FEMA 306, 1999 (b) out-of-

plane failure of a parapet, EERI, 1996

POUNDING

When adjacent roof levels of two buildings and vertical brick work faces flush with one another,the pounding action causes structural distress due to out-of-plane vibrations. Such a failure ischaracterized as shown in Figure 6.

a bFigure 6: Pounding failure (a) characterization of failure (b) minor pounding damage between

buildings of different heights, EERI, 1993.

l Vertical cracks in the adjacent wallsl Diagonal cracks due to different levels in the structures

127

CRITERIA FOR EARTHQUAKE RESISTANT PROVISIONS

The past earthquakes have revealed that masonry construction remains susceptible toearthquake forces because of (i) lack of integral action, (ii) lack of strong and ductileconnections between walls, roof elements and foundation, (iii) inadequate strength for out-of-plane forces, (iv) low tensile and shear strength of mortar (v) high in plane stiffness of wall, (vi)low ductility and deformability capacity and (vii) heavy mass. In view of the continuous use ofsuch buildings, it is felt necessary to increase the seismic resistance of masonry construction byproviding some additional features known as earthquake resistant (ER) measures. The earth-quake resistant measures intended to increase the seismic resistance in terms of strength andductility. These earthquake resistant features alongwith the general guidelines are given in IS:4326 and IS: 13928. Actually, the major features of these codes are extracted from theMonograph on "Basic Concepts of Seismic Codes" prepared by "The International Associationfor Earthquake Engineering IAEE in 1980. IS 4326: 1993 deals with the selection of materials,special features of design and construction for earthquake resistant buildings including masonryconstruction using rectangular masonry units, timber construction and building withprefabricated flooring/roofing elements. Guidelines for construction of earthquake resistantbuildings using masonry of low strength particularly brick and stone masonry are covered in IS13828: 1993 and for earthen buildings are covered in separate code in IS 13927: 1993. The basicaim for providing the earthquake resistant features as recommended in the codes is basedon following concepts: (i) need of integral action (ii) strong and ductile connectionsbetween walls, roof elements and foundation (iii) improvement in strength for out-of-plane bend-ing (iv) strengthening of weaker sections by steel, timber or reinforced concrete and (v)improving the strength of mortar, quality of construction and insertion of bonding elements.However, to develop a better understanding of the efficacy, reliability and acceptability of thesemeasures, an experimental verification is necessary (Agarwal, 2002).

SALIENT FEATURES OF EARTHQUAKE RESISTANT PROVISIONS

The general features for improving the performance of non-engineered masonry constructionrecommended in IS 4326: 1993 and IS: 13828: 1993 are summarized in Table 1.

Table 1: Salient features of earthquake resistant provisions recommended in IS 4326: 1993 andIS 13928: 1993

128

Features Earthquake Resistant Design and Construction of Buildings – Code of Practice

(IS 4326: 1993)

Improving Earthquake Resistance of Low Strength Masonry Buildings – Guidelines (IS:

13928: 1993) General Principle

• Building should be light weight, particularly roof and upper storeys

• Integrity and continuity in construction such that it forms a continuous load path between the foundation and all diaphragm levels, and ties all portions of building together

• Projection/ suspended ceiling should be avoided, other reinforced and firmly attached with main structure

• Building plan& elevation should be symmetrical with respect to mass and stiffness, otherwise use separation joints

• Avoid close proximity (pounding), use separation

• Use separated staircase, otherwise enclosed with rigid walls, if it is not possible use sliding joint

• Sloping roof system should be adequately braced in both orthogonal direction (horizontal tie member and cross bracing) and should be adequately anchored into the RC band.

• Foundation of building should be firm and uniform, otherwise separate the building in units. In case of loose soil, improve the soil

• Building should be light weight, particularly roof and upper storeys

• Integrity and continuity in construction such that it forms a continuous load path between the foundation and all diaphragm levels, and ties all portions of building together

• Projection/ suspended ceiling should be avoided, other reinforced and firmly attached with main structure

• Building plan& elevation should be symmetrical with respect to mass and stiffness, otherwise use separation joints

• Use separated staircase, otherwise enclosed with rigid walls, if it is not possible use sliding joint

• Sloping roof system should be adequately braced in both orthogonal direction (horizontal tie member and cross bracing) and should be adequately anchored into the RC band.

• Gables ends of unreinforced masonry walls are anchored to all diaphragm level

• Foundation of building should be firm and uniform, otherwise separate the building in units. In case of loose soil, improve the soil

Masonry unit

• Well burnt bricks or solid concrete blocks having a crushing strength > 35 MPa

• Squared stone masonry, stone block masonry or hollow concrete block masonry, as specified in IS: 1597 (Part 2): 1992 of adequate strength

Brick Work in Weak Mortars • Fired bricks having a compressive strength

> 3.5 MPa Stone Masonry • Stone masonry of random rubble or dressed

stone type as IS 1597: 1967 Mortar • Category A: M2 (Cement-sand 1:6) or M3

(Lime-cinder 1:3) or even richer • M2 (Cement-lime- sand 1:2:9 or Cement

sand 1:6) or richer • H2 (Cement- sand 1:4) or M1 (Cement-

lime-sand 1:1:6) or richer

Brick Work in Weak Mortars • Lime sand (1:3) or clay mud of good

quality for brick work Stone Masonry • Cement sand (1:6), lime sand (1:3) or clay

mud of good quality in stone masonry Wall dimension and Number of stories

• Not greater than 15m subject to a maximum of four storey, with strengthening arrangements

• Straight and symmetrical in both the direction

• Checked in flexure as a plate or as vertical strip

Brickwork in weak mortar • Minimum wall thickness - one brick

(230mm) in single storeyed, one brick in top storey and 1.5 brick (350mm) in bottom storey of up to three storeyed

• Storey height < 3.0m, No. of storey for category A,B, and C – 3 storey, and category D – 2 storey

Stone masonry • Wall Thickness < 450mm preferably

350mm, height < 3.0 m, length < 5.0 m if exceed provide buttress, course height < 600 mm, inner and outer width should be interlocked with bond stone, Max. number of storey – 2.

129

Masonry Bond

• Usual bond but vertical joints should be broken properly from course to course

• Make a slopping joint by making the corner first to a height of 600mm and then bulging the wall in between them

• A toothed joint perpendicular walls, alternatively in lifts of about 450mm

Brickwork in weak mortar • Usual joints but vertical joints should be

broken properly from course to course • Make a slopping joint by making the corner

first to a height of 600mm and then bulging the wall in between them

• A toothed joint perpendicular walls, alternatively in lifts of about 450mm

Stone masonry • Use bond or through stone of full-length (or

a pair of about ¾ wall thickness) in every 600mm lift but < 1.2m horizontally. Other alternatives of bond stones are steel bars 8 to 10mm diameter bent to S-shape or wood bars of 38mm x 38 mm or concrete bars of 50mm x 50mm with an 8mm diameter rod placed centrally.

Openings • Door and window should be as small as possible and placed centrally as recommended

• Top level of openings should be same, covered with lintel band

• If do not comply with code, strengthened by RC lining with 2 HYSD of 8φ

• Avoid arches over the opening otherwise use steel ties

• Door and window should be small as possible and placed centrally as recommended

• Top level of openings should be same, covered with lintel band

• If do not comply with code, strengthened by RC lining with 2 HYSD of 8φ

• Avoid arches over the opening otherwise use steel ties

Seismic Strengthening Arrangements

a. Masonry mortar b. Lintel band c. Roof band and gable band d. Vertical steel at corners and junctions of

walls e. Vertical steel at jambs f. Bracing in plan at tie level of roof g. Plinth band h. Dowel bars Category A (up to 3 storey) use only a Category A (up to 4 storey) use a, b, & c Category B (up to 3 storey) use a, b, f & g Category B (up to 4 storey) use a, b, c, d, f & g Category C (up to 2 storey) use a, b, c, f & g Category C (up to 4 storey) use a to g Category D (up to 2 storey) use a to g Category D (up to 4 storey) use a to h Category E (up to 3 storey) use a to h

Brickwork and Stone Masonry b. Lintel band c. Roof band and gable band d. Vertical steel at corners and junctions of

walls f. Bracing in plan at tie level of roof g. Plinth band Category A (up to 2 storey) use c & f Category A (up to 3 storey) use b, c, f, g Category B (up to 2 storey) use b, c, f, g Category B (up to 3 storey) use b, c, d, f & g Category C (up to 1 storey) use b, c, f & g Category C (up to 3 storey) use b, c, d, f & g Category D (up to 2 storey) use b, c, d, f & g

Note: The categories of construction are defined in clause 7.1 depending upon the design seismic coefficient (αh) (Category: A (0.04 <αh <0.05), B (0.05 <αh <0.06), C (0.06 <αh <0.08), D (0.08 <αh <0.12) and E (0.12 ≤αh).

130

SEISMIC STRENGTHENING FEATURES

The non-engineered building construction system should be strengthened by horizontal bands orbond beams at critical levels and vertical reinforcing bars at corners and junctions of walls. Thebands form a horizontal framing systems that transfer the horizontal shear induced by the earth-quakes from the floors to structural walls. It also connects all the structural walls to improve theintegral action. Depending upon its location in the building it may be termed as roof, lintel, sill, andplinth band. The reinforcing details of these bands are available elsewhere (IS 4326, 13927,IAEE etc). In combination with vertical reinforcement, it improves the strength, ductility andenergy dissipation capacity of masonry walls. Although levels of strengthening arrangementsmay vary with the type of construction and seismic Zones. The descriptions of each strengthen-ing measure with its individual function are as follows:

Plinth Band

This band is provided at the plinth level of walls on the top of the foundation, which is useful insustaining differential settlements particularly when foundation soil is soft or has unevenproperties.

Gable Band

Gable band is provided at the top of gable masonry below the purlins. This band shall be madecontinuous with the roof band at the eave level. It restricts the out-of-plane failure of gable wall,which is susceptible to earthquake forces.

Roof Band

Roof band is similar to lintel band but it is provided below the roof or floors. It improves the in-plane rigidity of horizontal floor diaphragms. Such band need not be provided in case of rigiddiaphragm.

Lintel Band

This band is provided at lintel level on all internal and external longitudinal as well as cross wallsexcept partition walls. It provides integrity to the structure and resistance to out-of-plane wallbending. The lintel band if provided in partition walls will also enhance their stability. The purposeof lintel and roof band is to prevent the collapse of roof.

Sill Band

This band is similar to lintel band but it is provided at sill level. This band reduces the effectiveheight of masonry piers between openings. This is expected to reduce shear cracking in piers. Ithas not been recommended so far in codes.

Vertical Steel

The vertical steel is provided at corners and junctions of walls and around jambs of doors andwindows. The vertical steel in walls shall be embedded in plinth masonry of foundation, roof slabor band so as to develop its tensile strength in bond. It should pass through the lintel bands andfloor slabs in all stories. It is either a steel bar of 10mm to 12mm diameter or a bamboo. Forproviding vertical steel in stone masonry a casing pipe is recommended around which masonrybe built upto a height of 600mm. The pipe is raised and cavity is filled by 1:2:4 grade of concretemix in case of steel bar.

131

EARTHQUAKE RESISTANT DESIGN OF MASONRYBUILDINGS

Pankaj AgarwalAsstt. Professor, Department of Earthquake Engineering, IIT Roorkee, 247 667

INTRODUCTION

Masonry buildings are widely used for housing construction not only in India but many othercountries of the world. There are innumerable advantages of masonry construction over bothtypes of construction i.e. reinforced concrete and steel such as, thermal comfort, sound control,possibility of addition and alteration after construction, less formwork, easy and inexpensiverepair, use of locally available materials, need of less skilled labour, less engineering interventionetc. However, there are some disadvantages as well, particularly, when it is built in seismicenvironment The seismic resistance capacity of masonry construction is relatively low in com-parison to engineered constructions. Therefore, many developed nations have imposed certainrestrictions on the use of unreinforced masonry constructions. However, in developing nationsunreinforced masonry construction is still being used frequently. In India, masonry constructionsare generally made by using locally available materials like stone, brick, timber, adobe, mud etc.and are constructed in a traditional manner with or without the earthquake resistant featuresmentioned in IS: 4326 and 13927. Therefore, this type of construction is treated as non-engi-neered construction and most of the casualties are due to collapse of these constructions inearthquakes. Moreover the plight is that even after gaining knowledge of earthquake engineer-ing since the last three decades, neither a proper method has been developed for the seismicanalysis and design of masonry buildings nor the topic is fairly covered in the current Indiancurriculum in spite of the fact that about 90% population of India lives in masonry buildings. Thepresent and subsequent chapters are a step towards this with regard to develop a procedure forseismic analysis and designing of masonry buildings. The procedure is divided into several dis-tinctive steps in order to create a solid feeling and confidence that masonry buildings may also bedesigned as engineered construction.

PROCEDURE FOR LATERAL LOAD ANALYSIS OF MASONRY BUILDINGS

To understand the proper design procedure for low-rise masonry buildings, this procedure isdivided into several distinctive steps. In actual practice, these various steps may not be so clearlydelineated nor so distinctly separated, but at this stage, at least, this step-by-step procedure isrecommended in order to understand it properly (Schneider and Dickey, 1994). Figure 1 showsmasonry building subjected to a lateral load and its resisting mechanism. In load bearing masonry

Chapter 10

132

buildings, the walls, which carry gravity loads, also act as shear walls to resist lateral load. Thestructural walls parallel to lateral force and subjected to in-plane (shear) forces and bending arecalled shear wall. The walls perpendicular to seismic force/ lateral force and subjected to out-of-plane bending are called flexural walls. Following are the major steps for the lateral loadanalysis of masonry buildings:

Step 1: Determination of lateral load based on IS 1893 (Part 1): 2002Step 2: Distribution of lateral forces on the basis of flexibility of diaphragmsStep 3: Determination of rigidity of shear wall by considering the openingsStep 4: Determination of direct and torsional forces in shear wallsStep 5: Determination of increase in axial load in piers due to overturningStep 6: Check the stability of flexural wall for out-of-plane forces

Figure 1: Force resisting mechanism in masonry building

STEP 1: DETERMINATION OF LATERAL LOADS

Earthquake Load

One of the most important lateral forces on a structure is due to earthquake, which arises frominertia (mass) of the structure. These earthquake loads are sudden, dynamic and can be ofimmense intensity. The magnitude of lateral force mainly depends upon the seismic zone, type ofsoil or ground condition and fundamental characteristics. The design base shear shall first becomputed as a whole, then be distributed along the height of the buildings based on simpleformulas appropriate for buildings with regular distribution of mass and stiffness. The designlateral force obtained at each floor level shall then be distributed to individual lateral load resistingelement depending upon floor diaphragm action. Following are the major steps for determiningthe lateral forcesDesign Seismic Base Shear

133

The seismic base shear force, VB that acts on the building in a given direction is as follows VB = αh W

αh = The design horizontal seismic coefficient for a structure. It is determined by the following expression (Z/2) (I/R) (Sa/g), provided that for any structure with ,1.0 ST ≤ the value of αh will not be taken less than Z/2 what ever be the value of I/R. Where, (Z/2) = Z is the Zone factor, based on maximum considered earthquake (MCE) and service life of structure in a Zone. Factor 2 in the denominator of Z is used so as to reduce the MCEzone factor to the factor of Design Basis Earthquake (DBE). The country is divided in to four zones and the values of Z ranges from 0.10 to 0.36. Zone factors for different zones are given in Table 2 of IS: 1893: 2002.

(I/R) = Ratio of Importance factor and Response reduction factor. The values of importance factor and response reduction factors are given in Table 6 & 7 of IS: 1893 (Part 1): 2002. The ratio of (I/R) shall not be greater than 1.0.

(Sa/g) = Average response acceleration coefficient for rock and soil sites based on appropriate natural period and damping of the structures. The equations of (Sa/g) for different type of soil in different ranges of period are given in clause 6.4.2 of IS 1893 (Part 1): 2002. The value of time period of the building may be determined as follows

dh

T a09.0

=

Where, h is the height of building, in m, d is the base dimension of the building at plinth level, in m, along considered direction of the lateral force. W = Seismic weight of the building as per 7.4.2.

Vertical Distribution of Base Shear to different Floor Levels

The design base shear ( BV ) computed shall be distributed along the height of the building (Figure 2) as per the following expression.

∑=

= n

jjj

iiBi

hW

hWVQ

1

2

2

Where, iQ = Design lateral force at floor i,

iW = Seismic weight of floors i,

ih = Height of floor i, measured from base, and n = Number of stories in the building is the number of levels at which the masses are located.

134

a b cFigure 2: (a) Seismic shear on building (b) Seismic load (c) Storey shear

Example: Determine the lateral forces on two storey un-reinforced brick masonry building situated at Roorkee

Building Data Plan size: 20m x 20m,

Total height of building = 6m (each storey height = 3.0 m) Weight of roof = 2.5 kN/m2

Weight of walls = 5.0 kN/m2

Live load at Roof = 0,

Live load at floors = 1 kN/m2 (25% of imposed load if imposed load lesser than 3.0 kN/m2 as per Table 8 IS 1893 Zone factor (Z) = 0.24 Importance factor (I)= 1.0

Response reduction factor = 1.5 Spectral acceleration (Sa/g) = 2.5

Soil = Type II (Medium soil)

The seismic dead load at roof level (Wf)

Weight of roof = 2.5 x 20 x 20 =1000 kN Weight of walls

= (5 x 4 x 20 x 3)/2 = 600 kN Weight at roof level (Wr)

= 1000 +600 = 1600 kN The seismic dead load at roof level (wf) Weight of roof

= 2.5 x 20 x 20 =1000 kN Weight of walls

= (5 x 4 x 20 x 3) = 1200 kN Weight of live load = 1x20 x 20 x 0.25 = 100 kN

Total weight at second floor (W2) = 1000+ 1200 + 100= 2300 kN Total weight of building

= 1600 + 2300 = 3900 kN

135

(a) (b)

Elevation of masonry building and lateral force

The natural period of building as per IS 1893 (Part 1): 2002

5.2/,12.020/609.0/09.0 =⇒=== gSxdhT a

The base shear is [ ] [ ] kNWgSRIZWAV ahB 7803900)5.2)(5.1/0.1)(2/24.0()/)(/)(2/( ====

Vertical distribution of base shear to different floor levels is At roof level

kNxx

x

hW

hWVQ n

jjj

iiBr 74.573

)3230061600(61600

780 22

2

1

2

2

=+

==∑

=

kNxx

x

hW

hWVQ n

jjj

iiB 26.206

)3230061600(32300

780 22

2

1

2

2

2 =+

==∑

=

STEP 2: DISTRIBUTION OF LATERAL FORCES

Figure 3 shows the distribution of lateral forces in box type shear wall buildings. In order totransfer the seismic forces to the ground there should be a continuous load path in the building.The general load path is as follows: earthquake forces, which originate in all the elements of thebuilding, are delivered through the transverse wall of the building and it is bent between thefloors. The lateral loads are transmitted from these transverse walls to the side shear wall byhorizontal floor and roof diaphragms. The diaphragms distribute these forces to vertical resistingcomponents such as shear walls and vertical resisting elements if any, which transfer the forcesinto the foundation. The diaphragms must have adequate stiffness and strength to transmit theseforces. The distribution of lateral forces in the masonry building will depend upon the flexibility ofhorizontal diaphragm i.e. how rigid the walls are compared to the rigidity of the diaphragm. Therigidity of the diaphragms is classified into two groups on relative flexibility: rigid and flexiblediaphragm.

136

(a) (b) (c) (d)

Figure 3: Lateral force distribution in a box type building (a) box type masonry building subjected to lateral load (b) bend of first storey/second storey transverse walls (c) distribution of lateral forces in second storey (d) distribution of lateral forces in first storey Rigid Diaphragms

A diaphragm may be considered rigid when its midpoint displacement, under lateral load, is lessthan twice the average displacements at its ends. Rigid diaphragm distributes the horizontalforces to the vertical resisting elements in direct proportion to the relative rigidities. It is based onthe assumption that the diaphragm does not deform itself and will cause each vertical element todeflect the same amount. Rigid diaphragms capable of transferring torsional and sheardeflections and forces are also based on the assumption that the diaphragm and shear wallsundergo rigid body rotation and this produces additional shear forces in the shear wall. Rigiddiaphragms consist of reinforced concrete diaphragms, precast concrete diaphragms, andcomposite steel deck.

Figure 4: Comparison between flexible and rigid diaphragm

137

Flexible Diaphragms

A diaphragm is considered flexible, when the midpoint displacement, under lateral load, exceedstwice the average displacement of the end supports. It is assumed here that the relative stiffnessof these non-yielding end supports is very great compared to that of the diaphragm. Therefore,diaphragms are often designed as simple beams between end supports, and distribution of thelateral forces to the vertical resisting elements on a tributary width, rather than relative stiffness.Flexible diaphragm is not considered to be capable of distributing torsional and rotational forces.Flexible diaphragms consist of diagonally sheated wood diaphragms, sheathed diaphragms etc.Figure 4 provides a comparison between flexible and rigid diaphragms (Willams, 2003).

Example: Distribute a seismic load of 100 kN in end shear walls A, B & C in case of (i) rigid diaphragm (ii) flexible diaphragms Rigid diaphragm Wall A = (100 x 5)/ (5+3+2) = 50 kN Wall B = (100 x 3)/ (5+3+2) = 30 kN Wall C = (100 x 2)/ (5+3+2) = 20 kN Flexible diaphragm Wall A = (100 x 2.5)/ (10) = 25 kN Wall B = (100 x 2.5)/ (10) = 25 kN Wall C = (100 x (2.5+2.5)/ (10) = 50 kN

STEP 3: DETERMINATION OF RIGIDITY OF SHEAR WALL

The lateral load capacity of shear wall is mainly dependent on the in-plane resistance rather thanout-of-plane stiffness. The distribution of lateral load to the shear walls is based on the relativewall rigidities if a rigid diaphragm supports the walls and the segment of wall deflects equally.The rigidity of a shear wall is dependent on its dimensions, modulus of elasticity (Em), modulus ofrigidity (Gm) and the support condition.

Pier Analysis

In masonry structures, it is generally assumed that in one and two storey buildings the walls maybe considered cantilevered and the segment of the walls between adjacent openings are calledpiers and might be considered fixed at top and bottom, depending on the relative rigidities of thewalls versus those of the floor diaphragms.

Assumptions

(a) Rotational deformations of the portions above and below the openings are much smallerthan those of the piers between the openings and are neglected.

(b) Points of contra flexure are assumed at the mid points of the piers and shears are assumedto be carried among the piers such that their top deflects by equal amount.

(c) Lateral forces will be transformed to the various parallel resisting elements in direct propor-tion to their stiffness (Schneider and Dickey, 1994)

138

(d) Large portion of the total lateral force is required to reduce same deflection in a stiffer wallcompared to that of a more flexible one.

(e) Stiffness, refers to the lateral force magnitude required to produce a unit deflection(f) Relative, rather than absolute, stiffness can be computed since each wall is only being

compared to the combined stiffness of the entire wall system

Cantilever Pier or wall If the pier or wall fixed only at the bottom and top is free to translate and rotate, it is considered a cantilevered wall. When a force (P) is applied at the top of a pier, it will produce a deflection, ∆ , that is the sum of the deflections due to bending moment ( ∆ m) plus that due to shear ( ∆ v), Figure 5 (Amrhein, 1998).

Figure 5: (a) Wall pier displaced at top and cantilevering from fixed bottom (b)

Deflection of walls due to bending and shear deformations

∆ c = ∆ m + ∆ v ∆

= mAGPhImEPh /2.13/3 +

Where, ∆ m = deflection due to flexural bending ∆ v = deflection due to shear P = lateral force on pier h = height of pier A = cross section of pier Em = modulus of elasticity in compression Gm = modulus of elasticity in shear (shear modulus)

For masonry, Gm = 0.4 Em

∆ c = [ ])/(3)/(4 3 dhdhtE

P

m

+

Rigidity of cantilever pier Rc = 1/ ∆ c = ))/(33)/(4/( dhdhtmE + Fixed Pier or wall For a wall/pier fixed at top and the bottom, the deflection from a force, P is, Figure 6., Amrhein 1998.

139

Figure 6: (a) Wall pier with top displaced and fixed at top and bottom (b) Deflection

of walls due to bending and shear deformations

∆ f = ∆ m + ∆ v

= mAGPhImEPh /2.112/3 +

For masonry, Gm = 0.4 Em

∆ f = [ ])d/h(3)d/h(tE

P 3

m+

Rigidity of fixed pier

Rf = 1/∆ f = ))/(33)//(( dhdhtmE +

Effect of Aspect ratio on Deflection due to Shear

Aspect ratio (h/d) % deflection due to shear Cantilever wall Fixed end wall

0.25 92 98 …..(i) 1 43 5 2 16 43 …..(ii) 4 5 16 8 1 4.3 …..(iii)

i. Very squat shear wall (h/d < 0.25), rigidities based on shear deformation are

reasonably accurate ii. For intermediate height of shear wall (0.25<h/d<4.0), including both the components

of deflection iii. For high h/d ratio, the effect of shear deformation is very small and rigidity based on

flexural stiffness is reasonably accurate (Drydale, Hamid and Baker, 1994).

Horizontal and Vertical Combinations of Shear Wall Segments

If the shear wall segments are combined horizontally, the combined rigidity R = Rc1 + Rc2 + Rc3, if the segments are combined vertically, the combined rigidity 1/R = 1/Rc1 + 1/Rc2 + 1/Rc3., Figure 7

140

Figure 7: (a) Horizontal combination of wall segments (b) Parallel combination of wall

segments

Method for Calculating the Rigidity of wall with Opening The following steps are required for calculating the rigidity of wall with opening (Drydale, Hamid and Baker, 1994).

- Calculate the deflection of the solid wall as a cantilever (for one or two storey building) is determined ( )( cSolid∆ )

- Calculate the cantilever deflection of an interior strip, having a height equal to that of the highest opening, is calculated and subtracted from the solid wall deflection. This step removes the entire portion of the wall containing all the openings ( )( copeninghigestofStrip∆ )

- Calculate the deflections of all the piers as fixed within that interior strip being determined by their own individual rigidities ( )( fPeirs∆ )

- Add deflection of piers to the modified wall deflection to arrive at the total deflection of the actual wall with openings ( total∆ )

- The reciprocal of this value becomes the relative rigidity of the wall (total

R∆

= 1)

Example: Determine the rigidity of the shear wall, as shown, in terms of Et

141

)f(7,6,5,4,3,2)c(AStrip)c(WallSolidWall ∆∆∆∆ +−=

)(/1 )(7,6,5,4,3,2)(7,6,5,4,3,2 ff R=∆

)(7)(6,5,4,3)(2)(7,6,5,4,3,2 ffff RRRR ++=

)(6,5,4,3)(6,5,4,3 /1 ffR ∆=

)(5,4,3)()(6,5,4,3)(6,5,4,3 ffBstripfsolidf ∆+∆−∆=∆

)(5)(4)(3)(5,4,3

1

ffff RRR ++

=∆

48.010

2.16.3dh

ForET/882.1dh

3dh

4Et1 3

)c(Solid =+

==

+

=∆

36.010

6.3dh

ForET/266.1dh

3dh

4Et1 3

)c(AStrip ===

+

=∆

2.10.12.1

dh

ForET/187.0dh

3dh

/EtRRR3

)f(5)f(4)f(3 ===

+

===

Et/782.1)Et187.0(3/1)f(5,4,3 ==∆

67.04.56.3

dh

ForET/311.2dh

3dh

Et1 3

)f(6,5,4,3 ===

+

=∆

22.04.52.1

dhForEt/671.0

dh3

dh

Et1 3

)f(BStrip ===

+

=∆

Et/422.3Et/782.1Et/671.0Et/311.2)f(6,5,4,3 =+−=∆

Et292.0R )f(6,5,4,3 =

6.30.16.3

dhForET017.0

dh3

dh/EtR

3

)f(2 ===

+

=

0.32.16.3

dhForET028.0

dh3

dh/EtR

3

)f(7 ===

+

=

Et/337.0Et028.0Et292.0Et017.0R )f(7,6,5,4,3,2 =++=

Et/967.2)f(7,6,5,4,3,2 =∆

Et/583.3Et/967.2Et/266.1Et/882.1wall =+−=∆

Et279.0RWall =

142

STEP 4: DETERMINATION OF DIRECT SHEAR FORCE AND TORSIONAL FORCES

Direct forces

In case of rigid diaphragm it is assumed that the walls are tied together with the diaphragm, the lateral force (P) will be distributed to the walls in proportion to their relative stiffness. For any wall i, the relative stiffness is given by

n

n

iii kkkkR ++= ∑

=..../ 2

11

Direct shear forces on parallel walls are equal to PRV iiD =)(

Torsional Shear Forces

When the centre of mass and centre of rigidity do not coincide, torsional shear forces will be induced on the wall in addition to the direct shear force. The horizontal load, P, will be at the centre of mass, thus a torsional moment, Mt, is induced that is equal to Py x ex, where ex equals the distance between the line of force (centre of mass) and the centre of rigidity. Even in symmetrical structure, where e = 0, a minimum eccentricity amounting to 5 % of the building dimension is assumed which is called accidental eccentricity, Figure 8 (Amrhein, 1998).

Centre of Mass

Centre of mass mX is found by taking statical moments about a point, say south-west corner, using the respective lumped weights of walls as forces in the moment summation (Figure 9).

∑+++= WxLWxLWxLWxLWX EsNRm /)2/2/2/( Where,

)( wEsNR WWWWWW ++++=∑ ,

WR , WN, WS, WE and WW represents the weight of roof and respective walls Similarly,

)/()2/2/2/( BWERNWERm WWWWxBWxBWxBWxBWY ++++++= Where,

)( SNWER WWWWWW ++++=∑ , WR, WE, WW, WN and WS represent the weight of roof and respective walls

143

Figure 8: Torsional shear determination

Figure 9: Lumped model for torsional shear determination

Centre of Rigidity

The centre of rigidity, CRX and CRY , is calculated by taking statical moments about a point, say south- west corner, using the relative stiffnesses of the walls parallel to the y-axis as forces in the moment summation (Figure 9). The stiffness of slab is not considered in the determination of centre of rigidity.

)()()0(

Ew

E

Ew

Ew

y

yr RR

xLRRR

xLRxRR

xRX

+=

++

=∑

∑= …..Center of rigidity

Since the walls parallel to the x-direction do not contribute significantly to the lateral resistance in the y-direction, these relative rigidity terms do not appear in this summation. On the other hand, the y co-ordinate of the centre of rigidity ,rY entails the use of the Rx terms (in -plane lateral stiffness of the wall in the x-direction) as follows:

144

)RR(xBR

)RR()0xRxBR(

RyR

YSN

N

SN

sN

x

xr +

=++

=∑

∑= …..Center of rigidity

Torsional eccentricity, rmyrmx YYeXXe −=−= and .

Total Shear Forces on Parallel Walls The total horizontal shear, ( )

iyP , resisted by a particular wall element, with an axis parallel

to the y-direction, due to the applied horizontal load, (Py)i, may be obtained from the expression

( ) xyr

yy

y

y

iy ePJ

xRP

R

RP ±

∑= ….Total wall shear

direct shear torsional shear where, x or y = perpendicular distance from the centre of rigidity, CR, to the axis of wall in question Σ Ry or Σ Rx = 1.00 Similarly, for an applied horizontal force in the x-direction

( ) yxr

x

x

xix eP

JyR

PR

RP +

∑=

In the preceding equations, Jr, equals the relative rotational stiffness of all the walls in the storey under consideration. It corresponds to a polar moment of inertia and may be found by the expression Jr ( )22 xRyR yx +∑= …. Polar moment of inertia

Note that the torsional forces are always plus sign. This stems from the fact that, since thehorizontal load P is reversible, the code generally states that the effect of torsional moment beconsidered only when they tend to increase the direct stress

145

Example: Calculating the torsional shear forces in one storey shear wall masonry structure with a rigid diaphragm roof. The relative rigidity of each shear wall is given.

Given: Building is a one storey box system; All walls are a total of 5m high; 4m

upto roof level and 1m parapet. Seismic Zone V, Z = 0.36, I = 1.0, RW = 1.5, Sa/ g = 2.5 Weights: Roof = 3.0 kN/m2, Wall = 5 kN/m2 Base Shear = 300 kN

Table 3: Calculation of Centre of Rigidity

To calculate the shear forces due to torsion, first to calculate the locations of the centre of mass and the centre of rigidity.

Location of the Centre of Mass

Centre of mass, CMX and CMY , is calculated by taking statical moments about a point, say south west corner , using the respective weights of walls as forces in the moment summation. as shown in Table 2.

Table 2: Calculation of Centre of Mass

Item Weight (kN) X (m) Y (m) WX (kN-m) WY (kN-m) Roof slab N- Wall S - Wall E - Wall W - Wall

10x15x3 = 450 5x5x5 = 125 15x5x5 = 375 5x5x5 = 125 10x5x5 = 250

7.5 7.5 7.5 15 0.0

5.0 10 0.0 5.0 5.0

3375 937.5

2812.5 1875

0

2250 1250

0 625

1250 Σ W = 1325 Σ WX =

9000 Σ WY = 5375

∑ ∑ == m79.6W/WXX CM from West wall

∑ ∑ == m06.4W/WYY CM from South wall

Location of the Centre of Rigidity

The centre of rigidity, CRX and CRY , is calculated by taking statical moments about a point, say south- west corner , using the relative stiffnesses of the walls as forces in the moment summation. The stiffness of slab and parapet height are not considered in the determination of centre of rigidity. The calculation for the center of rigidity is shown in Table 3.

Item RX Ry X (m) Y (m) Y Rx X Ry N- Wall S – Wall E – Wall W – Wall

0.16 0.84 - -

- - 0.246 0.754

- -

15 0.0

10 0.0 - -

1.6 0 - -

- - 3.69 0.0

146

Table 3: Calculation of Centre of Rigidity

Item RX Ry X (m) Y (m) Y Rx X Ry N- Wall S – Wall E – Wall W – Wall

0.16 0.84 - -

- - 0.246 0.754

- -

15 0.0

10 0.0 - -

1.6 0 - -

- - 3.69 0.0

Σ Rx = 1.0 Σ Ry = 1.0

Σ Y.Rx = 1.6 Σ X.Ry = 3.69

∑ ∑ == mRRXX yyCR 69.3/ from W- Wall

∑ ∑ == mRRYY xxCR 6.1/ from S- Wall

Torsional Eccentricity

Torsional Eccentricity in Y-direction

Eccentricity between centre of mass and center of rigidity

ey = 4.06 - 1.6 = 2.46mAdd minimum 5% accidental eccentricity0.05 x 10 = 0.50mTotal eccentricity = 2.46 + 0.50 = 2.96m

Torsional Eccentricity in X-direction

Eccentricity between center of mass and center of rigidityex = 6.79 - 3.69 = 3.10mAdd minimum 5% accidental eccentricity0.05 x 15 = 0.75mTotal eccentricity = 3.10 + 0.75 = 3.85m

Torsional Moment

The torsional moment due to E-W seismic force, rotate the building in Y - direction, henceMTX = Vx ey = 300 x 2.96 = 888 kN-m

Similarly, if considered seismic force in N-S directionMTY = VY eX = 300 x 3.85 = 1155 kN-m

Distribution of Direct Forces and Torsional Forces

If we consider the seismic force only in E-W direction, the walls in N-S direction will resist theforces and the walls in E-W direction may be ignored. Table 4 shows the calculation of distribu-tion of direct shear and torsional shear.

147

Table 4: Distribution of forces in North and South Shear Wall

Item RX dy*

(m) RX dy RX d2

y Direct Force (kN)

Torsional Force**

(kN)

Total Shear (kN)

N- Wall S - Wall

0.16 0.84

8.4 1.6

1.344 1.344

11.28 2.15

Σ 13.40

48 252

+ 89 - 89

137 252

* Distance of considered wall from center of rigidity (10 – 1.6 = 8.4m)

** Torsional forces in N-Wall= kN89888x40.13

344.1eV

dR

dRyx2

yx

yx ==∑

Torsional forces in S-Wall= kN89888x40.13

344.1eV

dR

dRyx2

yx

yx ==∑

Similarly, if considered seismic force in N-S direction

If we consider the seismic force only in N-S direction. The walls in E-W direction will resist theforces and the walls in N-S direction may be ignored. Table 5 shows the calculation ofdistribution of direct shear and torsional shear.

Table 5: Distribution of forces in East and West Shear Wall

Item Ry dx*

(m) Ry dx Ry d2

x Direct Force (kN)

Torsional Force**

(kN)

Total Shear (kN)

E- Wall W -Wall

0.246 0.754

11.31 3.69

2.78 2.78

31.46 10.26

Σ 41.72

73.8 226.2

- 76.96 + 76.96

150.76 226.20

* Distance of considered wall from center of rigidity (15 – 3.69 = 11.31m)

** Torsional forces in E-Wall= kN96.761155x72.4178.2

eVdR

dRxy2

xy

xy ==∑

Torsional forces in W-Wall= kN96.761155x72.4178.2

eVdR

dRxyx2

xy

xy ==∑

STEP 5: DETERMINATION OF INCREASE IN AXIAL LOAD DUE TO OVERTURN-ING MOMENT

In shear wall analysis, the principal forces are in-plane shear (direct + torsional), in-plane mo-ment (in-plane shear x ½ of height of pier) and dead and live load carried by the pier. In additionto these forces sometimes, the lateral forces from winds or earthquakes create severe overturn-ing moments on buildings. If the overturning moment is great enough, it may overcome the dead

148

weight of the structure and may cause tension at the ends of piers of shear walls. It may alsoinduce high compression forces in the pier of walls that may increase the axial load in addition todead load and live load. The increase in axial load in piers due to overturning moments may beevaluated in the following manner (Schneider, and Dickey, 1994).

Figure 10: Axial load on pier due to overturning

Overturning moment at second floor level (Figure 10) (Movt)2 = Vr (h2 + h3) + V3 h2

Then, total overturning moment on pier in the first storey

Movt = (Movt)2+ total V x distance to the second floor level from critical level of the pier in the first storey (Assume, at the sill height of piers hcr, as shown in Figure 10).

Thus the axial load on a pier due to overturning Povt, is Povt = (Movt)(li Ai)/ In

il = Distance from the centre of gravity of the net wall section in the first storey to the

centroid of the pier in question iAiAilh

i∑

=∑=

1

iA = Cross sectional area of pier in question

In= moment of inertia of net wall section in first storey = 21

iliAn

i=∑

149

Example: Determine the increase in axial load due to overturning effects of lateral forces in wall is shown in Figure

Taking the sum of moments about the centre line of axis of the vertical load:

( ) 23322 hVhhVMovt r ×++= ( ) kN1800320033200 =×++=

( ) hVtotalMovtMovt ×+= 2 kN330035001800 =×+= Centroid of Net Section of Wall Pier Area iA (m2) d/s from left edge of wall to

centroid of pier (m) Al (m3)

1 2 3

2 x1/4 3 x 1/4 3 x 1/4

20.2 mAi =∑

1m 2 + 1 + 1.5 = 4.5m 8.5m

0.5 3.375 6.375 ∑ = 25.10Al

Distance from left edge to centroid m

AAl

i

125.50.225.10 ==

∑∑=

Moment of Inertia of Net-section of Wall Pier )( 2mAi )(ml i )( 42 mlA ii

)(

124

3

mtd

I = In=

IlA ii +2 ii lA Povt

(kN)

1 2 3

0.5 0.75 0.75

4.125 0.625 3.375

8.5 0.29 8.54

0.167 0.562 0.562

8.667 0.852 9.102

2.06 0.47 2.53

365.07 83.29 448.36

Σ=18.621 Increase in axial load on the individual pier in the first storey

150

Povt = Movt . 621.18

Al3300IAl ii

n

ii = = 177.22 liAi

(a) (b) Figure 11: (a) Wall subjected to axial and out-of-plane loads (b) Linear interaction

diagrams

The relationship between the combined effects of axial load (P) and bending (M) can be related to the virtual eccentricity (e= M/P), and for linear elastic behaviour of section it can be expressed as, Figure 11(a)

SMAPFm // += Fm = Limiting (allowable) stresses for combined axial compression and bending

A= Area of section and S= section modulus

This equation can be used to define the linear interaction diagram and represented as shown in Figure 11(b), Drydale, Hamid, Baker, 1994.

If P0 = Fm. A is the section capacity at zero eccentricity and M0 = Fm. S is the moment that can be carried with zero axial load, the interaction can be represented by the unity equation as

1// 00 =+ MMPP The unity equation in some of masonry codes also be present in the form of

1// =+ bbaa FfFf

STEP 6: WALLS SUBJECTED TO OUT-OF-PLANE BENDING

In seismic design of masonry building, it is assumed that the total base shear induced by anearthquake will be resisted by the in-plane shear wall and transverse walls or flexural wallswhich will not resist any shear. However, the flexural wall will be checked for out-of-planeforces with the vertical loads. This action produces combined actions of axial compression andbending forces. Lateral stability of the walls needs to be checked for this combined effect.

151

Where, fa, fb = compressive stresses due to applied axial load and bending, respectively Fa, Fb = allowable axial and bending compressive stresses, respectively Both these equations are used for describing linear behaviour of section. For masonry, the effects of tensile cracking, non-linear stress-strain behaviour of masonry, the equation is conservative. However, the unity equation can be useful for working stress design of cracked sections where the limiting compressive stresses under axial compression and bending are not equal. For unreinforced masonry, the allowable compressive stresses, Fa and Fb are given as follows:

[ ] 99/)140/(125.0

99/)/70(25.0

3/1

2'

2'

'

≤−=

>=

=

rhforrhf

rhforhrfF

andfF

m

ma

mb

Where, h/r is slenderness ratio of the wall Nominal allowable load carrying capacity Pn of the wall in out-of-plane is

))/61/(1( tebtfP mn +′= for 0<e<t/6

)21(43(

tefbtP mn −′= for e>t/6

Example: Check the stability of a 9" (230mm) thick brick masonry wall under a verticalload of 3 kN/m with a 4.5" (115mm) eccentricity in addition to a lateral load of 1 kN/m2.The wall is 3.5m high, assuming simple supported at top and bottom.Consider a design compressive strength of masonry 15 N/mm2 and permissible tensilestrength is 0.21 N/mm2 (The allowable flexural tension is 0.16 N/mm2, with the 1/3 in-crease allowed for seismic loading, amounts to 0.21 N/mm2)

152

Check for Flexure tension Face area = 230mm x 1000mm = 2.3 x 105 mm2/m of wall (Assuming 1m width of wall) I = bt3/12 = 1000 x 2303/12 = 1.014 x 109 mm4/m of wall Z = I/y = 1.014 x 109 /115 =8.816 x 106 mm3

Wall weight = 20 x 0.23 x 3.5 = 16 kN/m Moment at mid height = moment due to lateral

load + moment due to eccentric load = 1(3.5)2/8 + (3x 0.115)/2 = 1.75625 kN.m/m Checking the flexural tension stress: P at mid height = 16/2 + 3 = 11 kN/m ft = P/A – My/I = 11 x 1000/2.3 x 105 –1.75 x 106 x 115/ 1.014 x 109 = .048 -.198 = 0.15 N/mm2 < 0.21 N/mm2

Therefore, the 9” thick wall is adequate for tension.

Check for unity equation

33.1054.095.4

10x816.8/10x75.136.3

10x3.2/10x11F

Z/MF

A/PFf

Ff

mm/N95.4f33.0F

mm/N23.3))140/18.52(1(15x25.0)r140

h(1(f25.0F

thatso,9918.524.66

3500rh

therefore,mm4.6610x3.2

10x014.1A/Ir

6653

bab

b

a

a

2'mb

222'ma

5

9

<=+=+=+

==

=−=−=

<=====

REFERENCES

Amrhein, J. E. (1998). "Reinforced Masonry Engineering Handbook," Masonry Institute ofAmerica, CRC Press

Dry dale, R.G. Hamid, A. H. and Baker, L.R. (1994). "Masonry Structure: Behaviour and De-sign," Prentice Hall, Englewood Cliffs, New Jersey 07632.

Schneider, R.R. and Dickey, W.L. (1994). "Reinforced Masonry Design", 3rd Ed., Prentice HallInc., New Jersey.

Williams, Alan. (2003). "Seismic Design of Buildings and Bridges," Oxford University Press.STP 992 (1988). "Masonry: Materials, Design, Construction & Maintenance," (editor: Harry A.Harris), ASTM, 1916 Race Street, Philadelphia, PA 19103Agarwal P. and Thakkar S.K. (2003) "Seismic Evaluation of Strengthening and RetrofittingMeasures in Stone Masonry Houses under Shock Loading" Workshop on Retrofitting of Struc-tures, IIT Roorkee, Oct. 2003.

153

Agarwal, P. and Thakkar, S. K. (2002) "An Experimental Study of Effectiveness of SeismicStrengthening and Retrofitting Measures in Stone Masonry Buildings", Journal of European Earth-quake Engineering, pp. 48-64..

Agarwal, P. and Thakkar, S. K. (2001) "Study of Adequacy of Earthquake Resistance andRetrofitting Measures of Stone Masonry Buildings", Research Highlights in Earth Systems Sci-ence, DST Special Vol.2, on 'Seismicity' (Editor O. P. Verma), Published by Indian GeologicalCongress,(August 2001), pp 327-335.

Agarwal, P. and Thakkar, S. K. (1998) "Seismic Evaluation of Strengthening Measures in StoneMasonry Houses", Eleventh Symposium on Earthquake Engineering, University of Roorkee,Roorkee, December 17-19,1998BIS (1993). "IS 13828: Improving Earthquake Resistance of Low Strength Masonry Buildings-Guidelines", Bureau of Indian Standards, Manak Bhawan, New Delhi.BIS (1993). "IS 14326: Earthquake Resistant Design and Construction of Buildings - Code ofPractice", Bureau of Indian Standards, Manak Bhawan, New Delhi.IAEE, (1980). "Basic Concepts of Seismic Codes - Vol. I" The International Association forEarthquake Engineering, Tokyo, Japan.Keightley, W.O. (1977). "Report on Indo-U.S. Subcommission on Education & Culture",Department of Earthquake Engineering, University of Roorkee, Roorkee.Thakkar, S. K. and Agarwal, P. (1999) "Seismic Evaluation of Earthquake Resistant and Retro-fitting Measures of Stone Masonry Houses", Paper No.110, 12th WCEE, February 2000, Auckland,New Zealand.

154

155

EARTHQUAKE RESISTANT DESIGN ANDDETAILING OF RC BUILDINGS

Yogendra SinghAsstt. Professor, Department of Earthquake Engineering, IIT Roorkee, 247 667

INTRODUCTION

As brought out in the previous Chapters, the structures are to be designed to have sufficientstrength and ductility for safety against earthquake forces. Both strength and ductility are impor-tant for seismic safety. The current codal practice of design of RC buildings is based on a linearanalysis and Limit State Design philosophy. The effect of ductility is considered in the form of a"Response Reduction Factor", which is used to reduce the earthquake forces for design.

The RC members are to be designed for three actions: (i) Axial Force, (ii) Shear Force, and (iii)Bending Moment. Beams are generally monolithic with slabs and these are not designed foraxial load. On the other hand, the columns are to be designed for an interaction of axial load andbending moment. The design for Shear is independent.

Concrete is known to be brittle material. Typical to brittle materials, it has much lower strength intension, than in compression. The behaviour of concrete can be greatly enhanced by confining it.The ductility of concrete can be significantly improved by proper detailing of the reinforcement.This Chapter deals with important aspects of the design and detailing of RC buildings.

VARIABILITY OF STRENGTH AND OVER-STRENGTH

If we test 100 cubes of same batch of concrete, they will not give the same strength. Similarly ifwe test 100 rods of steel of same grade or test 100 beams made of same concrete and samesteel, they will fail at different loads. This is due to inherent variability of strength of materials. Totake this into account we consider a lower than average strength of materials in design. Ourcode defines this as "Characteristic Strength". It is the estimate of strength below which notmore than 5% samples will fall. Further a partial factor of safety (1.15 for steel and 1.5 forconcrete) is used to estimate the design strength. Therefore, it is clear that the actual strength ofa member is higher than the force for which we have designed as per our current design prac-tice. This higher strength is termed as over-strength and it is kept as reserve strength in case ofgravity and wind load. In case of earthquake load, this strength is also utilized to resist theearthquake forces. In fact, the forces resulting from the earthquake are much larger than theactual strength of the members and the members yield under such forces. Our normal linearanalysis procedure can not predict the behaviour of structures for yielding members and we

Chapter 11

156

require non-linear analysis procedures. However, there are some simplified procedures, whichcan be used to approximately predict the non-linear behaviour from the linear behaviour. Theresponse reduction factor given in our code is one such procedure which takes into account theover-strength and ductility.

DESIGN FOR DUCTILITY

As mentioned earlier, the Response Reduction Factor, used in the design of structures dependson ductility of the structure. The ductility of structures, in tern, depends on the ductility of indi-vidual components and structural configuration, including relative strength of different compo-nents and redundancy. These two aspects of ductile design are described below.

Ductile Design of Individual Components

The ductility of structure depends on the ductility of individual components. In RC members, theductility of components can be enhanced in flexure but there are limitations on ductility in axialaction and shear action. In flexure, the ductility can be achieved by making under-reinforcedsections and by providing proper confinement at the locations where maximum moments areexpected and the component is expected to yield. The member ends near the joints are the mostprobable locations of yielding under earthquakes. Further, it should be ensured that the membershould yield in flexure and not in shear or axial action. This can be ensured by providing higherstrength in shear and axial action, than that required for yielding of the member in flexure.

Ductile Design of Structural System

A structure can yield in a variety of modes depending on the relative strength of various compo-nents and joints and structural configuration. As some of the members have to yield underearthquake, redundancy of structural system is very important. The structure with higher degreeof redundancy can afford to have larger number of plastic hinges before collapse and thereforeit will exhibit higher ductility. On the other hand a determinate structure will become unstable onthe formation of first plastic hinge, without showing much ductility.

The local failure mechanism resulting due to formation of plastic hinges in columns prior to thosein beams causes brittle failure of structure and should be avoided. This can be avoided bydesigning the columns to be stronger than the beams. Failure of joints is another cause of poorseismic performance of structures. If the joints fail in shear which is a brittle mode of failure andif joints fail prior to yielding of components, the ductility can not be achieved. This requiresproper detailing of the reinforcement in joints.

Capacity Design Concept

The capacity design is the art of avoiding failure of structure in brittle mode. This can be achievedby designing the brittle modes of failure to have higher strength than ductile modes. In a RCbuilding this can be achieved by following the following design sequence:

i. First design the beams in flexure for the moments obtained from the analysis for Gravity,Wind and earthquake Loads.

157

ii. Calculate the provided flexural strength of beams and the corresponding shear strength re-quirement.

iii. Design the beams for higher of the shear obtained above in (ii) and that obtained from analy-sis.

iv. Calculate the flexural strength requirement of the columns by considering the strength ofbeams joining the columns. The combined flexural strength of columns joining at a node mustbe higher than the combined flexural strength of beams joining at the node.

v. Design the columns for the higher of the moment obtained in (iv) above and that obtainedfrom analysis.

vi. Design the columns for the shear force higher of that obtained from the flexural capacity andobtained from analysis.

SPECIAL REINFORCEMENT DETAILING FOR DUCTILITY

As discussed in the previous section, ductile buildings can be designed even with concrete, whicha non-ductile material. This can be achieved by providing proper amount of steel reinforcementat proper location. The following sections describe the reinforcement detailing for ductility

Anchorage and Splicing of Reinforcement

Joints are subjected to very large earthquakeforces and it has been observed that the beamreinforcement pulls out of columns and thebuilding collapses. To avoid this, the code IS:13920 recommends that the beam reinforce-ment should be anchored into columns by alength ld + 10φ (Fig. 1). The increase of 10 φto the development length is to take into ac-count the loss of bond due to cracking of con-crete during earthquake.

Similarly, care should be taken in splicing thereinforcement. The splicing should not be donenear the beam column joints as these locationsare subjected to high bending moments and

concrete may crack and bond may be lost at these locations. Further, the code require that notmore than 50% of reinforcement should be spliced at one location.

Special confining reinforcement

As discussed above, it is the confinement of concrete, which makes it ductile. Code requiresspecial confining reinforcement at the location where moment hinges are likely to occur. Thediameter and spacing of these hoops special confining reinforcement is to be calculated accord-ing to codal requirements, but in no case this spacing of stirrups should be more than 100 mm forcolumns and it should not be more than 150 mm for beams. Figs. 2 & 3 summarize the require-ments of special confining reinforcements of and columns.

Fig. 1 Anchorage of beam reinforcement

158

Fig. 3 Special confining reinforcement

Reinforcement in Shear Walls

Shear walls are similar to a wide column and these have reinforcement grid, generally on bothfaces. These walls resist large shear forces and bending moments and the reinforcement shouldbe provided to resist both shear and bending moment.

Fig. 4 Boundary Elements Fig. 5 Reinforcement at openings

The code requires that if the stress in the shear wall exceeds 0.2 fck then these should beprovided with boundary elements. These boundary elements are similar to columns but mono-lithic with shear walls. (Fig 4). The width these boundary elements may be same as the thicknessof the shear wall or it may be more.

Fig. 2 Arrangement of stirrups

159

Special care is required at openings in the shearwalls. Concentration of stresses takes place nearopening. To take care of this, additional rein-forcement (Fig. 5) should be provided aroundthe openings. In case of coupled shear walls,the coupling beams are subjected to very highshear forces. Due to reversal of stresses underearthquake conditions, the concrete in couplingbeams gets crushed. To take care of the shearforce, diagonal reinforcement should be provided

(Fig. 6) in the coupling beams . This diagonalreinforcement should be anchored by 1.5 times the full development length, into the shear wallconcrete.

Detailing requirements in special conditions

There are two commonly found conditions in RC buildings, which need special attention in detail-ing. First, whenever, there is an abrupt change in stiffness of members, special confining rein-forcement should be provided. Two such cases are encountered when the shear wall is sup-ported on columns (Fig. 7) or columns are supported on shear wall (Fig. 8). The first case is notdesirable from earthquake safety point of view and must be avoided. In both the conditions, thecolumns should be provided with special confining reinforcement throughout the length.

Fig.6 Reinforcement in coupling beam

Fig. 7 Shear wall on columns

Fig. 8 Columns on shear walls

160

The second case is whenever, there is a possibility of short-column effect, due to partial infill ora mezzanine floor (Fig. 9), the columns should be provided with special confining reinforcementthroughout the length.

Fig. 9 Short-column effect

PRECAUTIONS DURING CONSTRUCTION

For satisfactory performance of buildings, during earthquake, construction supervision is alsoequally important. Several failures have been observed due to faulty construction.

- The most important point during construction is the construction joint. To avoid failure at theconstruction joints, shear keys should be provided at construction joints. Before placing thenew concrete, the surface of old concrete should be thoroughly cleaned by water jets.Wooden blocks should be used for making shear keys and these blocks should be removedafter initial sitting of concrete. These blocks should never be left in place.

- Splicing of reinforcement during construction is very important. As discussed above, notmore than half of the reinforcing bars should be spliced at the location and splicing should beavoided near the joints.

- Anchorage of stirrups in the most important factors on which the safety of building depends.In no case the stirrups should be anchored at 90o as these open up during earthquake andconfinement is lost (Fig. 10).

Fig. 10 Loss of confinement due to improper anchoring of hoops and ties

161

- Alignment of columns is also very important as any eccentricity will give rise to high bendingmoments in columns.

- There is considerable congestion of reinforcement at the joints. Compaction of concrete atjoints is a difficult task and honeycombed concrete at joints is quite common. Special careshould be taken to compact the concrete at joints, as joints are the highly stressed parts of abuilding.

REFERENCES

IS 1893-2002, Criteria for Earthquake Resistant Design of Structures, Part 1 General Provisionsand Buildings, Bureau of Indian Standards, New Delhi.

IS 456-2000, Plain and Reinforce Concrete - Code of Practice, Bureau of Indian Standards,New Delhi.

IS 13920-1993, Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces- Code of Practice, Bureau of Indian Standards, New Delhi.

Key, David, 1988, Earthquake Design Practice for Buildings, Thomas Telford, London.Penelis, George G., and Kappos, Andreas J., 1997, Earthquake Resistant Concrete Structures, E& FN Spon.Paulay T., and Priestley, M.J.N., 1992, Seismic Design of Reinforced Concrete and MasonryBuildings," John Wiley & sons, Inc., New York.

162

ARCHITECTURAL CONSIDERATIONS ANDGUIDELINES FOR EARTHQUAKE RESISTANT

DESIGN OF BUILDINGS

Yogendra SinghAsstt. Professor, Department of Earthquake Engineering, IIT Roorkee, 247 667

INTRODUCTION

Performance of a building during earthquake depends on its shape, structural configuration,strength and ductility. All these factors are to be considered while designing a building. 'Regular-ity' and 'Continuity' are the basic rules for seismic safety of a building. It has been observed thatperformance of a symmetric and regular building is much better than that of a building withasymmetric or irregular configuration having even much higher strength or ductility.

IS 4326 provides some guidelines for design of buildings against earthquake forces. These guide-lines, if followed in design and construction, can enhance the performance of building signifi-cantly and ensure their safety against collapse. A collection and illustration of such guidelinesavailable in IS 4326 and elsewhere is presented here.

GENERAL GUIDELINES

Some precautions are required to be followed in design and construction of all the buildings,irrespective of the material and structural type. Following are some of such guidelines:

- Buildings should be sufficiently away from steep upside and downside slopes. During earth-quakes, the slopes may become unstable and may slide. A building near the slopes may bewashed away by the landslide (Fig. 1).

Fig. 1 Building near slopes Fig. 2 Building on filled site

Chapter 12

163

- Special care should be taken in case reclaimed and filled-up sites. In a partially filled-up site,settlements of original firm soil and filled-up soil will be different and building will be sub-jected to differential settlement. To avoid this, the foundation of the building should be onoriginal firm soil. Raft on pile foundations should be used for this purpose (Fig. 2).

- Loose, cohessionless soils in saturated condition loose their strength under shaking due toearthquake. The phenomenon is termed as Liquefaction. Such conditions exist near therivers and in dried beds of rivers. Wherever liquefaction is suspected, raft or pile foundationsshould be used.

Building shape is very important for its earthquake resistance. Asymmetric buildings are sub-jected to torsion, which results in excessive forces in extreme columns. Buildings with T, L, C orX shaped plans are prone to damage during earthquakes. Such planforms should be divided intosymmetric rectangular parts by providing suitable separations (Fig. 3). Fig. 4 shows the differ-ence in behaviour of separated buildings and the buildings with irregular shapes.

Fig. 3 Symmetric building plans are desirable

164

Fig. 4 Behaviour of asymmetric and symmetric buildings

Irregular shaped buildings are subjected to high forces at the corners. As shown in the figure, thebehaviour can be greatly enhanced by separating the adjacent parts of the building.

- Adjacent buildings should be sufficiently away from each other, so that these do not collideduring earthquake. This collision termed as "pounding" results in severe damage and severalfailures due to pounding have been reported during past earthquakes. The separation be-tween adjacent buildings should be adequate to accommodate the total non-linear displace-ments of both the buildings during earthquake. The effect of pounding is more severe, if thefloor slabs of the two adjacent buildings are at different levels. To minimize the damage, thefloors of adjacent buildings should be at the same level, as far as possible.

Another type of damage, which has been observed in case of adjacent buildings is that resultingfrom falling objects/ components of adjacent buildings. Water tanks kept at the top of buildingshave been seen to fall over the adjacent buildings and result in severe damage.

Fig. 6 Sudden Change in building size

Fig. 5 Inverted pendulum buildings

165

- Buildings with irregularities such as high height to width ratio, inverted pendulum configura-tion (Fig. 5) and with sudden change in shape and stiffness (Fig. 6) should be avoided.Sometimes, it is possible to remove the stiffness or shape irregularity by a judicial use ofseparations as shown in Fig. 6.

- The separations should be properly designed for material of construction and height of build-ing. Minimum gap between adjacent buildings/portions of buildings should be as given below:

Type of Construction Minimum Gap Per Storey (mm)

Load Bearing Building 15RCC Frame Building 20Steel Frame Building 30

Fig. 7 Separation details at walls Fig. 8 Separation details at roof

Fig. 9 Separation details at floors

166

The gap should be properly detailed (Figs. 7-9) to avoid water leakage and at the same time itshould be functional during earthquake to allow free relative movement of the adjacent building

- Large overhangs and balconies (Fig. 10) are subjected to much higher forces and should beavoided. Buildings with floating columns (Fig. 11) are particularly dangerous from earth-quake safety point of view and should be avoided.

Fig. 10 Overhang & Balconies Fig. 11 Floating columns

- Sloping roofs (Fig. 12) have thetendency to open out and exertexcessive forces on walls duringearthquake. In case of Pitchedroofs (sloping in two directions),gable ends are much more proneto damage and therefore Hippedroofs (sloping in four directions)are preferable. Sloping roofsshould be provided with horizontaltie members to avoid the opening up effect. To avoid the relative motion of the top andbottom members, appropriate cross bracings should be used.

- Staircases have sloping flights,which act as diagonal bracingsunder lateral load. Thesebracings provided large stiff-ness to staircase against lat-eral loads. Due to this effectstaircases attract very high lat-eral force during earthquakes,which result in collapse ofstaircases and punching offloor slabs by stair case flights.Two alternatives have beensuggested to avoid this:

Fig. 12 Sloping roofs

Fig. 13 Separation of staircase

167

- (i) Complete separation of the staircase from the remaining building (Fig.13), and (ii) enclosure of the staircaseby rigid (minimum one brick thick)walls (Fig. 14).As shown in Fig. 13, for structurallyseparating the staircase, the staircaseneed not be outside the building. Byproper use of separation joints, this canbe achieved at any location in thebuilding. This arrangement has additional advantage that the staircase may

be provided at any location in the building without causing asymmetry.

- Parapet is a neglected element in a buildingand this is the first element to fall during earth-quake. This may hit the people running out ofthe building or moving on the street duringearthquake and in many cases result in fatalinjuries. To avoid falling of parapets, its heightshould be small and it should be properly se-cured with the building by providing a closedloop of steel reinforcement through the cop-ing (Fig. 15).

- Swimming pools and large water storage tanks at the terrace (Fig. 16) cause severe massirregularity and have resulted in collapse of buildings in past earthquakes. These should beavoided. If smaller water tanks are to be provided at the roof, these should be properlyanchored against sliding and overturning. Similarly, large mass at any floor of the buildingalso results in mass irregularity and it should be avoided.

Fig. 14 Enclosed staircase

Fig. 15 Securing of parapet

Fig. 16 Large mass at roof is undesirable

168

GUIDELINES FOR MASONRY BUILDINGS

Most of the masonry buildings in India are constructed without any calculation of strength orductility. This type of construction is called non-engineered construction. It has been observed inlaboratory and during past earthquakes that if some simple measures are taken, collapse of suchbuildings during earthquake can be avoided.

- Masonry buildings are weakenedby presence of openings for doorsand windows. These openingsshould be as small as possible.Total length of openings in a wallshould not be more than 50% ofwall length in a single storey build-ing and it should not be more than33% of the wall length in a multi-storey building. Further, the mini-mum horizontal and vertical dis-tance between openings shouldnot be less that 600 mm (Fig. 17).Similarly, the openings should notbe very close to corners. The dis-tance of the opening from cornershould be preferably larger than 500 mm.

- Resistance of a masonry buildingto lateral loads is provided by itsintegral box action. The buildingshould behave like a box ratherthan four individual walls duringearthquake.

This can be achieved by integrat-ing the four walls using RC bandat plinth, lintel and roof level. Outof these, the lintel band is the mostcrucial. The plinth band and roofband can be avoided in case ofhard soil and flat RC roof, respec-tively. In addition to bands, verti-cal reinforcement at corners andjoints and along the jambs of openings provides additional strength. Fig. 18 shows the generalarrangement of bands and vertical reinforcement in a building. Fig. 19 shows the detailing ofreinforcement at joints and corners. It should be noted that no splicing of the reinforcementis allowed at joints. In case of masonry in mud mortar, timber band can be provided.

Fig. 17 Location of openings in masonry buildings

Fig. 18 Earthquake bands and vertical reinforcement

169

Fig. 19 Detailing of reinforcement in bands

- Openings are source of weakness in masonry buildings.Therefore special reinforcement is required around the open-ings. This reinforcement can be provided either in the formof vertical reinforcement and bands at sill and lintel level(Fig. 18), or it can be provided in the form of loops aroundthe openings as shown in Fig. 20.

- In case of rubble stone masonry, there is a tendency in thewalls to split. To avoid splitting, sufficient number of throughstones (Fig. 21) should be used. If longer stones to be usedas through stones are not available RC elements may beused. Similar elements are also to be used at corners andjoints to avoid separation of orthogonal walls under lateral loads.

Fig. 20 Strengthening ofopenings

Fig. 21 Corner stones and through stones in rubble masonry

GUIDELINES FOR RC BUILDINGS

Frames and shear walls are the two major lateral load resisting components in RC buildings,which resist the earthquake forces. Proper placing of these elements is very important to avoidasymmetry and torsion in the building. Even in a rectangular building, if the lateral load resistingelements are not placed symmetrically, these may result in torsion. Fig. 22, shows some of thedesirable and undesirable configurations of RC buildings.

170

Fig. 22 Desirable and undesirable configurations

- In case of hilly areas, it is common tohave buildings as shown in Fig. 23. Insuch buildings, the length of the columnson down-slope direction is more, com-pared to columns on the up-slope di-rection. Longer columns have lowerstiffness compared to smaller columns.This results in torsion in the building.To avoid this, shear walls should be usedon down-slope direction to compensatefor the loss of stiffness due to longercolumns.

- In cities, parking space is a big problemand usually the ground storey of a multi-storey building is kept open forparking. Such configuration results insoft ground storey buildings. The per-formance of soft storey buildings hasbeen observed to be very poor, as thesoft storey undergoes large non-lineardisplacements and the building col-lapses. To avoid this the ground storeyshould be provided with shear walls (Fig.24), so that the stiffness and strengthof ground storey match with those ofupper storeys. Alternatively, the ground

Fig. 23 Buildings on slopes

Fig. 24 Soft ground storey buildings

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storey columns should be designed 2.5 times stron-ger than upper storey columns (Fig. 25)Ductility is the key of safety of RC buildingsagainst earthquake. Concrete is known to be abrittle material, but it can be made to behave in aductile manner by providing proper amount ofreinforcement at proper place. IS: 139208 pro-vides some information about ductile detailing ofRC buildings. Some more information is availablein other references. The following guidelines pro-vide some useful tips about design and detailingof RC buildings:

- RC members can fail in Flexure, Shear of Axial crushing modes. Out of these modes onlyflexure mode is known to provide ductility. Other failures are non-ductile failures. Further,two modes of failure of frames have been observed: (i) in local failure mode (Fig. 26), thecolumns of a storey fail resulting in collapse of the building; and (ii) in global failure mode(Fig. 27), all the beams yield before formation of hinges at column

- bases and collapse of the building. The second mode of failure is desirable as it providesmuch more ductility than the first mode without jeopardising the safety against vertical loads.The failure in second mode can be ensured by a weak beam - strong column design in thefollowing manner:

Fig. 25 Stronger ground storey columns

Fig. 26 Local failure mode Fig. 27 Global failure mode Undesirable configuration Desirable configuration

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- Beams should have sufficient strength in shear so that these do not fail in shear before failingin flexure or the capacity of beams in shear should be more than the capacity in flexure.

Fig. 28 Detailing at exterior joint Fig. 29 detailing at interior joint

- Columns should fail neither in shear nor in flexure before formation of flexural hinges inbeams or the shear and flexural capacity of columns should be more than the combinedcapacity of all the beams meeting the column.

- Ductility of RC members depends on the amount of confinement and proper anchorage ofreinforcement. For an effective confinement, all the stirrups should be anchored at 1350 andnot at 900. The 900 stirrups open up during earthquake and confinement is lost. Specialconfining reinforcement is required in the beams and columns near the joints, as the ductilitydemand is high near the joints. Figs. 28 & 29 show the reinforcement detailing at exterior andinterior joints, respectively.

- Presence of masonry infills in RC buildings makes their behaviour complex and care shouldbe taken in design of such buildings. Columns in partially infilled frames or in frames withmezzanine floors suffer damage due to short column effect (Fig. 30). Such columns shouldbe provided with special confining reinforcement throughout length.

Fig. 30 Damage in columns with partial infill and mezzanine floor

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NON-STRUCTURAL ELEMENTS, SERVICES AND EQUIPMENT

Buildings have a large number of non-structural components, equipment and services, which arealso very important from earthquake safety view point. This is particularly important for build-ings with post-earthquake importance, such as hospitals, where failure of services can renderthe hospital non-functional at the time when it is needed most. Safety of non-structural compo-nents is also very important in case of structures which store hazardous substances. Leakage ofhazardous gases, liquids, radiation etc. can cause severe health risk in post-earthquake period.Some basic principles should be followed in installation of services and equipment. The installa-tion should be properly secured with the main structure; at the same time it should be flexibleenough to accommodate the movement of structure.

- Piping is the most important part of building services. During earthquake, different parts ofbuilding undergo relative displacements. Piping systems should be designed to accommo-date these displacements (Fig. 31).

- In hospitals and other buildings, there is a number of important equipment, which should beproperly secured, so that it does not fall during earthquake. The equipment mounted onwalls should be positively connected to walls as shown in Fig. 32. Equipment place at floorshould be secured with floor and walls (Fig, 33).

Fig. 31Pipe connections to allow relative displacement

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Fig. 32 Mounting of equipment on wall Fig. 33 Mounting of equipment on floor

- There are some vital movable equipment which may fall during the earthquake. Arrange-ment (Fig. 34) should be made at appropriate locations in the building, so that these equip-ment may be secured with walls.

- Hospitals store a large number of medicines and glassware, which can damage by fallingduring the earthquake. The shelves storing these items should have arrangement (Fig. 35) toprevent falling of these items.

Fig. 34 Mounting of movable equipment Fig. 35. Securing of falling objects

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Fig. 36 Securing lights and hanging objects

- False ceilings and lights should be secured (Fig. 36) with the main structure, so that these donot swing during the earthquake.

- It has been observed during the past earthquakes that the counterweights of lifts jump fromthe guiding rails. The counterweights should be mounted (Fig. 37) on guiding rails in such away that these do not jump during the earthquakes.

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REFERENCES

IS 4326-1993, Earthquake Resistant Design and Construction of buildings - Code of practice,Bureau of India Standards, New Delhi.D.K. Paul, et. al. (2002), Guidelines for Earthquake Resistant Buildings, TATA Steel, Jamshedpurand Department of Earthquake Engineering, IIT, Roorkee.Paulay and Priestlay (1992), Seismic Design of Reinforced Concrete and Masonry Buildings,John Wiley & Sons.David Key (1988), Earthquake Design Practice for Buildings, Thomas Telford, London.

Farzad Naeim (2001), The Seismic Design Handbook, Kluwer Academic Publishers.Penelis and Kappos ((1997), Eartquake-resistant Concrete Structures, E & FN Spon.Arya, A.S. et al. (1986), Guidelines for Earthquake Resistant Non-Engineered Construction,IAEE Committee, The International Association for Earthquake Engineering, Tokyo.IS 13920-1993, Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces- Code of Practice, Bureau of India Standards, New Delhi.IS 1893-2002, Criteria for Earthquake Resistant Design of Structures, Part-1: General Provi-sions and Buildings, Bureau of India Standards, New Delhi.Gary L. McGavin, (1981) Earthquake Protection of Essential Building Equipment, John Wiley &Sons.

Fig. 37 Mounting of lift counte- weights

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APPENDIX: Earthquake resistant measures in masonry buildings

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SEISMIC VULNERABILITY ASSESSMENTOF EXISTING BUILDINGS

Yogendra Singh and D.K. PaulDepartment of Earthquake Engineering, IIT Roorkee, Roorkee, 247 667

INTRODUCTION

Problem of assessment of safety of existing structures against various loads, including earth-quake load, has been recognised world over. In developing countries, about 50% of the construc-tion industry resources are being utilised for problems associated with existing structures. Manycountries have developed standards for assessment of existing structures. In India also theproblem is slowly showing its extent. Performance of our structures in the recent earthquakeshas also forced us to think on this issue. Many agencies, within the country, are working on thedifferent aspects of this problem.

Assessment of an existing structure is much more difficult a task than evaluation of a design onpaper. Firstly, the construction of the structure is never exactly as per designer's specificationsand a number of defects and uncertainties crop up during the construction. Secondly, the qualityof the material deteriorates with time and the assessment of an existing structure becomes atime dependent problem. The problem of the assessment involves not only the current status ofthe structure but also its extrapolation in the life of the structure with or without repairs. Thereare three sources of deficiencies in structures:

1. Defects arising from the original design, such as under estimation of loads as per old stan-dards/practices, inadequate section/reinforcement, inadequate reinforcement anchorage anddetailing.

2. Defects arising from original construction, such as under strength concrete, poor compac-tion, poor construction joints, improper placing of reinforcement and honeycombing.

3. Deterioration since the completion of the construction due to reinforcement corrosion, al-kali-aggregate reaction, etc.

In Indian conditions, it is generally a combination of all the three deficiencies and the retrofittingof the structure has to take care of all the three.

If the design documents are available, the first type of deficiencies can be assessed with asatisfactory level of confidence. However, if the design details are not available, it makes thetask of assessment, next to impossible. Till date no testing technique with sufficient reliability isavailable to completely outline the reinforcement detailing inside the concrete.

Chapter 13

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A number of techniques have been developed to detect the other two types of deficiencies.However, almost all of them depend on indirect measurements and have a low reliability. Further,the variation of test results is large and interpretation of results requires experience and skill.

Evaluation is an integral component of Seismic Retrofitting. The very first question to be an-swered by a retrofit engineer is whether retrofitting is required for a particular building? This cannot be answered without a systematic Seismic Evaluation. Evaluation is required to estimate thestrength of the existing structure, so that the need, extent, strategy and system of retrofitting canbe decided. Evaluation is also required to assess the adequacy of the proposed retrofit scheme.

SEISMIC EVALUATION OF BUILDINGS

According to the Vulnerability Atlas of the country, more than 80% houses are non-engineeredconstruction, which are mainly load bearing buildings. However, there are many RC framedurban buildings which have been constructed without any consideration to resist earthquakeforces or without using the current codal practices on Earthquake Resistant Design. For such alarge number of seismically deficient buildings, a quick assessment method and guidelines haveto be developed together with training and capacity building.

The difficulties faced in the seismic evaluation of a building are threefold. There is no quick andreliable method to estimate the in-situ strength of the material and components of the building.Analytical methods to model the behaviour of the building during an earthquake are eitherunreliable or too complex to handle with the generally available tools. The third difficulty isunavailability of a reliable estimate of the earthquake parameters, to which the building isexpected to be subjected during its residual life. The ground motion parameters available in thepresent code have been estimated at a macro level and do not take into account the effect oflocal soil conditions which are known to greatly modify the earthquake ground motion.

Evaluation of an existing building is a difficult task, involving considerable cost and efforts andrequires skills in different disciplines of structural engineering including materials. Procedureswith different sophistications are available for evaluation of existing buildings. It is very impor-tant to decide an appropriate evaluation strategy depending on the seismicity of area, vulnerabil-ity, scale and importance of the buildings and the funds available.

Seismic Evaluation consists of the following three phases

Rapid Visual Screening (RVS)

- For mass scale evaluation of buildings in a city- Only visual inspection and limited addition information- Based on behaviour of buildings in past earthquakes- Rapid, based on checklists- Inspecting a building from out side-"Side Walk Survey" to come to a conclusion whether the

building is probably adequate for earthquake forces likely to occur at site or there are reasonable doubts that the building may not perform satisfactorily

- Should the building be subjected to more detailed evaluation?

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Steps

i) Identify the building class.ii) Estimations of Performance based on past experience and expert opinion.

What we should look for?

i) Common Deficiencies in planning and site conditionsii) Common Deficiencies in Masonry Buildingsiii) Common Deficiencies in RC Buildings

Checklists and Data Collection Forms (Appendix 2-I)Checklists for Indian Seismic Zones (Appendix 2-II)

- Classification of building classes for Masonry and RC buildings- Damage pattern using MSK scale

Simplified Vulnerability Assessment (SVA)

- Based on Limited Engineering Analysis- Calculations based on structural drawings or on site measurements- Information regarding size and strength of lateral load resisting members- Simplified analysis to estimate building drift

Detailed Vulnerability Assessment (DVA)

- Insitu Strength Estimations- Computer Modelling- Linear or non-linear static or dynamic analysis

Detailed Vulnerability Assessment is recommended in the following conditions:

1. Buildings failing Simplified Vulnerability Assessment.2. Building has more than 6 storeys for RC & Steel and more than 3 storeys for URM.3. Building is located on in competent or liquefiable soils located near active faults with inad-

equate foundations details.4. Buildings with inadequate connection with primary structural systems e.g. with pre cast

elements.

DETAILED IN-SITU INVESTIGATION

Visual inspection and preliminary evaluation of a building provides some insight into the potentialdesign and construction deficiencies and causes of deterioration. A detailed investigation is re-quired to estimate the in-situ strength of material and extent of deterioration. A number of testingmethods have been developed for estimating in-situ strength of RC. Some of these techniqueshave also been used for masonry.

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Fig.1 Single flat jack test Fig. 2 Two jack test

In-Situ Testing Methods For Masonry

FEMA-306 provides a detailed description of various methods available for in-situ testing ofmasonry. The most important method for in-situ masonry testing is taking out a core and testingin split tension test. The mortar joints are kept at 450 to the loading to get the shear strength ofjoints directly. Correction for normal stress is to be made for getting the in-situ shear strength.Alternatively a square sample may be removed from the masonry wall and tested for compres-sive and shear strength.

Another method, which can provide some vital information about in-situ strength of masonry, isFlat Jack Test. A single flat jack (Fig. 1) may be used to determine the state of stress in themasonry. A two-jack test (Fig. 2) may be used to estimate the in-situ stress-strain behaviour ofmasonry in compression.

Most of the methods have been basically developed for concrete and their performance formasonry is not satisfactory. These methods are being described in the following sections forconcrete.

In-Situ Testing Methods for Concrete

Estimation of in-situ strength of concrete is the first step towards quantification of the existingstrength of a building. It is a complex problem, which requires the knowledge of both science andart of in-situ testing. A number of techniques are now available for this purpose, but these aremostly based on some indirect measurement of the strength. Skill is required to interpret theresults of testing. The various techniques available and their suitability in different conditions arebeing discussed here.

The following Table gives a list of in-situ testing methods available for masonry.

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Tests for assessment of structural performance and integrityThe different tests in this category and the type of equipment required, are listed below:

The static load test can be performed either in-situ, which is generally non-destructive or it canbe performed in the laboratory on the removed member, which is generally destructive. The aimof the test is to demonstrate satisfactory performance of a member or a group of members of thestructure under an overload above the design values. The selection of the members to be testedis to be made based on importance of members and with the help of other techniques to locateweakest zones. A static load is applied incrementally and the deflection, strains and crack widthsare measured during loading as well as unloading. The loads and deflection limits have beenspecified by various codes. The test requires elaborate arrangements for application of load,measurements and safety of testing personals. The test is particularly valuable in restoring thepublic confidence but it disrupts the normal usage of the structure and it is an expensive test. Along-term measurement of the structural response under service loads can also be done, but it iscostly and cumbersome.

In dynamic response testing, response of the entire structure is studied under dynamic loadsfrom exciting machines, impact or under ambient vibrations. The response is compared with thatof the theoretical model and overall stiffness deficiency due to damage, defects and deterioration

Sl. Property under Test Equipment TypeNo. Investigation1. Integrity Sounding Mechanical2. Strength Rebound Hammer Mechanical3. Strength Ultrasonic Pulse Mechanical/Electrical

Velocity Test4. Integrity Impact-Echo Mechanical/Electrical5. Integrity Penetrating Radar Electromegnatic/Mechanical6. Strength Core Testing Mechanical7. Shear Strength In-situ Shear Mechanical8. Strength In-situ Flat Jack Mechanical

Testing

Sl. No. Test Equipment Type1. Static Load Test Mechanical/Electronic/Electrical2. Dynamic Response Mechanical/Electronic/Electrical3. Pulse-Echo Mechanical/Electronic4. Strain or Crack Measurement Optical/Mechanical/Electrical5. Reinforcement Location Electromagnetic6. Radar Electromegnatic7. Thermoluminescence Chemical8. Thermography Infra-red Photography9. Acoustic Emission Electronic

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can be estimated. The method can also be used to assess the improvement by repair and retro-fitting. Long term measurement of dynamic signatures of structure can give the deterioration ofstructure with time. The method gives only an indication of the overall health of the structure andit is difficult to assess the condition of individual members. Research is going on to locate theweak zones in the structure by analyzing its dynamic response and free vibration characteristics.The test requires carefully placed accelerometers and complex signal processing equipment.

The pulse-echo method is a useful technique to detect cavities, cracks and delaminations withinthe concrete. Shock waves are induced in the concrete by a surface hammer blow, which arereflected by any discontinuity present within the concrete. The reflected shock waves areanalyzed to determine the depth, extent and width of the discontinuity. An Artificial NeuralNetwork based system has been patented in the trade name "DOCTOR" and is commerciallyavailable.

Electromagnetic equipment is commercially available to measure the cover and diameter ofreinforcement bars. The accuracy of the equipment has improved in the recent years. Theequipment is useful in detecting the adequacy of the cover provided to reinforcement.

The other techniques listed above require specialized equipment and skill. These are not widelyused and are at research stage at present.

Tests for assessment of in-situ quality

After identification of weak zones in a structure, detailed assessment of the in-situ quality of thematerial is to be done. A number of tests have been developed and standardized for differentproperties of concrete. Suitable tests are to be selected based on the aims of testing. A list ofvarious available tests is given below:

Sl. No.

Property under investigation

Test Equipment type

1 . Cores Mechanical 2 . Pull-out Mechanical 3 . Pull-off Mechanical 4 . Break -off Mechanical 5 . Internal fracture Mechanical 6 . ESCOT Mechanical 7 . Penetration resistance Mechanical 8 . Maturity Chemical/Electrical 9 .

Concrete Strength

Temperature -matched curing

Electrical/electronic

10. Surface hardness Mechanical 11. Ultrasonic pulse velocity Electronic 12. Radiography Radioactive 13. Radiometry Radioactive 14. Neutron absorption Radioactive 15.

Concrete quality, durability and deterioration

Relative humidity Chemical/electronic

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16. Permeability Hydraulic 17. Absorption Hydraulic 18. Petrography Microscopic 19. Sulphate content Chemical 20. Expansion Mechanical 21. Air content Microscopic 22. Cement type and content Chemical/microscopic 23.

Abrasion resistance Mechanical 24. Half-cell potential Electrical 25. Resistivity Electrical 26. Cover depth Electromagnetic 27. Carbonation depth Chemical/microscopic 28.

Corrosion of embedded steel

Chloride Concentration Chemical/electrical

Tests for concrete strength

Concrete strength is the most important parameter in assessing the safety of a structure againstloading. Due to lack of construction supervision, sometimes, very low strength concrete may beencountered in existing structures. Such locations are to be identified and suitable remedialmeasures to be taken. The testing methods for concrete strength vary from very indirect surfacehardness test to the direct testing of concrete strength by removing cores. Broadly, these testscan be divided into three categories:

Non-destructive tests

These tests are based on indirect measurement of concrete strength through measurement ofsurface hardness and dynamic modulus of elasticity. Calibration curves relating these propertieswith the strength of concrete are available. For surface hardness rebound of an impact from theconcrete surface is measured.

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A simple equipment known as Rebound Hammer or Schmidt Hammer is used for this purpose.The details of the equipment are shown in Fig. 3.

The dynamic modulus of elasticity of concrete is measured by measuring the velocity ofultrasonic pulse through concrete. The test equipment has provisions for generating ultrasonicpulse, transmitting it to concrete, receiving and amplifying the pulse and measuring anddisplaying the pulse travel time. The details of the equipment are shown in Fig. 4. Good acousticcoupling between the transducers and concrete is to be established for correct measurement ofthe speed.

Fig. 4 UPV Testing equipment

These two equipments are robust and straightforward in application. The tests are inexpensiveand fast. However, there is no direct theoretical relationship between the properties measured inthe tests and the strength of concrete. Further, the test results are affected by a number ofparameters and test conditions. This reduces the reliability of the tests and these are suitable foronly comparative survey of the quality of the concrete. For more reliable estimation of concretestrength, other tests should be used at selected locations.

Partially destructive tests

These are surface zone tests18, which require access to one exposed concrete face and causesome localized damage. This damage is sufficiently small to cause no loss in structural perfor-mance. The strength of concrete is estimated with the help of correlation charts, which aresensitive to lesser number of parameters compared to the surface hardness and ultrasonic pulsevelocity tests. Hence reliability of these tests is higher. The advantage compared to core test isthat these are faster and less disruptive and damaging. Different tests in this category are basedon penetration resistance, pull-out pull-off and break-off.

In penetration resistance testing, a specially designed bolt is fired into concrete with the help ofa standardized explosive cartridge. The equipment and testing procedure have been standard-

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ized by ASTM C803. A consistent correlation of the depth of the penetration with the strength ofconcrete has been found. The details of failure process of concrete under penetration have beenshown in Fig. 5.

Fig. 5 Penetration resistance test

In pull-out testing, the force needed to pull a bolt or some similar device embedded into concreteis measured and correlated with the strength of concrete. This correlation has been shown to beunaffected by the mix characteristics and the curing history. The bolt may be inserted at the timeof casting of the concrete or it may be epoxy grouted into a hole drilled into hardened concrete.The testing has high reliability and it is accepted by a number of public agencies in some coun-tries as equivalent to cylinders for acceptance testing. The details of insert and failure zone areshown in Fig. 6. Different versions of this test are in practice in different parts of the world, suchas, Lok-test, North American Pull-out Method, Internal fracture test, ESCOT, CAPO test etc.

Fig. 6 (a) Pull - out test Fig. 6 (b) CAPO test

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Fig7 Pull - off test

The pull-off method is based on the measurement of in-situ tensile strength of concrete. Thecompressive strength of concrete is well known to be related with the tensile strength. Anotherapplication of the test is in testing of the bond between original and new concrete in repairs andstrengthening. The details of the test are shown in Fig. 7. Two versions of the test are possible.In first case a metallic disk is glued directly to the surface of concrete and pulled off to measurethe force necessary to pull a piece of concrete away from the surface. In the second case partialcoring is done with a standard diameter of 75mm and the above procedure is repeated by gluingthe disk at the top of the partial core. For assessing the bonding strength of the repairs with theoriginal concrete, the depth of the partial coring should be below the surface of the originalconcrete.

The break-off test measures thein-situ flexural strength of theconcrete at a plane parallel toand at a distance from the sur-face of concrete. In the test, apartial core is broken off by atransverse force acting at the topsurface as shown in Fig. 8. Thebreak-off strength of concretehas been shown to have a linearcorrelation with the modulus ofrupture of prism specimens.

Fig. 8 Break - off test

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Core tests

The core test provides the visual inspection of the interior of the concrete and direct measure-ment of the compressive strength. Other physical properties, such as, density, water absorption,indirect tensile strength and expansion due to alkali-aggregate reaction can also be measured.After strength testing, these can be used as samples for chemical analysis. The procedure hasbeen standardized by BS, ASTM and ACI codes.

In core testing, the determination of core size and location is a crucial factor. The test should betaken at points where minimum strength and maximum stress are likely to coincide. But, at thesame time, the core cutting causes some damage to the member and may impair the futureperformance of the member. Therefore, in slender members, the core should be taken awayfrom the critical section. For compression testing, the diameter of the core should be at leastthree times the nominal maximum aggregate size. The accuracy of the test increases with theratio of core diameter to the aggregate size. The generally recommended length to diameter ratioof the cores is between 1 to 2.

Tests for concrete quality, durability and deterioration

The aim of these tests is to identify the cause of deterioration. Most of these tests requirespecialized laboratory facilities. The accuracy of these tests, at present, is low. ASTM and BSstandards are available for common chemical tests. These include cement content, aggregatecontent and grading, aggregate type, cement type, original water content, as well as chloride,sulphate and alkali contents. Microscopic study of a prepared concrete surface or a thin sectionof concrete may also be useful in detecting the defects. In addition to these, there are severalspecialized instrumental methods.

Tests for detecting corrosion of reinforcement and pre-stressing steel

Corrosion of embedded steel is the major cause of deterioration of RC structures. This results inweakening of structure due to loss of steel cross-section, loss of bond between steel and con-crete and surface staining, cracking and spalling of concrete. A good indicator of risk of corro-sion in the embedded steel is the potential of the steel to a reference half-cell placed at theconcrete surface. Zones of varying degrees of corrosion risk are identified from the potentialcontours drawn with the help of a moving half-cell. Percentage chance of active corrosion fordifferent values of half-cell potential, have been specified.

Another approach for estimation of corrosion risk is based on the measurement of various pa-rameters affecting/responsible for corrosion. These include reinforcement cover, concrete pH/depth of carbonation, concrete resistivity, absorption and permeability, chloride and sulphatecontent and moisture movement.

DETAILED ANALYSIS FOR EARTHQUAKE FORCES

A lot of research has taken place in the area of analysis of buildings for earthquake forces. Theanalysis methods can be broadly classified into Linear and Non-linear methods. Earthquakeresistance design relies heavily on the ductility or post yielding behaviour of the structure andtherefore, the non-linear methods appear to be more reliable. However, these methods also have

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inherent assumptions and require skill and computer software, as these are computation inten-sive. Another classification is based on the type of load considered in the analysis. Static analysisprocedures consider equivalent static force, while the dynamic analysis procedures take intoaccount the time varying nature of the earthquake forces. The dynamic analysis is nearer toreality but require high degree of computation. On the other hand, the static analysis procedureis simple, easy to use and provide insight in to behaviour of structure. For regular buildings, astatic linear procedure is considered to be sufficient, but for buildings with irregular configura-tions, a linear or non-linear dynamic analysis may be necessary.

REFERENCES

IS 1893-2002, Criteria for Earthquake Resistant Design of Structures, Part 1 General Provisionsand Buildings, Bureau of India Standards, New Delhi.

IS 4326-1993, Earthquake Resistant Design and Construction of buildings - Code of practice,Bureau of India Standards, New Delhi.

IS 13920-1993, Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces- Code of Practice, Bureau of India Standards, New Delhi.

J. H. Bungey, 1989, The Testing of Concrete in Structures, Surrey University Press.ACI Committee 437, 1991, Strength Evaluation of Existing Concrete Buildings, American Con-crete Institute.IS 13311 (Part 1): 1992, Non-Destructive Testing of Concrete - Methods of Test, Part - 1, UltraSound Pulse Velocity, Bureau of Indian Standards.IS 13311 (Part 2): 1992, Non-Destructive Testing of Concrete - Methods of Test, Part - 2,Rebound Hammer, Bureau of Indian Standards.

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APPENDIX 2-I: CHECKLISTS AND DATA COLLECTION FORMS

FORMAT FOR RECONNAISSANCE OF EXISTING BUILDINGS Building Identification

GENERAL INFORMATION

Building name:_________________________________________________

Ownership: Public Private

Owner name: __________________________________________________

Address: ______________________________________________________

_____________________________________________________________

1. Building type

Earthen building Stone in mud mortar

Stone in cement mortar Brick masonry in mud mortar

Brick masonry in cement mortar RCC Frame Building

RCC Frame-shear wall Building

Mixed construction (specify): ______________________________________

2. Usage of the building

Residential Business Offices Public Storage

Others (specify): _______________________________

3. Number of occupants (approximately): ______________________

4. Building location

Isolated Internal End Corner

5. Plan dimensions:

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6. Total number of stories:

7. Average inter-storey height:

8. Construction age:

9. Building construction quality

Good Average Poor

10. Building condition and maintenance

Good Average Poor

11. Building site located at

Hill top High slope of hill Mild slope Plain

River bed Others (specify):_________________________________

12. Soil type

Rock / Hard soil Medium soil Soft soil

Reclaimed/filled land Partially filled land Loose sand

Others (Specify): _____________________________

13. Ground Water Table depth _______________________

14. Gap between adjacent buildings: ________________________

15. Plan of building:

Symmetric Asymmetric

16. Reentrant corner is:

Less than 15% more than 15%

17. Regularity in elevation:

Regular irregular

If irregular, approximate Shape in elevation is of the type

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Percentage reduction/increase in dimensions: ________________________

18. Location of staircase in building is

Symmetric Asymmetric

19. Staircase is

Separated from main structure Connected to main structure

If connected to main structure, it is

Enclosed by rigid walls Not enclosed by rigid walls

20. Basement provided: Yes No

21. Overhang length: ________________________________________

22 Parapet height: ___________

Secured against falling Yes No

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MASONRY BUILDING

1. Type of construction Engineered construction Non-Engineered construction

2. Layout of masonry (Tick in case of stone masonry)

Random rubble Half dressed stone Ashlar masonry

3. Through stones (Tick in case of random rubble masonry)

Provided Not provided

4. Earthquake resistant features provided in the building

Lintel band Roof band Plinth band

Vertical reinforcement at

Corners Junctions Jambs of openings

5. Minimum pier dimensions: ___________________________________

6. Existence of floating walls

Yes No

7. Foundation type

Strip foundation Raft foundation Pile foundation

Others (specify): _____________________________________

8. Foundation material used for construction

Stone Brick Cement concrete RCC

9. Percentage openings in main walls: ________________________

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10. Thickness of

Exterior wall _________mm Interior wall _________mm

11. Roof type

Flat roof Sloping roof Hipped Roof

12. If sloping provided, then

Ties provided Yes No

Bracing provided Yes No

Roof and Gable band provided Yes No

13. Roofing material used

RCC slab Tiles Corrugated iron sheeting

Asbestos sheeting Others, specify ____________________

14. Inspection accuracy

From outside only Contacted owner also

Partial Complete

Names of the surveyors: Date:

1. 2.

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RCC BUILDING

1. Irregularities in structure

Open ground floor Floating columns Mezzanine floor

Heavy mass at roof Partially filled panels Floating shear walls

Any other (Specify): _______________________________________________

_______________________________________________________________

2. Designed by

Architect Structural engineer Mason

3. Enhanced ductility design: Yes No

4. Column size at ground floor: __________________

5. Beam size: ________________________________

6. Spans between columns: _____________________

7. Shear wall(s) provided: Yes No

If provided, Symmetrically asymmetrically

Thickness of shear wall ________________________________

8. Infill type

Brick masonry Stone masonry Solid concrete blocks

Hollow concrete blocks Timber

Any other (Specify): ___________________________________

Thickness of Infill

Exterior________________ Interior________________

9. Basement Provided: Yes No

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10. Inspection accuracy

From outside only Contacted owner also

Partial Complete

Names of the surveyors: Date:

1. 2.

Sketch of building plan

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Sketch of building elevation

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APPENDIX 2-II: RAPID VISUAL SCREENING OFBUILDINGS IN VARIOUS SEISMIC ZONES IN INDIA*

Basis of the Methodology

The methodology proposed herebelow is based on the classification of buildings as per MSKIntensity scale as well as the new European Intensity scale, modified by the author to someextent based on his experience of buildings in India. The Grades of damage are also based on thetwo Intensity scales taken together. The relationship of the MSK Intensities adopted in IS: 1893-2002 (Part 1) and that adopted in the European scale have been studied and made use of indeveloping the table of damage grades of various building types under Intensities VI to IX.Based on this table the rapid visual screening of buildings in various seismic zones has beenarrived at.

Seismic Zones in India (IS:1893-2002, Part 1)

Zone V - MSK Intensity IX or higher (Destructive or Very Destructive intensities)Zone IV - MSK Intensity VIII (Heavily damaging intensity)Zone III - MSK Intensity VII (Damaging intensity)Zone II - MSK Intensity VI or lower (Slightly damaging or no damage intensities)Note : In a zone of higher intensity occurrence, lower intensities will occur around higher intensity area.

Building Types in India

From the damage vulnerability consideration the buildings can be classified as follows :

Masonry load bearing wall buildingsBuilding Type

Description

A Rubble (Field stone) in mud mortar or earthen walls A+ As above but one storey only having light roof B Semi-dressed, rubble, brought to courses, with through stones and

long corner stones; unreinforced brick walls with country type wooden roofs; unreinforced CC block walls

B+ As above of only single storeys and/or better quality of construction C Fully dressed (ashler) stone masonry or CC block or burnt brick walls

built using good lime or cement mortar.Unreinforced walls but having RC floor/roof.

C+ As at C but having horizontal RC bands (IS: 4329, 13828). D Masonry construction as at C but reinforced with bands & vertical

reinforcement, etc (IS: 4329), or confined masonry using horizontal & vertical reinforcing of walls.

Note: In rural areas, there are huts or shacks made from bio-mass & metal sheets etc. Their vulnerabilityto earthquakes is very very low.

*Anand S. Arya, Capacity Building Advisor, GOI-UNDP (DRM), New Delhi, 24/11/03

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Reinforced Concrete Frame Buildings (RCF) and Steel Frames (SF)

Frame Type

Description

C RCF without ERD or WRD, built in non-engineered way; RCF with hollow plinth (open ground storey); SF without bracings having hinge joints; RCF of ordinary design without ERD or WRD, SF of ordinary design without ERD or WRD

C+ MR-RCF/MR-SF of ordinary design without ERD or WRD

D MR-RCF with ordinary ERD without special details as per IS: 13920, with ordinary infill walls (such walls may fail earlier similar to C in masonry buildings; MR-SF with ordinary ERD without special details as per plastic design hand book SP:6(6)-1972.

E MR-RCF with high level of ERD as per IS: 1893-2002 & special details as per IS: 13920 MR-SF with high level of ERD as per IS: 1893-2002 & special details as per Plastic design hand book, SP:6(6)-1972

E+ MR-RCF as at E with well designed infills walls MR-SF as at E with well designed braces

F MR-RCF as at E with well designed & detailed RC shear walls MR-SF as at E with well designed & detailed steel braces & cladding; MR-RCF/MR-SF with well designed base isolation.

Notes: RCF = Reinforced concrete column- beam frame system

SF = Steel column- beam frame systemERD = Earthquake Resistant DesignWRD= Wind Resistant DesignMR = Moment Resistant jointed frame

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Grades of Damage to Buildings

Classification of damage to masonry buildings

Classification of damage to buildings of reinforced concrete

Grade 1: Negligible to slight damage (no structural damage, slight non-structural damage)

Hair-line cracks in very few walls.

Fall of small pieces of plaster only.

Fall of loose stones from upper parts of buildings in very few cases.

Grade 1: Negligible to slight damage (no structural damage, slight non-structural damage) Fine cracks in plaster over frame members or in walls at the base. Fine cracks in partitions & infills.

Grade 2: Moderate damage (Slight structural damage, moderate non-structural damage) Cracks in many walls. Fall of fairly large pieces of plaster. Partial collapse of smoke chimneys on roofs.

Grade 2: Moderate damage (Slight structural damage, moderate non-structural damage)

Cracks in columns & beams of frames & in structural walls.

Cracks in partition & infill walls; fall of brittle cladding & plaster. Falling mortar from the joints of wall panels.

Grade 3: Substantial to heavy damage (moderate structural damage, heavy non-structural damage) Large & extensive cracks in most walls. Roof tiles detach. Chimneys fracture at the roof line; failure of individual non-structural elements (partitions, gable walls).

Grade 3: Substantial to heavy damage (moderate structural damage, heavy non-structural damage)

Cracks in columns & beam column joints of frames at the base & at joints of coupled walls. Spalling of concrete cover, buckling of reinforced rods.

Large cracks in partition & infill walls, failure of individual infill panels.

Grade 4: Very heavy damage (heavy structural damage, very heavy non-structural damage) Serious failure of walls (gaps in walls), inner walls collapse; partial structural failure of roofs & floors.

Grade 4: Very heavy damage (heavy structural damage, very heavy non-structural damage)

Large cracks in structural elements with compression failure of concrete & fracture of rebars; bond failure of beam reinforcing bars; tilting of columns. Collapse of a few columns or of a single upper floor.

Grade 5: Destruction (very heavy structural damage) Total or near total collapse of the building.

Grade 5: Destruction (very heavy structural damage)

Collapse of ground floor parts (eg. Wings) of the building.

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Relationship of Seismic Intensity, Building Type & Damage Grades Few : Less than(15±5)%; Many: Between(15±5) to(55±5)%; Most: Between (55±5) to100%

Type of Building

Zone II MSK VI or less

Zone III MSK VII

Zone IV MSK VIII

Zone V MSK IX or More

A and A+

Many of grade 1 Few of grade 2 (rest no damage)

Most of grade 3 Few of grade 4 (rest of grade2or1)

Most of grade 4 Few of grade 5 (rest of grade 3,2)

Many of grade 5 (rest of grade 4&3)

B and B+

Many of grade 1 Few of grade 2 (rest no damage)

Many of grade 2 Few of grade 3 (rest of grade 1)

Most of grade 3 Few of grade 4 (rest of grade 2)

Many of grade 4 Few of grade 5 (rest of grade 3)

C and C+

Few of grade 1 (rest no damage)

Many of grade 1 Few of grade 2 (rest of grade 1,0)

Most of grade 2 Few of grade 3 (rest of grade 1)

Many of grade 3 Few of grade 4 (rest of grade 2)

M A S O N R Y B U I L D I N G S

D Few of grade 1 Few of grade 2 Many of grade 2 Few of grade 3 (rest of grade 1)

C and C+

Few of grade 1 (rest no damage)

Few of grade 2 (rest of grade 1,0)

Many of grade 2 Few of grade 3 (rest of grade 1)

Many of grade 3 Few of grade 4 (rest of grade 2)

D -

Few of grade 1 Few of grade 2 Many of grade 2 Few of grade 3 (rest of grade 1)

E and E+

-

-

-

Few of grade 2 (rest of grade 1 or 0)

R C F / S F B U I L D I N G S

F - - - Few of grade 1

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Rapid Visual Screening of Indian Buildings for Potential Seismic Hazards

Seismic Zone II

Plan to Scale

Probable Maximum Grade of Damage Masonry Building RC or Steel Frame Building Building

Type A,A+ B,B+ C,C+ D C,C+ D E,E+ F URM infill

Wood

Damage grade in Zone II

G2

G2

G1

-

G1

-

-

-

G1

-

Note: +sign indicates higher strength hence somewhat lower damage expected than that stated. Also average damage in one building type in the area may be lower by one grade point than the probable maximum indicated.

Surveyor will identify the Building Type, encircle it, also the corresponding damage grade. Recommended Action : Surveyor’s Signature __________________

1) Ensure adequate maintenance Name __________________________

Date ______________

OCCUPANCY SITE FALLING HAZARDS

Resi:Ord/Imp. School Max. Number of Persons High W.T. (within 3 m) Health Assembly Office 0-10 11-50 51-100 >100 Liquefiable (if sandy soil) Commercial Historic Residents _____ Land Slide Prone Emer. Service Industrial Floating __________ Chimneys Parapets Cladding Other

Elevation to Scale

Building Name __________________

Use _________________________

Address: ______________________

_________________Pin _________

Other Identifiers ________________

No. Stories ________ Year Built ____

Total Floor Area (sq.m)____________

PHOTOGRAPH

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Rapid Visual Screening of Indian Buildings for Potential Seismic Hazards

Seismic Zone III

Elevation to Scale

Building Name __________________

Use _________________________

Address: ______________________

_________________Pin _________

Other Identifiers ________________

No. Stories ________ Year Built ____

Total Floor Area (sq.m)____________

PHOTOGRAPH

Plan to Scale

Probable Maximum Grade of Damage Masonry Building RC or Steel Frame Building Building

Type A,A+ B,B+ C,C+ D C,C+ D E,E+ F URM infill

Wood

Damage grade in Zone III

G4

G3

G2

G1

G2

G1

-

-

G2

-

Note: +sign indicates higher strength hence somewhat lower damage expected than that stated. Also average damage in one building type in the area may be lower by one grade point than the probable maximum indicated. Surveyor will identify the Building Type, encircle it, also the corresponding damage grade, and tick mark the recommendation

Recommended Action :

OCCUPANCY SITE FALLING HAZARDS

Resi:Ord/Imp. School Max. Number of Persons High W.T. (within 3 m) Health Assembly Office 0-10 11 -50 51 -100 >100 Liquefiable (if sandy soil) Commercial Historic Residents _____ Land Slide Prone Emer. Service Industrial Floating __________ Chimneys Parapets Cladding Other

1) Ensure adequate maintenance 2) Detailed evaluation of B,B+ types for need for retrofitting 3) Detailed evaluation of A, A+ types for need for

reconstruction or possible retrofitting

Surveyor’s signature: ___________________

Name : _______________________________

Date :

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Rapid Visual Screening of Indian Buildings for Potential Seismic Hazards

Seismic Zone IV

Plan to Scale

Probable Maximum Grade of Damage Masonry Building RC or Steel Frame Building Building

Type A,A+ B,B+ C,C+ D C,C+ D E,E+ F URM infill

Wood

Damage grade in Zone IV

G5

G4

G3

G2

G3

G2

-

-

G3

G2

Note: +sign indicates higher strength hence somewhat lower damage expected than that stated. Also average damage in one building type in the area may be lower by one grade point than the probable maximum indicated. Surveyor will identify the Building Type, encircle it, also the corresponding damage grade and tick mark the recommendation Recommended Action :

OCCUPANCY SITE FALLING HAZARDS Resi:Ord/Imp. School Max. Number of Persons High W.T. (within 6 m) Health Assembly Office 0-10 11-50 51-100 >100 Liquefiable (if sandy soil) Commercial Historic Residents _____ Land Slide Prone Emer. Service Industrial Floating __________ Chimneys Parapets Cladding Other

1) A, A+ or B, B+: evaluate in detail for need of reconstruction or possible retrofitting to achieve type C or D

2) C, C+ : evaluate in detail for need for retrofitting 3) URM infill : evaluate in detail for need for retrofitting 4)

Surveyor’s signature : ____________________ Name : ________________________________ Date: __________________________________

Elevation to Scale

Building Name __________________

Use _________________________

Address: ______________________

_________________Pin _________

Other Identifiers ________________

No. Stories ________ Year Built ____

Total Floor Area (sq.m)____________

PHOTOGRAPH

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Rapid Visual Screening of Indian Buildings for Potential Seismic Hazards

Seismic Zone V

Elevation to Scale

Building Name __________________

Use _________________________

Address: ______________________

_________________Pin _________

Other Identifiers ________________

No. Stories ________ Year Built ____

Total Floor Area (sq.m)____________

PHOTOGRAPH

Plan to Scale

Probable Maximum Grade of Damage Masonry Building RC or Steel Frame Building Building

Type A,A+ B,B+ C,C+ D C,C+ D E,E+ F URM infill

Wood

Damage grade in Zone V

G5

G5

G4

G3

G4

G3

G2

G1

G4

G4

Note: +sign indicates higher strength hence somewhat lower damage expected than that stated. Also average damage in one building type in the area may be lower by one grade point than the probable maximum indicated. Surveyor will identify the Building Type, encircle it, also the corresponding damage grade and tick mark the recommendation. Recommended Action :

OCCUPANCY SITE FALLING HAZARDS

Resi:Ord/Imp. School Max. Number of Persons High W.T. (with in 8 m) Health Assembly Office 0-10 11-50 51-100 >100 Liquefiable (if sandy soil) Commercial Historic Residents _____ Land Slide Prone Emer. Service Industrial Floating __________ Chimneys Parapets Cladding Other

1) A, A+ or B, B+ : evaluate in detail for need of reconstruction or possible retrofitting to achieve type C or D

2) C, C+: evaluate in detail for need of retrofitting to achieve type D 3) URM infill: evaluate for need of reconstruction or possible

retrofitting to level D

Surveyor’s Signature _________________ Name:_____________________________ Date: ______________________________

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ASSESSMENT OF EXISTING MULTISTOREYEDBUILDINGS FOR DESIRED SEISMIC PERFORMANCE

D.K. PaulProfessor, Department of Earthquake Engineering, IIT Roorkee, Roorkee, 247 667

INTRODUCTION

Cities are growing, urban seismic risk is rapidly growing, and particularly in developing coun-ties the cities are more and more vulnerable to disasters. Direct strike by a major earthquake intowns and cities specially mega cities, losses could be in billions. Mega cities are like tickingtime bomb.

Poor performance of urban and semi urban buildings in recent Indian earthquakes has beenobserved specially in the Gujarat earthquake where multi-storey buildings had crumbled likehouses of cards within a matter of seconds. Generally, most of the multi-storey buildings are inthe urban areas and most of them are less than 20 storeys. Since, the IS codes on earthquakeresistant design are not mandatory and construction practices are not very strict, most of theexisting multi-storey buildings may not have been design for earthquake forces and thereforevulnerable to earthquakes. Many of the existing old structures may have faulty original design,extensions, alterations, encroachments, degradation of material over the time and thereforepose enormous seismic risk in particular to human lives and property. It is therefore importantto assess the vulnerability of the existing multi storey buildings using well-established acceptedprocedures.

We may accept minor damages but no collapse during a major earthquake. That is a goodenough reason why we should get our homes assessed for its earthquake resistance. But wheredo we begin? Depending upon the structure and seismic zone, only a Structural Engineer willbe in a position to tell whether the basic design of the building is adequate or not which willdepend upon the availability of structural drawings and soil data.

BUILDING PERFORMANCE LEVELS

The building performance is evaluated based on the performance of structural components andnon-structural components. The structural performance levels for which buildings have to beevaluated are (i) Immediate Occupancy -S1 i.e. the building undergo virtually no damage andcan be occupied immediately after the earthquake, (ii) Damage Control - S2, (iii) Life Safety -S3, (iv) Limited Safety - S4 and (v) Collapse Prevention - S5 i.e. the building undergoes severedamage but not collapsed. The non-structural performance levels for which non-structural ele-ments have to be evaluated are (i) Operational Performance - NA, (ii) Immediate Occupancy -

Chapter 14

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NB, (iii) Life Safety Performance - NC, and (iv) Hazard Reduced Performance - ND. Theoverall building performance is obtained as (i) Operational (S1+NA) i.e. very little damage orvirtually undamaged, (ii) Immediate Occupancy (S1+NB) i.e. minor repairs required, safe toreoccupy, (iii) Life Safety (S3+NC) i.e. structure remain stable, and (iv) Collapse Prevention(S5+ND) i.e. building barely standing.

SEISMIC EVALUATION

According to the Vulnerability Atlas of the country, more than 80% houses are non-engineeredconstruction, which are mainly load bearing buildings. However, there are many RC framedurban buildings which have been constructed without any consideration to resist earthquakeforces or without using the current codal practices on Earthquake Resistant Design. For such alarge number of seismically deficient buildings, a quick assessment method is required.

Assessment is a complex process, which has to take into account not only the design of thebuilding but also the deterioration of the material and damage caused to the building, if any. Thedifficulties faced in the seismic assessment of a building are threefold. (i)There is no reliablemethod to estimate the in-situ strength of the material and components of the building. (ii)Analytical methods to model the behaviour of the building during an earthquake are eitherunreliable or too complex to handle with the available tools. (iii) The third difficulty is unavail-ability of a reliable estimate of the earthquake parameters, to which the building is expected tobe subjected during its residual life. The ground motion parameters available in the presentcode have been estimated at a macro level and do not take into account the effect of local soilconditions, which are known to greatly modify the earthquake ground motion.

The condition survey of a building has to take into account three types of deficiencies: (i)deficiency arising from the original design, (ii) deficiency due to construction defects and dam-age due to earthquake or fire, and (iii) deficiency due to deterioration of material with time. Thedeficiencies in the original design can be identified by studying the drawings and design calcu-lations.

Earthquake Intensity

Vulnerability assessment depends upon the earthquake intensity and therefore need to be as-sessed very carefully. Earthquake intensity at a site can be estimated from the seismic zoningmap of India. For better estimation site-specific studies are carried out. Seismic microzonationof major cities of India are being carried out. Once the seismic microzonation maps are avail-able more accurate estimation of earthquake intensities will be possible. The site where thestructure is located is assigned a modified Mercalli Intensity (MMI) for the expected earth-quake. If this data is not available, then it will have to be worked out based on geological andseismological studies.

Methods of Evaluation

The evaluation of a building has to take into account a number of parameters described above.A rigorous evaluation of a building is involved and time consuming. Several methods of evalu-ation have been suggested namely, Screening method, Field Evaluation method, Approximate

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Analytical method and Detailed Analytical Evaluation method for assessing the seismic vulner-ability of an existing building. Visual inspection is the most important tool in the study of theactual condition of a building. Study of drawings is another source of information.

Rapid Visual Screening - RVA (L1)

The Rapid Visual screening is carried out for all considered buildings. It permits quick visualvulnerability assessment. Assessment during RVS i.e. level 1 assessment requires engineeringjudgment and training.

The purpose of the Rapid Visual Assessment (RVA) is to determine the adequacy of the struc-tural facility as to whether the facility will be able to withstand the expected earthquake. Forscenario earthquakes, performance levels for existing building stock need to be assessed. Rapidvulnerability assessment is the first necessary step but may not be sufficient to establish build-ing stock performance levels.

Necessary information for evaluation of structure is obtained either by conducting a field sur-vey or from building typology, if available or from both. If plan is available, then the field partymust check and verify the present status of the building.

In this method, buildings are evaluated qualitatively in terms of structural characteristics, struc-tural configuration, and the degree of deterioration of the building. This method is rapid andinexpensive and helps in identifying structures, which are clearly hazardous and the structuresfor which detailed hazard evaluation is sought.

The most pertinent information required to establish rating for building is collected. The infor-mation required are (i) general data, (ii) site related data, (iii) structural data and (iv) data aboutnon-structural elements. Based on these data, capacity ratio of each structure is worked out interms of intensity rating and structural system rating. If the capacity ratio is worked out to bemore than one than the structure is expected to withstand the expected earthquake, otherwisethe structure is weak and need strengthening. This method is considered reasonable and quiteadequate for a large scale survey of building in areas of potential seismic danger.

Simplified Vulnerability Assessment - SVA (L2)

It requires limited simplified engineering analysis to estimates the building drift. In this method,the capacity of an existing building to resist lateral forces is evaluated by determining the stressratios of its structural and non-structural elements. Stress ratio of an element is defined as theratio of the stresses induced by the seismic loading to that of the allowable stresses. In brief, themethod involves identification of critical elements of the

building such as columns, walls, chord members etc., and determining their stress-ratios for acombination of lateral and vertical loads. These are then compared with the acceptable ratingsand the capacity of the building assessed as good, fair, poor or very poor.

Detailed Vulnerability Assessment - DVA (L3)

It requires detailed nonlinear analysis on computer as for new buildings. It is recommended forimportant buildings A detailed evaluation is made for those structures which are found deficient

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in the initial assessment. For detailed assessment, static and seismic analysis may have to becarried out. Detailed analytical method evaluates the damage on the basis of energy capacity ofthe structure and expresses as percentage of total damage on a storey-by-storey basis. Damageis computed separately for structural, non-structural and glass elements. For structural and glasselements the damage is evaluated as a function of inter storey drift while for non-structuralelements the damage is evaluated on the basis of an estimated floor Modified Mercalli Intensity.The maximum dynamic response of the building to the applied load is calculated by responsespectrum approach considering the fundamental mode only. The performance of the building isevaluated with regard to strength and ductility.

SIMPLE ASSESSMENT GUIDELINES

The building typologies and epochs of their construction need to be studied with their vulnerabil-ity analysis under various seismic intensities. Various factors contributing to the vulnerability ofbuildings are (i) construction of non-engineered r.c. frame buildings where engineers are notconsulted; (ii) faulty original design -lack of lateral resisting elements e.g. inadequate framesand shear walls; (iii) upgrading of Codal practices; (iv) inadequate detailing of reinforcement;(iv) extensions, alterations and encroachment; (v) increase in load during to usage; (vi) poor anddeficient construction; (vii) lack of regular maintenance; and (viii) degradation of building mate-rial/ Corrosion.

Possible Deficiencies

While carrying out assessment identification of structural system is foremost important. Thebuildings based on gravity and lateral load resisting system can be classified into (i) RC MomentResisting Frame and (ii) RC Frame with Masonry Infill. Next step is to identify earthquakeresistant features; potential deficiencies and weak links in each type and benchmark the perfor-mance in recent Indian earthquake for sample building in each category. Followings are theimportant deficiencies contributing to the vulnerability of buildings.

Inadequate Application Technical Knowhow

Inadequate application of available engineering know-how due to ignorance, negligence andeconomic constraints has resulted in many seismic unsafe buildings. In the event of practicallynon-existent legislation about safe construction, the IS codes of practice are not beingcomplied.

Inadequate planning and design

Following are the important factors contributing to poor planning and design. (i) Unsymmetricalbuildings; (ii) Floating columns leading to sudden change in load path and stiffness; (iii) Smallwidth of columns (120 to 200 mm) aligned to walls; (iv) Weakness due to orientation of columns/beams; (v) Columns designed for axial load without any moments; (vi) Walls resting on slabwithout any beam; and (vii) Long cantilever.

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Inadequate Foundation System

Buildings have to be assessed for its performance, settlement, depth of foundation, deteriorationdue to weathering or age, capacity of foundation, stability against overturning, ties betweenfoundation elements, load path for transfer of seismic forces to soil and special requirements insloping sites. Buildings situated near the steep slope, on the filled up ground or loose ground,liquefiable soil are vulnerable. Foundation adequacy has to be checked for such conditions.Safety of buildings lying close to the zone of landslide/rock slide area, near known fault ruptureand shear zone should also be checked. There is a need for seismic microzonation to assessvulnerability to greater detail and accuracy.

Inadequate detailing and construction

Following are the important factors contributing to poor detailing and construction. (i) Lack oflateral load resisting elements: moment resisting frame, shear walls; and (ii) inadequate detail-ing of reinforcement from ductility considerations e.g. anchorage of longitudinal reinforce-ment, beam-column joint regions, lap splices placed in potential plastic hinge regions and trans-verse reinforcement in beams and columns (iii) inadequate diaphragm action of roof & floors;(iv) inadequate strength and ductility in soft storey; (v) poor quality of construction materialand technology; (vi) treatment of non-structural components - infill walls, staircases, watertanks on roof; (vii) inadequate strength of footings and/or piles, and (viii) local constructionpractice - Strength of concrete, reinforcement properties, detailing practices, compressive strengthof bricks and mortar. A building with discontinuity is subjected to concentration of forces anddeformations at the point of discontinuity, which may lead to failure of members at the junctionand finally lead to collapse of the building.

Non-uniform Configuration & Torsion

Experience in past earthquakes has shown that the buildings with simple and uniform configu-rations are subjected to less damage. The geometric irregularities in horizontal and verticaldirections weaken the building. However, due to practical and architectural considerations, it isnot always possible to have a regular structural configuration in the horizontal and verticalplanes and it has been the root of conflict between the architects and the structural engineers.

The important factors contributing to the torsional behaviour of building are (i) unsymmetricalin plan and elevation e.g. shape of the building - L, E, T or irregular plans, (ii) location of liftcore, (iii) regularity of columns on a typical floor, (iv) position of water tanks, heavy equipment onroof; (v) locations of infill walls.

Large Openings and Glazing

Large openings in the infill walls and glazing reduce the stiffness.

High walls

High walls such as theatre, auditorium, churches, and temples arevulnerable. The transfer of seismic load to the ground has to beassessed in terms of lateral resisting systems such as frames, shearwalls, stiffness, connections, joints and support conditions. Fig.1 Floating columns

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Floating Columns

The vertical and lateral load carried by the columns should be transferred to the soil through thefoundation system. The columns floating (resting) on beam as shown in the Fig.1 is likely todamage severely since the beam supporting the column will be subjected to very high loads.Such construction should be avoided as far as possible.

Soft and weak storeys

Due to scarcity of parking space in the cities the ground storey is kept open for parking. Inframed buildings, usually the upper storeys have masonry infills as partitions, while the groundstorey is having only the bare frame. This type of structural system, termed as Stilt, is verycommon in commercial, as well as, residential multi-storey buildings of big cities. The masonryinfills present in the upper storeys act as diagonal braces and increase the storey stiffness sig-nificantly. The absence of stiffness due to infills, at the ground storey makes it a soft storey.Sometimes, the ground storey is designed to house plazas and commercial compounds withlarge floor areas and high ceilings.

Fig.2 Soft ground storey failure Fig.3 Ten storey stilt building withcollapse of third storey

A few of the columns and shear walls are dropped at first storey and the height of the remainingcolumns is more than usual. This type of system results in further reduced stiffness of theground storey, and poses great risk to the safety of the building, specially when subjected toearthquake forces. Such architectural designs resulting in extremely soft ground storeys may bedebatable, but the scarcity of parking space in big cities is a real problem and there does notseem to be a solution for that, other than the stilt type construction. Therefore, the open groundstorey buildings are going to remain and the structural engineers have to find solutions to thisproblem. Figure 2 shows failure of ground floor stilt columns and Fig.3 shows failure of tenstorey silt building with collapse of third weak storey.

The recent earthquake of Bhuj has once again focused on the problem of soft storey buildings,as a number of soft ground storey buildings in Ahmedabad, which is about 300 km away fromthe epicentre, have collapsed. Of course, the major reasons of collapse have been the non compliceof the codal practices, poor detailing of reinforcement and the poor quality of construction.

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There were many other modes of failure of multi-storey RC framed buildings. But these weak-nesses have been widely exposed in most of the soft ground storey buildings. This has forced toconsider alternative designs providing sufficient stiffness to the ground storey and, at the sametime, not hindering the parking functional requirement of parking space.

The soft and weak storey in commercial, apartment or office building or stilt parking withoutadequate stiff support at ground floor may result in a weak storey causing partial or total col-lapse. In masonry infilled frame building with open ground storey configuration the stiffness ofthe upper storeys is about three times the stiffness of the ground storey. This ratio

may be much higher in case of buildings with columns or shear walls dropped at ground storey.Such storeys were originally visualized [(Fintal & Khan(1969)] to have shock absorbing effectto reduce the earthquake forces on multi-storey buildings. However the performance of suchbuildings in past earthquakes has shown that the ductility demand of first storey in such build-ings is beyond the practicable limits [Chopra et al.(1973), Pekau(1975)]. The soft first storeybuilding behaves as an inverted pendulum, resulting in very large lateral deformation of groundstorey. Most of the lateral deformation of the building is concentrated in the ground storey,while the upper portion of the buildings is well within the elastic limit. The energy dissipationtakes place mainly through the first storey.

In a multi-storey building the ground storey is subjected to the maximum storey shear. If the firststorey is soft storey the columns of the first storey yield, first. The upper storeys of the buildingfloat over the first storey and this reduces the storey shear in the upper storeys due to baseisolation effect. Therefore, the upper storeys, including the masonry infills, remain intact andelastic. Most of the energy dissipation takes place through the first storey only, which requiresvery large displacement, usually much beyond the capacity of the columns. The beams of theground storey are protected from large inelastic deformations, due to presence of infills above.This essentially results in formation of an unstable mechanism. P-delta effect due to excessivedisplacement of the first storey and heavy weight of the upper storeys further increases theinstability.

The codes [IS:1893(2002), IBC(20000] define a soft storey as having lateral stiffness less than70% of the stiffness of the storey above or less than 80% of the average lateral stiffness of thethree storeys above. If the stiffness is less than 60% of the stiffness of the storey above or lessthan 70% of the average lateral stiffness of the three storeys above, then, it is termed as Ex-treme Soft Storey.

Masonry Infill

The behaviour of masonry infills in frame buildings is complex. Several problems associated withthe masonry infills have been observed in the past earthquakes. Out of these the soft storeyeffect is the most glaring. In developed countries the seismic codes tend to discourage the use ofmasonry panels as partitions. At the same time, some beneficial effects of masonry have alsobeen reported. Some of the buildings with masonry infills have shown excellent seismic behaviourduring past earthquakes. In developing countries masonry is still quite common to be used aspartitions in framed buildings.

The complexity of the behaviour of masonry is further complicated by the absence of rigidcontact between the masonry panel and the beam above and presence of openings for windows.

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The infills are brittle and weak compared to the RC frame members. These contribute to thestiffness of the structure in the initial stage of loading, but generally fail before the ultimatecapacity of frame is reached. Therefore, the usual design practice has been to ignore the stiff-ness and strength contribution of the infills and design the bare frame for the earthquake loads.

If a frame has uniform in-fills throughout its height, it increases the stiffness of the buildinguniformly. This reduces the time period of the building and pushes it into the higher accelera-tion zone of the response spectra. Therefore, some of the codes require that the infilled framesshould be designed for the average of the time periods of the infilled and the uncracked bareframe. However, it has been reported [Fardis(1998)] that such requirements may be too conser-vative and unduly penalizes the infilled structures.

Problem arises in case of a building with open first storey; where the first storey is kept open forparking purposes. The stiffness of the upper storeys, having masonry infills increases severaltimes the stiffness of the first storey, resulting in the soft first storey configuration. Even abuilding with uniform masonry infills, throughout its height may result in soft first storey con-figuration. As the masonry panels are weak, they fail, first in the ground storey. The failure ofmasonry infills in the ground storey changes the configuration to the soft first storey configura-tion where the ground storey columns yield while the upper storeys remain elastic, as the storeyshear in the upper stories is reduced due to base isolation effect. Seismic evaluation of adjacentbuildings is also important. Adjacent buildings of unequal height may collide; floors may im-pact or fall over the adjacent buildings causing damage.

Estimation of in-situ strength, defects and degree of damage/deterioration is an important andcomplex task. A number of non-destructive techniques are available for this purpose such asultra sonic pulse velocity, rebound hammer test etc. Most of the non-destructive tests are basedon indirect measurement and require experience and skill in interpretation of results. Use ofstatistical methods is helpful in concluding about the in-situ strength of concrete and overallcondition of the building.

Vulnerability of Existing Buildings

Many buildings constructed during 60's and 70's have not been designed for earthquake forces.Building design at various time interval followed different revision of the code varying not onlyin the seismic force level but also in the provisions for ductility. Table 1 gives the comparison offorces in ratio of the current code i.e. IS: 1893-2002 which indicates the buildings designed asper 1962 codes are about 25% of the forces of the current code.

Table -1 Comparison of forces in ratio of the current code i.e. IS: 1893-2002

Seismic Zone 1962/2002 1966/2002 1970/2002 1975/2002 1984/2002

III 0.28 0.53 0.60 0.72 1.0

IV 0.23 0.44 0.50 0.60 1.0

V 0.25 0.47 0.50 0.65 1.0

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Note: For comparison the importance factor is taken as 1.0; In 1984 Performance factor K=1.6;Soil type III; In 1970, 1975, 1984 Beeta is taken 1.5; Seismic coefficient for 2002 evaluated forR=3

Quick Assessment Guidelines

A quick way of assessing vulnerability is as follows: (i) Relative size - slenderness ratio; (ii)Weak storey - strength less than 80% of adjacent storey; (iii) Soft storey - stiffness less than70% of adjacent storey; (v) Infill - if the height/ thickness ratio (h/t) > 15 then out of planefailure is possible (iv) Geometry - not more than 30% change in horizontal dimension of thelateral resisting system; (v) Mass - not more than 50% change in effective mass from one storeyto the next; (vi) Torsion - eccentricity not more than 1.5 times the building width; (vii) Dia-phragm continuity - no sudden discontinuity; (viii) Plan irregularities projection of the

structure beyond re-entrant corner greater than 15% of its plan dimension; (ix) Redundancy -structure should have large indeterminacy; and (x) Strong column-weak beam - the sum of themoment capacity of the columns shall be 10% more than that of beams at the frame joint.

Non-structural components

Parapets, sunshades, projections, fixtures, cladding. etc. have to be assessed for their capacityto withstand earthquake forces. Safety of non-structural components is particularly important incase of buildings such as Hospitals, Telephone exchanges, control buildings, etc. The failure offixtures and connections may lead to not only the disruption of the function but also the loss oflife due to disruption as well as due to direct injury from the falling component.

Partitions and infills are another component, which are usually considered as non-structural inthe design. Their safety is not ensured in design. Failure of masonry infills in out of planebending may be fatal to the inmates.

DIAGNOSIS

Actual diagnosis of vulnerability can be assessed by working out the capacity and demand of abuilding. Capacity curve is a plot of seismic shear "V" and lateral deflection of the building atroof level obtained by an approximate nonlinear, incremental analysis. A computer model iscreated and then lateral storey forces are applied either at the top or along the height of thebuilding proportional to fundamental mode shape of the building. The building is then analyzedfor gravity and lateral loads to obtain member forces including p-delta effects. The base shear,roof displacement, member forces and member displacements are recorded. Cumulative baseshear, roof displacement, member forces and member displacements are calculated. Analsis isrepeated till the structure becomes unstable or the deformation of members is such that loss ofgravity load carrying capacity takes place.

The demand curve is elastic response spectra reduced to the damping ratio corresponding to thedeformation stage of the building. For comparison the demand capacity needs to be plotted inthe same coordinates. The acceleration-displacement response spectra (adrs) is the convenientformat for this purpose.

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CONCLUSIONS

The problem of seismic evaluation and assessment of buildings based on desired seismicperformance levels has been discussed. Different methods of seismic evaluation of buildingshave been presented. Building features and factors contributing to vulnerability has beenpresented in detail.

REFERENCES

Arnold, C., (1984) "Soft First Storey: Truths and Myths," Proc. Of the Eighth World Confer-ence on Earthquake Engineering, San Francisco, Vol. 5, pp 943-949.ATC 40, Seismic Evaluation and Retrofit of Concrete Buildings, Vol. 1&2, Applied TechnologyCouncil, Report No. SSC 96-01, 1996..FEMA Seismic Evaluation Handbook, Published by Federal Emergency management Agency,USAChopra, A.K., Clough, D.P. and Clough, R.W., (1973) "Earthquake Resistance of a Buildingwith Soft First Storey," International Journal of Earthquake Engineering and Structural Dynam-ics, Apr-June, pp 347-355.Dolsek, Matjaz, Fazfar, Peter, (2001) "Soft Storey Effects in Uniformly Infilled ReinforcedConcrete Frames," Journal of Earthquake Engineering, Vol. 5, No. 1, Jan., pp 1-12.Fardis, M.N., (1998) "Design of RC infilled structures," Proc. of the 11th European Conferenceon Earthquake Engineering, Rotterdam.Fintel, M. and Khan F.R., (1969) "Shock Absorbing Soft Storey Concept for Multi-Storey Earth-quake Structures," ACI J., May, 381-390.IS 13311 (Part 1) : 1992, Non-Destructive Testing of Concrete - Methods of Test, Part - 1, UltraSound Pulse Velocity, Bureau of Indian Standards.IS 13311 (Part 2) : 1992, Non-Destructive Testing of Concrete - Methods of Test, Part - 2,Rebound Hammer, Bureau of Indian Standards.IS: 1893 (2000), Draft Code on Criteria for Earthquake resistant Design of structures, Part-1:General Provisions and Buildings, to be published, Bureau of Indian Standards, New Delhi, pp23, 31.

IBC (2000), International Building Code, International Code Council, Inc., USA, pp 357.Pekau, O.A., (1975) "Behaviour of Yielding Soft-Storey Structures," Proc. of Fifth EuropeanConference on Earthquake engineering, Ankara, Vol. 1, Paper No. 72.Rapid Visual Screening of Buildings for Potential Hazards, Published by Federal Emergencymanagement Agency, USA.

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RETROFITTING OF MASONRY BUILDINGS

Yogendra Singh & D.K. PaulDepartment of Earthquake Engineering, IIT Roorkee, 247 667

GENERAL

In India, a large population in rural as well as in sub-urban areas resides in buildings made ofmud, rubble and coursed stone masonry, and unreinforced brick and block masonry. Construc-tion of such buildings is done by local artisans, employing local construction practices andmaterials. No strength calculations or engineering practice is involved in construction of suchbuildings, which are termed as "Non - engineered buildings".

These buildings have been observed to cause large scale devastations during past earthquakes,due to lack of strength and ductility against earthquake loads. However, it has also been ob-served in laboratory, as well as in practice that if some safety measures are taken, collapse ofsuch buildings during earthquakes can be avoided and large scale loss of life can be prevented.This chapter deals with the techniques and measures for safety of existing non-engineered build-ings against earthquakes.

PRINCIPLE OF SEISMIC SAFETY OF LOAD-BEARING WALL BUILDINGS

Buildings in which the roof and floor slabs are directly supported on the walls are called load-bearing wall buildings. These walls serve as partitions and also bear the load from slabs. Thelateral load resulting from earthquake and wind is also resisted by these walls and transferred toground. Individual unreinforced masonry or mud walls are very weak in out-of-plane bendingdue to lack of tensile strength. These are generally not capable of bearing out-of-plane bendingmoment, even resulting from their own inertia. These walls act as shear-walls in their in-planeaction and possess sufficient in-plane strength, if not weakened by too many openings. In abuilding, there are four or more than four walls, which act as a box under lateral load. The wallsparallel to the lateral load, act as webs and the walls orthogonal to load act as flanges. Theresistance of box is much higher than the resistance of individual walls. The box action involvesconsiderable interaction between webs and flanges at corners of building. It has been observedin past earthquakes that in many cases, the damage initiates at corners, resulting in loss of boxaction and walls start acting independently leading to collapse of building. The basic principleof seismic safety of load bearing wall buildings, lies in their integral box action during earth-quake. In new buildings it can be ensured by providing seismic bands. In existing buildings, theintegral box action is to be ensured by providing external bandage or prestressing.

Openings in walls are the source of weakness. Openings result in reduction of effective cross-sectional area of wall resisting lateral loads. If the openings are very near to the integral box

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action by weakeningthe joints. The piersbetween openingsare subjected tohigher stresses thanthe portion of thewall above and be-low the openings. It

has been observed in the past earthquakes

that diagonal X- shaped cracks in the piers originate from the corners of openings, Figure. 1. Toavoid this damage the opening perimeter need to be strengthened by proper reinforcement.

In addition to these two basic principles, the shape of building and regularity and continuity ofconstruction are also very important for seismic safety of buildings. Asymmetric buildings un-dergo torsion and result in warping of walls. Warping produces very high out of plane forces inwalls leading to collapse. The stiffness of walls is affected by openings and therefore openingsneed to be symmetrically located. Regular rectangular shaped buildings have been observed tobehave better during earthquakes. Continuity of construction particularly at joints and cornersis very important due to interaction between different component walls at these locations.

These are the basic causes of damage and principles of seismic safety of load bearing wallbuildings. In addition to these, there are several other aspects of seismic safety of such build-ings, which have been covered in the following sections.

RETROFITTING OF EARTHEN BUILDINGS

In India earthen buildingsare still a reality. Suchhouses are not considered tobe safe against earthquakeand wide spread damagehas been observed in pastearthquakes. But due toeconomic constraints, alarge population, particu-larly in rural areas, is livingin earthen buildings. Thesebuildings pose a severe riskto life and need to be retro-fitted.

Figure 2 shows a typicalarrangement for retrofittingof earthen buildings. Thewalls are quite weak in out

Fig. 1 Cracks originating from corners of opening

Fig. 2 Retrofitting of earthen buildings

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of plane action. Also, the walls loose their vertical load carrying capacity after cracking. There-fore, the walls and roof are to be supported by timber members as shown in the figure. Slopingroofs are particularly vulnerable in case of earthen buildings. The roofs are also to be retrofit-ted, in addition to the support to walls. The retrofitting of sloping roofs has been discussed laterin this Chapter.

Earthen buildings with asymmetry, with more than one storey and with high walls have beenknown to be very vulnerable in seismic zones. Such buildings should be modified/ rebuilt withsymmetric plan and regular construction and height should be limited to single storey.

RETROFITTING OF MASONRY BUILDINGS

Retrofitting of Rubble Masonry Buildings

Stone is a very common construction material in hilly androcky areas. Most of the buildings in stone have rubble ma-sonry with mud or cement-sand mortar. The stone walls areusually made in two wythes giving smooth finish along thetwo faces of wall and the space, in between, is filled withsmaller stone pieces. If the two wythes are not interconnectedby sufficient number of 'through stones' these split duringearthquake shaking and result in collapse of wall. This hasbeen observed as the major failure mode in case of rubblestone masonry buildings. To avoid this splitting, sufficientnumber of 'Headers' or through elements are to be added inexisting rubble masonry walls. These Header elements canbe of stones, RC or wood. In case of wooden elements, theseshould be properly treated to avoid decay. The easiest optionis to provide RC elements (Fig. 3). For this purpose, a hole in

the wall is to be made at selected location by gently remov ing the stones from the two sides of the wall. Care has to be

taken in removing the stones, so that the wall is not damaged. The space created by removal ofstones, is filled with concrete and a steel rod bent at two ends as shown in the Fig 3.

In addition to the above strengthening of individual walls, unsupported length and height ofwalls and size and placing of openings is also to be controlled as per codal requirements. Theintegral box action of the building is to be ensured by providing seismic belts at lintel and rooflevel and vertical reinforcement at corners and junctions is to be provided. These are same asfor other masonry buildings and described in following sections.

Retrofitting of Buildings Made of Rectangular Masonry Units

Rectangular masonry units of stone, brick or concrete blocks are also very common in India.Masonry made of such units is much stronger than rubble stone masonry and performs betterduring earthquakes. However, the strength and quality of bricks, stones and concrete blocksvaries in different parts of country. This masonry is also constructed in cement- sand or mudmortar. Some old construction also exists in lime-surkhi mortar. Retrofitting of these buildingsincludes strengthening of individual walls and ensuring of integral box action.

Fig. 3 Providing RC throughelements

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Strengthening of Walls

Individual walls in a building are to be checked for strength, quality of construction, unsupportedlength and height, and size and placing of openings. In ordinary buildings, the strength and qualityof construction can be judged by visual inspection of masonry units and mortar. If the walls havecracks, voids, loose pockets or degradation of mortar, these need to strengthen by grouting.Grouting can be done using shrinkage compensated cement slurry or polymer grouts or epoxy.Polymer/epoxy grouting is costly and is normally not used in masonry. Shrinkage compensatedcement slurry grouting is considered satisfactory in ordinary masonry buildings. Larger voids canbe directly filled by shrinkage compensated cement-sand mortar or polymer modified mortar.For grouting of cracks, voids, loose pockets and pockets with degraded mortar, following proce-dure is followed:

(i) The plaster from the area to be grouted is removed and holes are drilled (2 to 4 holes in eachsquare meter). In case of cracks, the cracks outcrops at the wall surfaces are chiseled to Vshape groove. The dust and loose material is removed by air blasting.

(ii) Nipples (plastic/Aluminum pipes of 12 mm dia and about 30 to 40 mm long) are placed inthe drilled holes and sealed by polymer mortar. In case of major cracks, T shaped nipplesare placed directly above the V grooves and sealed by polymer mortar. The cracks aresealed on both the faces using polymer mortar.

(iii) After the sealing mortar gets strength, water is injected through nipples. This washes thedust and saturates the masonry to avoid excessive withdrawal of water by the masonry fromthe grout. The injection of water is started from the highest nipple moving down. Aftersaturating the masonry with water, grout injection is started from the lowest nipple, till itcomes out from the upper nipple. Then the nipple is sealed using polymer mortar and injec-tion is started from upper nipple.

The grout to be injected usually consists of shrinkage compensated cement and water withflowable consistency. In case of wide cracks (width > 5 mm) or voids, fine sand (1-cement : 2-fine sand) is also added to the grout.

After setting of the grout, the nipples are cut and the surface is plastered.To provide additional strengths to severely damaged/ deteriorated masonry walls, Ferro-cementplated can be provided on both the faces of walls. Figure 4 shows the details of Ferro-cementstrengthening. It consists of following steps:

(i) The plaster on both sides of the wall is removed, the mortar joints are raked out up to 15-20mm depth, surface is cleaned and wet with water and a coat of cement slurry or polymerenhanced cement slurry is applied.

(ii) A 10 mm thick coat of cement sand plaster (1:3 - cement : coarse sand) or 1:1.5:3 microconcrete is applied. The surface of the plaster is roughened to have good bond with thesecond coat.

(iii) Welded wire mesh is fixed on the surface of plaster/ micro concrete using 150 mm longnails. The wiremesh and nails are galvanized to protect them from corrosion. Alternatively

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the wiremesh on the two sides of the wall can be anchored together using 3 mm galvanizedwire or 'J' bolts passing through holes drilled in the wall. The anchors are used at every 450mm. After clamping the wiremesh on the two sides of wall, the wires/bolts are grouted inthe holes.

(iv) After fixing the wiremesh, second coat of plaster or micro-concrete (16-20 mm thick) isapplied.

Fig. 4 Strengthening by Ferro-cement

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Fig. 5 Strengthening of masonry walls using FRP strips

Masonry walls can also be strengthened by epoxy glueing the FRP strips (Fig 5). Another methodof strengthening masonry walls has been employed in some historical buildings. In this method,vertical cores are drilled in the thick masonry walls and filled with reinforced concrete. Thevertical RC members so created, act as columns and support the masonry wall, without affect-ing its external appearance.

Control of Unsupported Length, Height and Openings

Masonry walls are weak in out of plane action. Large unsupported lengths and heights need tobe supported laterally. These supports can be provided either by cross walls (Fig 6) or by but-tresses (Fig 7). The cross walls and buttresses need to be properly connected with the existingwall. Figure 8 shows the details of connecting a buttress with an existing wall and Fig. 9 showsthe details of connecting a new masonry wall with an existing rubble/stone masonry wall.

Fig. 6 Supporting long walls by cross walls

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Fig. 7 Supporting long walls by buttresses

Fig. 8 Connection of buttresses with original walls

Fig..9 Connecting a new masonry wall with an existing stone wall

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Fig. 10 Closing of an opening

Openings result in weakness in masonry walls. The total length of openings in masonry shouldgenerally be restricted to one third of the length of the wall. Further the openings should not bevery close (less then 600 mm) to each other or corners. If these conditions are not satisfied,some of the openings have to be closed completely or partially. Proper bond of old and newmasonry has to be ensured while closing the opening. Teething (Fig 10) and steel anchors can beused for this purpose.

Integral Box Action

For integral box action of the building under lateral load, all the walls of the building are requiredto be tied together using continuous ferro-cement seismic belts at lintel and roof/floor level (Fig4.4). The procedure of application of ferro-cement belts is the same as described above inSection 4.4.3. In case of flat RC roofs and floors, the belt at roof/floor level can be omitted. Incase of pitched roofs, the seismic belt is to be provided at eaves level and at the top of gable wall.

Another method of achieving integral box action is prestressing of walls as shown in Fig. 11.Prestressing also results in strengthening of the walls.

To enhance the integrity and bond of orthogonal walls, vertical ferro-cement plates are alsoprovided along corners and joints, as shown in Fig 4.4. These plates provide better interconnec-tion of orthogonal walls and provide vertical reinforcement at corners. Alternatively, verticalreinforcement can also be added in the form of steel bars in the corners as shown in Fig 12. Thebar has to be properly connected with walls, roof/floor and is to be grout anchored to the foun-dation. As explained earlier, the openings result in weakness in wall and cracks initiate from thecorners of openings. Therefore the masonry around the windows and doors need to be strength-ened using Ferro-cement strips (Fig 4.4).

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Fig. 11 Prestressing of masonry walls

Fig. 12 Corner reinforcement

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Strengthening of Foundations

Strengthening of foundations is a costly affair and require skill, but sometimes it may be neces-sary. The bearing area of the strip footing is increased by providing RC beams on both sides ofthe wall (Fig 13). These beams are interconnected at several locations through gaps created inthe wall. This results in effective transfer of load from the wall to the added RC beams.

Fig. 13 Strengthening of foundation

In addition to this, the drainage condition around the building is to be improved to avoid saturationof soil. A concrete apron around the building is helpful in avoiding direct soaking of the soil in thevicinity of the foundation.

Strengthening of Arches

Old buildings have masonry arches over the openings. Under severe shaking during earthquake,these arches get loosened and arch action is lost. To avoid this, steel tie-rods are provided at thespringing of the arches, by drilling holes and grouting the steel bars (Fig 14). Another method ofstrengthening masonry arches is relieving them from the load by providing a RC or steel linteljust below or above the arch. Fig 15 shows two alternatives for providing a RC lintel below anarch. For providing a steel lintel above an arch, two steel channels or I-sections are provided onboth sides of walls by partially removing the masonry and interconnected by bolts and coveredby concrete.

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Fig. 14 Strengthening of masonry arch by tie bars

Fig. 15(a) Strengthening of masonry arch by RC lintel (Alternative-1)

Fig. 15(b) Strengthening of masonry arch by RC lintel (Alternative-2)

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RETROFITTING OF ROOFS AND FLOORS

Pitched roofs have been observed to be most vulnerable during past earthquakes. Usually theseroofs are cladded with heavy, brittle and loose stone pieces or tiles. These tiles fall duringshaking and cause injury. These should be replaced by lightweight corrugated GI sheets.Properly connected tiles can also be used. Sloping roofs have tendency to open up duringearthquakes and there is a relative motion between eaves and crown, which results in looseningand falling of cladding. For seismic safety, the roof should move as a whole and there shouldnot be any relative motion of different members. This can be achieved by proper bracing inhorizontal, vertical and inclined plane as shown in Fig 16. The roofs also need to

be connected properly with the walls. Anchor bolts are to be grouted into roof band/wall to holdthe rafters. In case of seismic belts, the bolts may be anchored to the roof belt.

Fig. 16 Horizontal and vertical bracing in sloping roofs

In case of wooden floors, the relative movement of planks and beams can be avoided by provid-ing bracings and another layer of planks perpendicular to the existing planks. Floor consistingof precast elements or steel sections with stones or tiles need to be integrated. This integrationcan be achieved by providing a RC topping on the floor and a RC edge beam with partialbearing on masonry walls (Fig 17).

In case of jack-arch roofs and floors, supported by steel girders, the girders undergo relativemovement during earthquake and result in collapse of roof. The relative motions is to be seizedby providing steel strips or bars welded to the bottom of girders. These bars/strips should alsobe properly anchored to the walls.

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Fig. 17 Integration of floor with walls

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REFERENCES

IS 13827-1993, Improving Earthquake Resistance of Earthen Buildings - Guidelines, Bureau ofIndian Standards, New Delhi.Arya, A.S., Retrofitting of Stone Houses in Marathwada Area of Maharashtra, Building Materi-als and Technology Promotion Council, Ministry of Urban Affairs and Employment, Govern-ment of India.

IS 13828-1993, Improving Earthquake Resistance of Low Strength Masonry Buildings - Guide-lines, Bureau of Indian Standards, New Delhi.

Arya, A.S., 2001, A Manual of Earthquake Resistant Non-Engineered Construction, Indian So-ciety of Earthquake Technology.

Arya, A.S., Guidelines for Seismic Retrofitting of Existing Housing, School and Health Build-ings in Uttaranchal..

Arya, A.S., 1987, Protection of Educational Buildings against Earthquakes, Educational Build-ing Report 13, UNESCO, Bangkok.

IS 4326-1993, Earthquake Resistant Design and Construction of Buildings - Code of Practice,Bureau of Indian Standards, New Delhi.

IS 13935-1993, Repair and Seismic Strengthening of Buildings - Guidelines, Bureau of IndianStandards, New Delhi.

Hollaway, L.C., and Leeming, M.B., ed., 1999, Strengthening of Reinforced Concrete Struc-tures - using Externally Bonded FRP Composites in Structural and Civil Engineering, CRCPress.

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RETROFITTING OF RC BUILDINGS

Yogendra Singh & D.K. PaulDepartment of Earthquake Engineering, IIT Roorkee, 247 667

GENERAL

A large number of reinforced concrete buildings have been constructed in post-independenceIndia and the construction is going on at a much larger scale in the growing towns and devel-oped mega cities. These building are considered to be engineered buildings, as a structuralengineer is usually engaged in design and construction of these buildings. However, the marketforces tend to ignore the seismic safety of buildings, being constructed even now after fourdecades of Indian Code of Practice on Earthquake Resistance Design in existence, and afterwitnessing a number of damaging earthquakes. Not only the buildings constructed in sixtieslack in earthquake resistance measures, even the most modern buildings being constructed byprivate builders today, are seriously lacking in earthquake safety and pose a serious seismic riskto the occupants. This has been well exposed by the Bhuj earthquake of January 2001.

This large inventory of existing buildings needs systematic retrofitting to make it safe for theoccupants. This chapter deals with the techniques available for retrofitting of existing RC build-ings.

The retrofitting schemes for RC buildings are based on two principles (i) reduction inearthquake demand by reducing mass, by base-isolation or by supplemental energy dissipation,and (ii) enhancing the capacity of the structure to withstand the earthquake forces. The capacitymay be enhanced either by strengthening the deficient members or by improving the ductility anddeformation capacity resulting in increased hysteretic damping. There is another importantaspect of retrofitting-completion of load path and removal of configurational irregularities. Thefollowing sections describe different techniques based on the above principles.

COMPLETION OF LOAD PATH

A large number of buildings in India have incomplete load paths mainly to take advantage of theloopholes in the building by-laws and sometimes due to market compulsions and Architect'squest for creating new shapes. For example, floating column constructions are not uncommonin Indian cities to take maximum advantage of floor area with restrictions on ground area.

The general seismic load path in a building is as follows - the inertial forces originating throughoutthe building are first transferred to horizontal floor diaphragms, the diaphragms transfer theseforces to vertical framing system resisting lateral loads; the vertical framing system consisting

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of beam-column frames and shear walls, transfers the seismic force to foundation and support-ing soil. If there is a discontinuity in load path, the building is incapable of transferring the loadto ground and it is unable to resist the lateral load during earthquake, irrespective of strength ofexisting members.

The common examples of such building are those in which shear walls or columns are notstarted from ground but started at first floor (or at a higher level). Such columns are commonlyknown as floating columns. This is done to increase the floor area at first floor level or to havelarge open spaces at ground floor for commercial purposes. In such buildings, the first floorbeams are subjected to very high forces as the forces from floating columns/shear walls aretransferred to other columns and walls through these beams.

The remedy to this deficiency is to complete the load path by providing the missing part of thecolumn/shear wall. In case of a floating column, a new column is to be erected below thefloating column (Fig. 1). This column should have footing connected with the foundation of theexisting building and the reinforcement of the new column should be welded with the reinforce-ment of existing column. Shrinkage compensating agents should be used in the new concrete toavoid shortening of the new column resulting in separation between new and old concrete.

Fig. 1 Adding a new column below a floating column

Similarly, a shear wall panel (Fig. 2 (a)) is to be provided below the existing shear wall. Thispanel should have rigid shear connections with adjacent columns, beam/slab above it, andfoundation. For this purpose either epoxy grouted shear keys (Fig. 2. (b)) can be used or the newreinforcement can be welded with the reinforcement of the adjacent columns and slab(Fig. 2. (c)). Other precautions necessary for a proper bond between old and new concreteshould also be taken.

REMOVAL OF CONFIGURATIONAL IRREGULARITY

Experience during past earthquakes has shown that the configuration of building is very impor-tant for its performance during earthquake. Buildings with irregular plans and elevations suffermuch more damage compared to buildings with regular shapes. The types of common irregu-

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Fig. 2 (a) Adding a new shear wall at ground Floor

larities are (i) irregular geometry giving rise to torsion in building (ii) a weak or soft storey (iii)concentration of mass on a floor, and (iv) discontinuity in the lateral force resisting system.

Fig. 2 (a) Adding a new shear wall at ground Floor

Fig. 2 (b) Connection of shear wall panel with existing building components using epoxy grouted shear keys

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Fig. 2 (c) Connection of shear wall panel with existing building components by welding ofreinforcement

Soft/Weak Storey

If a storey in a building has lateral stiffness less than 70% of the stiffness of the adjacent storeyabove or less than 80% of the average stiffness of the three storeys above, it is termed as a softstorey. Similarly if a storey in a building has lateral strength less than 70% of the strength of theadjacent storey above or less than 80% of the average strength of the three storeys above, it istermed as weak storey. In buildings having soft/weak storey, most of the ductility demand isconcentrated in the soft/weak storey, resulting in excessive lateral displacement of the storeyleading to failure of the building due to formation of unstable mechanism.

In Indian cities there is a lack of parking space. In multistory buildings, the ground storey isusually kept open (free of masonry in-fills) while the upper storeys have masonry in-fills forpartitions. It has been seen that such a configuration results in the stiffness of the ground storeyabout one-third of the stiffness of the upper storeys. A classical failure of such a building duringBhuj earthquake of 2001 is shown in Fig. 3, where collapse of the building has taken place dueto failure of ground storey columns, while the upper portion of the building is almost intact.

IS: 1893-2002 has addressed this problem and suggests that either a non-linear analysis of suchbuildings should be performed or the ground storey beams and columns should be designed fora storey shear 2.5 times of that obtained from analysis of a bare frame without in-fills. In case ofexisting buildings the ground storey is to be stiffened so that the increased stiffness of theground storey is nearly equal to the stiffness of the upper storeys. Three options are availablefor this purpose:

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Fig. 3 Failure of a soft storey building

Jacketing of ground storey columns and beams

The ground storey columns and beams may be strengthened by providing RC Jacketing. Thereinforcement detailing at joints poses difficulty in this case. Moreover, it should be noted thatmasonry in-fills above the first floor beams restrain the beams from rotation and try to concen-trate the rotation in columns resulting in weak column and strong beam configuration. There-fore, the strengthening of columns is more beneficial than strengthening of beams.

Providing Shear Walls at ground storey

The stiffness of the ground storey canbe increased by providing shear wallsbetween some of the columns atground storey. The shear walls shouldbe located symmetrically (Fig. 4) andas far from the centre of the buildingas possible. Further, these should notcause hindrance to the normal usageof the ground storey as parking space.This option poses less difficulty in re-inforcement detailing than the jacket-ing. However, a perfect bond betweenthe shear wall and adjacent columnsand beam/floor is to be ensured. Thiscan be achieved either by providingepoxy grouted shear keys or by weld-

Fig. 4 Adding shear walls at ground storey

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ing the shear wall reinforcement to the longitudinal reinforcement of the columns and beams.Similar arrangement is to be provided at the foundation level. If the columns have isolatedfooting, additional foundation to the shear wall is to be provided and properly connected to thecolumn footings.

Providing Steel Braces at Ground Storey

Steel braces are an easy and quick alternative to shear walls (Fig. 5.5). The braces also need totransfer forces to beams, columns and foundation. This requires proper anchoring/bolting of thebraces with existing building.

The same constraints on location as for shear wallsalso apply for braces, however, braces result in lesserobstruction in ventilation.

Torsional Effects

Buildings with asymmetric plans are subjected totorsion during earthquakes, as the centre of massand centre of stiffness do not coincide in manyasymmetric buildings. Buildings with rectangularplans may also be subjected to torsion due to asym-metric location of masses, shear walls and staircases.

Torsion puts additional seismic demand4 on lateralload resisting vertical elements due to rotation offloor diaphragms. Even if this additional demand isconsidered in the design by considering additionalforces as per codal requirements, the building isnot expected to perform satisfactorily during earth quake. Therefore, it is advisable to balancethe building configuration rather than designing individual elements for additional forces dueto torsion.

Buildings with non-rectangular plans may be divided into rectangular parts by providing slits atproper locations (Fig. 6). For this purpose dummy columns are to be constructed at the locationof separation and the reinforcement of beams, cut to create slit, is to be properly welded andanchored into the columns. Another alternative to non-rectangular plan buildings is to makethem structurally symmetric by providing shear walls at suitable locations (Fig. 7). In this casea proper analysis of the building is to be performed to assess its behaviour to maintain symme-try even in non-linear range.

In hilly areas, due to sloping ground, the length of ground storey columns varies. This results invariation of their stiffness and shifting of centre of stiffness away form centre of mass. Thisresults in torsion in the building. To avoid this, shear-walls can be provided on ground storey onthe downhill side as shown in Fig. 8. The connection details of the shear wall with the existingbuilding should be followed as given in Fig. 2.

Fig. 5 Adding steel braces atground storey

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Fig. 6 Reducing torsional effects byseparation (Plan)

Fig. 7 Reducing torsion by providing shearwalls at ends (Plan)

Fig. 8 Reducing torsion at hill slopes

Mass Irregularity

If a storey has mass more than 150% of the mass of the adjacent storeys, it is considered to bea mass irregularity. Such mass irregularity affects the dynamic response of the structure andresults in large contribution from higher modes of vibration. Dynamic analysis is required tocalculate the forces accurately and the only remedy is to reduce the mass.

In India, there is a common practice to house large water storage tanks at the top of the building.In no case the weight of the water tank should be more than 50% of weight of a typical storey(consisting of self-weight of floor slab, beams, columns and partitions of one storey plus theweight of the permanent equipment, if any. Live load should not be considered for thiscomparison).

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Vertical Element Irregularity

Figure 9 shows out of plane irregularity in lateral load resisting vertical elements. The shearwalls above and below first floor are in different planes. This will cause very high seismicdemand in the columns below the shear wall resulting from overturning of the shear wall, and inthe first floor diaphragm due to transfer of cumulative lateral shear from the exterior shear wallto interior shear wall through the floor diaphragm.

Figure 10 shows in plane irregularity in vertical element, where the shear walls above andbelow the first floor are in the same plane but displaced in-plane. This also results in very highseismic demand on columns and floor slab of first storey, as explained above.

Fig. 9 Out-of-plane shear wall irregularity

Fig. 10 In-plane shear wall irregularity

The remedy to this irregularity is also completion of load path as explained in Section 5.2

Strengthening of Structure

Design practices, which have prevailed in India for decades, and are still prevailing in privatesector, either do not consider earthquake forces at all, or underestimate these grossly. Thisresults in quite inadequate lateral load resistance of the building structural system. The problemis further aggravated by inadequate detailing, which reduces the ductility and therefore, in-creases the effective force on the structure. Therefore, the lateral load resisting systems need tobe evaluated and strengthened in most of the cases.

Strengthening of a building structure can be done in several ways. While selecting a suitablescheme for strengthening, both economy as well as functional requirements are to be consid-ered. While considering the cost of retrofit, all the costs including the cost of disruption ofnormal usage should be considered to evaluate different alternative retrofit schemes.

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Addition of New Members

Addition of new members is per-haps the easiest option tostrengthen an existing building.Addition of new members ispossible externally, without dis-turbing the space inside thebuilding. The main concerns inaddition of new members arethe connection of new and oldmembers and foundation of thenew members. It is possible toprovide only a few stiff mem-bers to take most of the earth-quake force of the existingstructure, but the connectionsshould be capable of transferringthis load. Similarly, the foundations should be capable of transferring this load to the ground.

Several options, in the form of frames, shear walls and vertical trusses, are possible for strength-ening an existing building. Addition of new members changes dynamic characteristics of thebuilding. Sometimes, new members are also added to reduce eccentricity. Therefore, re-analy-sis of structure is required after addition of members.

Shear walls

Shear walls can be provided internally or externally in existing buildings. Sometimes, if spaceis available and there is a requirement, the existing building can be extended by a bay on sides(Fig. 11). The extended portion can have shear walls and it will support the existing building,laterally during an earthquake.

Addition of shear walls is the most commonly used method of strengthening of existing build-ings. This is also generally the most economical solution and can be provided in almost all typesof buildings. However, there are some points, which need careful consideration. A tall shearwall results in considerable overturning moment at the foundation. This overturning moment isto be balanced by the counterweight of the existing building. This requires a properly designedconnection between the foundation of the added shear wall and the foundation of existing build-ing. Addition of shear walls also results in high shear force in floor diaphragms. The existingdiaphragm strength needs to be evaluated and strengthening of diaphragm is to be done, ifrequired. Another disadvantage of shear walls is that the stiffness of the building is also in-creased significantly and this may reduce the time period of vibration and may result in in-creased seismic demand. Shear walls also result in significant impact on architectural charac-teristics of building, resulting in loss of windows and loss of large barrier free floor areas.

Fig. 11 Strengthening a building by an additional bav

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Braced Frames

Similar to shear walls, vertical trusses or braced frames can be provided externally (Fig. 12) orinternally (Fig. 13) in a building. Braced frames provide lesser strength and stiffness as com-pared to shear walls, but they also add less mass to building and hence result in lesser increasein the seismic demand. These also result in lesser disruption of the normal usage of building andlesser loss of window openings.

Fig. 12 Strengthening a RC building by external steel bracing

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Connection of braced steel frames with existing RC building is difficult and needs attention as alarge force is to be transferred between braced frames and the building. Long steel elements areused to connect the braced frame with building, providing larger number of bolts/anchors totransfer the load. These elements are termed as "drag elements" or "collector elements". Thesecollectors and their connection with building need to be properly designed to avoid "unzipping"type of failure in which the connectors along the length of the drag element fail in sequence.

Fig. 13 Strengthening by internal bracing

Buttresses

In some cases, it is important for the building to remain fully operational during retrofitting. Insuch cases no intervention inside the building is possible. If space is available outside the

building, shear walls or braced frames may beprovided perpendicular to external walls(Fig. 14). These are termed as "buttresses".

The problem with buttresses is that as theseare provided outside the building, thebuilding counter weight is not available tobalance the overturning moment at the base ofbuttresses. Separate foundation for buttressesis required. The buttresses have a significanteffect on the aesthetics of the building and areseldom suggested for historically/aestheticallyimportant buildings.

Moment Resisting Frames

Building strength can also be increased by providing moment resisting RC or steel frames. Theframes have the advantage that they add little stiffness and mass to the building. But, theserequire large deformations of the building before being effective.

For example, if a steel or RC frame is added to an existing building consisting of masonry in-filled RC frames, the stiffness of the existing building is much higher compared to the addedbare frame. It will require significant cracking of masonry in-fills to mobilize the strength of theadded frame.

Fig. 14 Strengthening by RC buttresses

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Strengthening of Existing Members

In most of the cases, strengthening of at least a few of existing members will be required inseismic retrofitting of a building. A number of techniques based on steel/FRP plate bonding,RCC jacketing and FRP jacketing are available for strengthening of individual members. Thechoice of the technique depends on the specific weakness and demand on the member.

Strengthening of individual members require good knowledge of the different materialsavailable in the market for repair and retrofitting. The load transfer mechanism between the oldand new material is complex and proper bonding between the two is difficult to be ensured.Following points are to be considered in strengthening of individual members:

- A variety of materials, discussed in 3rd chapter is available for strengthening of existingmembers. A detailed study of manufacturer claimed properties of these materials is re-quired before selecting a suitable material. Short-term as well as long-term properties are tobe considered.

- The load transfer between old and new material can take place through several mecha-nisms, such as, compression against pre-cracked interfaces, adhesion between non-metallicmaterials, friction between non-metallic materials, load transfer through resin/glue layers,clamping effect of steel, dowel effect of steel, etc. Modelling of this interaction is complexand not well understood, yet. It should be ensured that more than one mechanism of loadtransfer between new and old material are present.

- Anchorage lengths of reinforcement in new concrete should be as per codal specifications.However, in case of anchorage into old concrete, smaller anchorage lengths may besufficient if special grouts are used to anchor the bars in drilled holes. This anchoragelength should be in accordance with the manufacturer's specifications and should beverified by Pull-out tests.

- Anchoring of additional bars can also be accomplished by welding them with existing bars.For this purpose, spacers can be provided between old and new bars to provide a gap forintrusion of concrete. The weld is to be designed to develop full strength in the new bar.

Strengthening of Slabs

Usually in RC buildings, the slabs are strong enough to transfer load between different lateralload resisting vertical elements. However, if the building has irregularity in the form of openingin slab, the slab diaphragm may not be adequate to transfer forces between different elementsand strengthening is needed. In case of additional shear walls also the adequacy of the dia-phragm needs to be evaluated and strengthening is to be done, if required.

RC slabs can be strengthened either by over-laying or by under-laying. In over-laying (Fig.15),thickness of slab is increased by cast in place concrete on the upper side. In under-laying (Fig.16), additional reinforcement is placed below the slab and thickness is increased using shotcrete.

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Fig. 15 Strengthening a slab byoverlaying

Fig. 16 Strengthening a slab byunderlaying

Here, it is important to emphasize the need of ensuring bond between old and new concrete.Four alternatives are available for this purpose. The bond between old and new concrete can beenhanced by a layer of epoxy and coarse sand (Fig. 17 (a)). The bond can also be enhanced byproviding mechanical anchors in the form of bolts (Fig. 17 (b)), dowels (Fig. 17 (c)) and anglesections (Fig. 17 (d)). In addition to this some shear keys as shown in Fig. 5.15 are also need tobe provided. In case of hollow slabs, the voids can also be used as shear keys (Fig. 18).

Fig.17 Enhancing bond between old and new concrete

Fig. 18 Shear keys in a hollow slab

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In addition to these techniques, theslabs can also be strengthened byglueing steel or FRP plates (Fig. 19)on the bottom of the slab. This tech-nique is effective in enhancing theflexural strength of slab and not effec-tive in shear strength enhancement.

Strengthening of Beams

Beams can be strengthened either byRC jacketing or by gluing steel/FRPplates. In RC jackets, reinforcementand concrete can be added either onthree sides (Fig. 20) or on all the foursides (Fig. 21). In case of jacketingon three sides, the stirrups are either to be grout-anchored into slab (Fig. 20) or nailed into beamweb using a strand (Fig. 22).

Fig. 19 FRP strengthening of slab

Fig. 20 Beam jacketing on three sides Fig. 21 Beam jacketing on four sides

Figure 23 shows the details of RC jacketing on four sides of a beam. The longitudinal reinforce-ment of the jacket is to be welded with the existing longitudinal reinforcement through z shapedlinks. Alternatively, the longitudinal reinforcement is to be anchored through a collar as shownin Fig. 24.

Fig. 22 Stirrups anchored through strand Fig. 23 Jacketing on four sides of beamsand columns

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The stirrups in the jacket are to be placed in two pieces as shown in Fig. 21. For placing thestirrups, closely spaced holes are to be made through slab. These holes may also be used forpouring concrete from the top. In case of jacketing on three sides, pouring of the concrete fromtop is not possible and shotcrete is to be used.

Fig. 25 Strengthening of beams byunderlaying

Fig. 26 Strengthening of beams byplate bonding at bottom

Beams can also be strengthened by providing RC underlays on the lower face of the beam (Fig.25). However, these overlays can increase only flexural capacity of the beams. Jackets on threesides can increase the flexural and shear capacity of beams under vertical loads only. These arenot much effective under lateral loads, as the strengthening near joints is not effective. Jacket-ing on four sides of all the beams are columns meeting at a joints is the most effective solution,as it provides scope for strengthening of joint also.

Another alternative to strengthen beams is by bonding of steel plates or FRP sheets. Steel platesor FRP sheets can be glued at bottom (Fig. 26), on three sides (Fig. 27) or on four sides (Fig. 28)of a beam. For effective action under earthquake loading, the joint is also to be strengthened.

Fig. 24 Anchoring of longitudinal reinforcement using collar

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Strengthening of Columns

Similar to beams, in case of columns also, strengthening is possible by RC jacketing or by en-casement using steel plates or FRP sheets. RC jackets are most effective if applied on all thefour sides, but sometimes these may also applied on only one or more sides of column (Fig. 29).Two points are to be kept in mind in jacketing of columns (i) bond between the old and newmaterial and transfer of forces to new reinforcement, and (ii) confining of concrete throughproper placing and anchoring of transverse reinforcement.

Fig. 28 Strengthening of beams by plate bonding on four sides

Fig. 27 Strengthening of beams by plate bonding on three sides

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Fig. 29 (a) Jacketing on one side of column (Alternative-1)

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Fig. 29 (b) Jacketing on one side of column (Alternative-2)

Fig. 30 Jacketing on four sides of column

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Figure 30 shows two alternative arrangements of reinforcement for jacketing of columns on foursides. These jackets are very effective in increasing axial and shear strength. If the jackets arelimited to the storey height, these are not much effective against bending moment. Jacketsshould protrude through the slab (Fig. 31) to be effective in flexure.

Steel jackets have a problem that these tend to separate out due to Poisson's effect during load-ing. FRP encasement can be applied (i) by wrapping the columns using FRP straps (32(a)), (ii)by complete encasement by FRP sheets (32(b)), or (iii) by partial wrapping by FRP straps/sheets (32(c)). Wrapping by FRP straps provides the possibility of prestressing the strap andhence is more effective.

Fig. 31 Reinforcement detailing for column jacketing

(a) (b) (c)Fig. 32 FRP Strengthening of columns

Steel/FRP jackets are more effective if provided in elliptical shape as compared to rectangularshape. The column shape may be modified to elliptical shape (Fig. 33) for this purpose.

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Strengthening of Walls

Existing masonry and RC walls can also be strengthened by providing RC jackets on one orboth sides of the walls. It is customary to have half brick thick partitions in the interior ofbuildings. These partitions are unsafe under out of plane action during earthquake. Out of planestrengthening of partitions can be clubbed together with lateral strengthening of building byproviding RC jackets to the partitions (Fig. 34).

Fig. 33 Shape modification of columns for effective jacketing

Fig. 34 Strengthening of masonry infills

Fig. 35 Connection of wall jacket with floors and columns

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Some basic rules have been suggested5 for strengthening of walls by RC jacketing:

- The strength of new concrete must be at least 5 MPa greater than that of existing concrete- The minimum thickness of jacket should be 50 mm on each side.- The minimum horizontal and vertical reinforcement should be 0.25% of the jacket section.- The minimum reinforcement with which the ends of the wall are strengthened should be

0.25% of jacket section.- The diameter of the ties at the well ends should not be less than 8 mm with a maximum

spacing of 150 mm.- The jacket must be anchored to the old concrete with dowels spaced at no more than 600

mm in both directions.- It is also important that the jacket should be able to transfer forces to slab diaphragms. This

can be achieved by providing epoxy grouted anchors and diagonal connecting bars throughholes made in slabs, as shown in Figure. 35.

Fig. 36 Strengthening of joint by collar prestressing

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Strengthening of Joints

Strengthening of beam-column joints in RC building is perhaps the most difficult task in retrofit-ting of existing buildings. The joints are expected to behave rigidly during earthquake and theirfailure is to be avoided. In a planar joint (where two beams are meeting in plane) X-shapedcollars may be provided (Fig. 36). These collars have arrangement for prestressing. After pro-viding collars the joint is covered by welded wire mesh and gunnite.

Glued steel plates (Fig. 37) or FRP sheets can also be used to strengthen a joint. This methoddoes not alter the dimensions of the joint. There is a problem with steel plates that due toPoisson's effect, the jacket tends to separate from the concrete and confinement is not effective.To avoid this, crimpled steel jackets are suggested. These jackets develop smaller longitudinalstrains and hence smaller transverse strain and result in more effective confinement.

RC jacketing is a very effective method of joint strengthening. However, placement of newreinforcement with proper confinement at joints is quite difficult. Several holes are to be punchedthrough existing columns and beams for placing confining reinforcement. Pouring of concreteand getting a good bond between old and new concrete is also quite difficult.

Fig. 37 Strengthening of joint by plate/FRP bonding

Strengthening of Foundation

Strengthening of foundations has two aspects: (i) increasing the bearing area with or withoutstrengthening of column, and (ii) anchoring of column jacket reinforcement into foundationwith or without strengthening of footing. The column moments are maximum at base and thisrequires proper anchorage of jacket reinforcement in to the footing. This can be accomplishedby drilling holes into existing concrete of footing and epoxy grouting (Fig. 38) the longitudinalreinforcement of jacket. Another possibility is to provide full anchorage length for longitudinalreinforcement by extending the column jacket at the top of footing as shown in Fig. 39.

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Fig. 38 Anchoring of column reinforcement without foundation strengthening

Fig. 39 Anchoring of column reinforcement with foundation strengthening

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If the bearing area under a footing is not sufficient, it is to be increased by increasing the size ofthe footing. If the column is also being jacketed, it is easier to transfer the forces from theextended footing area to column jacket as shown in Fig. 40. As can be seen from the force flowdiagram in the figure., there is a component of force, which tends to split the new concrete fromold concrete. To avoid this splitting, sufficient number of closed rings with sufficient overlap orwelded connection are to be provided around the footing.

If the bearing area is to be increased without strengthening of the column, soil pressure on theextended area is to be transferred to the existing footing as shown in Fig. 41. This is difficult asexcavation is required below the existing footing. The building is to be properly supported andsettlement is to be avoided. As can be seen from the force flow diagram, in this case also, thereis a tendency of the new concrete to split from the old concrete. To avoid this, as in previouscase, sufficient numbers of well anchored/welded hoops are required.

Fig. 40 Increasing foundation area with column jacketing

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Fig. 41 Increasing foundation area without column jacketing

ENHANCING DEFORMATION CAPACITY

Post yielding deformation capacity of a building plays a very important role in reducing theeffective seismic force on the building. The members of a building are expected not to lose theirvertical load carrying capacity, while undergoing large plastic deformations in lateral direction.Sometimes, a few poorly designed members can limit the capacity of the whole building todeform laterally. These members may be modified to increase their deformation capacity andthis will result in large reduction in effective seismic force on the building. If the number of themembers to be modified is small, this strategy does not disrupt the functioning of the building.But, if a large number of members are to be modified, this becomes costly and disruptive.

Detailing Enhancement

Most of the buildings in our country do not have ductile detailing as per IS: 13920. Normally,the buildings suffer from two major deficiencies: (i) splicing of longitudinal reinforcement incolumns is inadequate and it is done very near to joints, and (ii) confining reinforcement inbeams and columns near the joints is inadequate. The longitudinal reinforcement is to be welded

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at the splice (Fig. 42) to develop full bar strength and additional confining reinforcement is to beadded in beams and columns near the joints (Fig. 43).

For providing confining reinforcement, cover is to be chipped off and holes are to be madethrough slab. Further, the stirrups are to be welded to develop full bar strength. These stirrupswill be effective if these are confining the concrete. To achieve confinement, the space betweenstirrups and existing concrete may be filled with expanding grout/concrete.

The ductility of joints can also be enhanced by confinement jacketing using steel/FRP, as dis-cussed earlier.

Fig. 42 Welding of splice

Fig. 43 Adding confining reinforcement near joints

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Avoiding Storey Mechanism of Failure

One of the fundamentals of earthquake resistant design is to avoid storey mechanism of buildingfailure by providing strong columns and weak beams. But in our country, it is not uncommon toencounter buildings with beams stronger than columns. In such cases, the plastic hinges from incolumns and building fail by formation of local storey mechanism. The columns, in such buildings,are to be strengthened to result in strong column-weak beam configuration. The methods ofstrengthening of columns have been discussed earlier. However, weak beam and strong columnconfiguration is difficult to achieve as the joints are also to be strengthened and it is to be ensuredthat beam behaves in a ductile manner under reversed lateral loading. The reinforcement detail-ing in beams usually does not permit reversing flexural yielding.

Reduction in Local Stiffness

Sometimes, the building may have some secondary structural/non-structural members, whichmay result in damage in the main structural members and hamper their functioning under lateralload. For example, some buildings may have deep spandrels at lintel level or partially in-filledframes. This may result in short column effect. To avoid this, a gap between these elements andthe main structural elements can be provided (Fig. 44). This allows larger deformation of thebuilding under lateral load without undesirable damage mechanism.

Similarly, some building may have very stiff non-structural walls or other architectural ele-ments. These elements may trigger undesirable failure mechanisms in the building. These ele-ments can be locally demolished or modified to avoid undesirable failure mechanism.

Fig. 44 Reducing local stiffness by providing a slit between structuraland non-structural members

Supplemental Supports

In the old design philosophy, some of the building elements are designed only for gravity loading.But due to compatibility, these members also deform laterally with the rest of the building, duringearthquake. As these are not designed for large lateral deformations, these may loose theircapacity to carry vertical loads and can jeopardize the building safety. Such locations in thebuilding need to be provided with supplemental support to transfer vertical load.

In case of flat slabs, lateral deformations may result in punching shear failure of slabs. Tosupport the slab after punching shear failure, supplemental supports as shown in Fig. 45 can beprovided.

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Fig. 45 Providing supplemental supports

EARTHQUAKE DEMAND REDUCTION

In the previous sections, different techniques for enhancing the capacity of the building to with-stand earthquake forces have been discussed. An alternative approach is to reduce the earth-quake demand (forces and displacements). This can be achieved either by reducing the mass ofthe building or using base-isolation/energy dissipation devices13. Reduction of building massis not always possible and it is mainly the use of base-isolation/supplemental energy dissipationdevices, which is employed to reduce the earthquake demand on the buildings.

Use of base-isolation/supplemental energy dissipation devices is costly and it is recommendedonly for those building which are required to have operational performance level after an earth-quake or which house sensitive equipment. Base-isolation has been found to be particularlyuseful for historic buildings, where it is not possible to modify the structure significantly. How-ever, it is important to note that base-isolation and supplemental energy damping cannot beused in all buildings. In many cases, the structure is also to be strengthened in addition to base-isolation/energy dissipation.

Seismic Base-Isolation

Base-isolation is based on the principle of elongating the time period of the building byproviding compliant bearings at the base of the building (Fig. 46). The bearings have sufficientstiffness and strength against vertical load, but relatively low stiffness and large deformationcapacity in lateral direction. Sometimes, these bearings are also provided with enhanced energydissipation characteristics or with additional dampers.

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Fig. 46 Schematic representation of a base isolated building

The base-isolation results in significant increase in fundamental time period of the structure anddamping. Further, as the stiffness of the bearings is much smaller compared to structure, thelateral deformation gets concentrated into bearings, resulting in greatly reduced earthquakedeformation demand in the portion of the structure above bearings.

Base-isolation is considered to be useful for buildings having a fundamental time period of onesecond or less, as it requires a relatively stiff building to have concentration of lateral deforma-tion in bearings only. Further, the building should remain elastic under the residual demandtransmitted to the structure by the isolators. In order to achieve this, in many cases, the buildingstructure is also required to be strengthened in addition to base-isolation.

Base-isolation is considered to be very effective for historical buildings, believing that no inter-vention/modification is required in the building, preserving its historical character. But, as de-scribed above, this belief may not be always true and significant strengthening of the structuremay be required.

Base-isolation provides an effective solution for retrofitting of buildings having enhanced per-formance objectives. Base-isolation results in significant reduction of displacement and forceresponse of the building. This is a preferable condition for better performance of sensitiveequipment, systems and other non-structural components.

Supplemental Energy Dissipation

Supplemental Energy Dissipation Systems dissipate the energy transmitted to the structure bythe earthquake, in addition to the energy dissipated by the structure in normal course. Thisresults in significant reduction in the displacement and acceleration response of structure. Forthis purpose, energy dissipation units (EDUs) are installed in the lateral load resisting system ofthe building (Fig. 47). These EDUs work either on viscous or on hysteretic damping.

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Fig. 47 EDU's in a building frame: (a) along the diagonal, (b) mounted on a platform

EDUs can be mounted either along the diagonals (Fig. 5.547(a)) of the frames or on a rigidplatform (Fig. 5.47(b)). Contrary to base-isolation, the energy dissipation system is moreeffective in flexible buildings with large lateral deformations, as the energy dissipated by EDUsis directly proportional to the force developed by EDUs and displacement across these EDUs.For a rigid building, the small lateral displacement during earthquake will results in smallerenergy dissipation and the reduction in effective earthquake forces will not be significant.

Similar to base-isolation, supplemental energy dissipation system is also a costly method and issuitable for buildings with high post-earthquake importance. The energy dissipation results inreduced seismic response of building and better performance of equipment, systems andnon-structural components.

a

b

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REFERENCES

ATC 40, 1996, Seismic Evaluation and Retrofit of Concrete Buildings, AppliedTechnology Council, California.FEMA 273, 1997, NEHRP Guidelines for the Seismic Rehabilitation of Buildings, FederalEmergency Management Agency, Building Seismic Safety Council, Washington, D.C.Hamberger, Ronald O., and Craig A. Cole, 2001, "Seismic Upgrading of ExistingStructures," The Seismic Design Handbook, Farzad Naeim, ed., Kluwer AcademicPublishers.

IS 1893-2002, Criteria for Earthquake Resistant Design of Structures, Part 1 GeneralProvisions and Buildings, Bureau of Indian Standards, New Delhi.

Penelis, George G., and Kappos, Andreas J., 1997, Earthquake Resistant ConcreteStructures, E & FN Spon.

Hollaway, L.C., and Leeming, M.B., ed., 1999, Strengthening of Reinforced Concrete Struc-tures - using Externally Bonded FRP Composites in Structural and CivilEngineering, CRC Press.Teng, J.G, Chen, J.F., Smith, S.T., and Lam, L., 2002, FRP Strengthened RC Structures, JohnWley & Sons, Ltd.IS 456-2000, Plain and Reinforce Concrete - Code of Practice, Bureau of IndianStandards, New Delhi.Bungey, J. H., 1989, The Testing of Concrete in Structures, Surrey University Press.

FEMA 172, 1992, NEHRP Handbook of Techniques for the Seismic Rehabilitation of ExistingBuildings, Federal Emergency Management Agency, Building Seismic Safety Council, Wash-ington, D.C.FEMA 308, 1999, Repair of Earthquake Damaged Concrete and Masonry Wall Buildings, Fed-eral Emergency Management Agency, Building Seismic Safety Council, Washington, D.C.IS 13920-1993, Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces- Code of Practice, Bureau of Indian Standards, New Delhi.Key, David, 1988, Earthquake Design Practice for Buildings, Thomas Telford, London.

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RETROFITTING MATERIALS

Yogendra Singh & D. K. PaulAsstt. Professor, Department of Earthquake Engineering, IIT Roorkee, 247 667

GENERAL

Considerable research has taken place in the field of repair and retrofitting materials and a largevariety suitable to different applications and working conditions is available. Most of the mate-rials are patented and available in brand names. The retrofit engineer needs to have informationabout these materials for designing the retrofit scheme.

The repair and retrofit materials can be broadly classified into three categories:

i. Grouts for repair of cracks, strengthening of masonry and honeycombed concrete.ii. Bonding agents for enhanced bonding between old and new concrete, and concrete and

reinforcement.iii. Replacement and jacketing materials for replacing the damaged portions, increasing the

size of members, enhancing the confinement and external reinforcement of the members.iv. A brief description of different materials available under these categories is given below.

INJECTION GROUTS

Grout is a flowable plastic material, which can be injected into a structural member underpressure. The grout should have negligible shrinkage to fill the gap/void completely and itshould remain stable without cracking, delamination or crumbling.

Injection grouts are used to fill interior space within the concrete or masonry created due tocracks, voids or honeycombs. In case of damaged concrete or masonry if the cracks are thin,these can be repaired by injection grouting, otherwise, if the cracks are wide, the material aroundthe cracks is to be removed and replaced by new material. Injection grouts can also be used forstrengthening of old masonry structures, in which mortar has degraded and in honeycombedconcrete. These are particularly useful in strengthening of monumental structures, but compat-ibility of original material and the grout must be ensured.

CEMENT-SAND GROUTS

Cement Sand grouts are the cheapest. For injection purpose, the grout requires high water andcement contents. This result in shrinkage and cracking of grout at hardening. Suitable shrinkagecompensating agents1 are required to minimize this. Fig. 1 shows the apparatus for cement-sand grouting. Use of cement-sand grouts is very common in masonry buildings, but not verycommon in concrete.

Chapter 17

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GAS-FORMING GROUTS

These grouts contain some ingredients (usually Aluminum and Carbon powder), which reactwith the cement liquor to generate gas bubbles. The gas expands the grout to compensate shrink-age. These grouts require proper confinement to develop strength and volume stability. Theseare temperature sensitive and not suitable for high temperature application, as the reactionforming gas bubbles may be too fast and may complete before placing of the grout.

SULFOALUMINATE GOUTS

In these grouts either shrinkage-compensating cement or anhydrous sulfoaluminate expansiveadditive is used with Portland cement. The dosages of additive are recommended at 6% to 10%by weight of cement. The additive results in expansion at hydration. This produces expansionafter the grout has set and is more reliable then gas-forming grouts. But the expansion of suchgrouts requires post-hardening curing and it will not be effective if moist curing is not available.

FIBRE-REINFORCED GROUTS

Polypropylene, steel or Glass fibres may be used in Portland cement or shrinkage compensatingmortar to provide improved flexural strength, impact resistance and ductility. The typical dos-age recommended1 for the three types of fibres is 4-6%, 2-3% and 2-4% by weight of cementfor propylene, steel glass fibres respectively. These grouts require skilled handling to avoidsegregation of fibres.

POLYMER GROUTS

The Polymer resin grouts are most commonly used in concrete. The commonly used polymersare polyester, epoxy, vinyl ester, polyurethane and acrylic. Out of these, epoxy is most popular.In case of underground and water seepage conditions, polyurethane and acrylic resins are used.These grouts come in three-component materials having (i) liquid resin content (ii) curing agentor hardener, and (iii) aggregate or dry filler; and two component materials having curing agentpackaged with the aggregate. The basic resins and curing agents are added with modifiers toachieve desired properties. Such modifier materials are unique specialty products, whose com-positions are trade secrets. These are mostly sold as pre-packaged units to avoid errors in pre-paring. The manufacturer's literature should be studied in details before specific use of suchmaterials.

Polymer grouts can be injected by pre-mixing the resins and hardener and injecting the mixturethrough a pressure gun fitted with a nozzle (Fig. 2). The automatic injection machine (Fig 3) hasa controlled supply of the resin and the hardener through two separate pipes. The two compo-nents are mixed prior to injection in a chamber just before the nozzle. Injection can be done byhand operated gun at low pressure (up to 1 MPa), or by pump operated gun at a high pressure(up to 20 MPa). If the crack width is small (0.1-0.5 mm), resin with hardener is used. If thecracks are wider then filler is also used.

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Fig. 2 Hand epoxy grouting Machine

INJECTION PROCEDURE

Before injecting grouts into crack, preparation of the crack is to be done as following:

(i) Cleaning of crack with compressed air and removal of loose material, if any.(ii) Drilling of holes (5 to 10 mm) at several places along the length of the crack.(iii) Placing of 'Ports' or 'Nipples' at the mouth of holes (Fig 4). If the cracks are wide and

accessible from surface 'T' ports (Fig .5) can be installed.

After injecting resins through the ports, the cracks and ports are sealed by quick hardening resinpaste.

On vertical surfaces, the injection is started from the lowest port till it comes out from the uppernipple. Then the port is sealed and injection is started from the upper port. After hardening ofthe epoxy in a day or so, the sealing resin paste is removed. The effectiveness of injectiongrouting in concrete can be tested either by USPV test or by visual inspection of cores drilledthrough the injected crack.

Nipple

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Fig. 3 Automatic epoxy grouting machine

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Fig. 4 Fixing of pipe ports for grouting

Fig. 5 Fixing of T-ports

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BONDING AGENTS

Bond between existing concrete, new concrete and reinforcement is very important foreffectiveness of repair/retrofitting. There are three methods available for enhancing the bond:

(i) Application of adhesives at the interface(ii) Surface interlocking(iii) Mechanical bonding

Polymers and epoxy are the adhesives used for bonding between old and new concrete andreinforcement. After removal of the concrete cover, the existing concrete surface and steel arecleaned by sand or water blasting. After cleaning and drying, concrete and steel is painted byepoxy/polymer or polymer modified cement grout. If the new steel is to be welded, it is weldedprior to coating of the concrete and steel. This coating provides enhanced bond between the oldand new material and reduces the risk of corrosion in steel.

To improve the surface interlocking, the existing concrete surface is coated with epoxy/polymerand a layer of coarse sand is applied above the coating. Mechanical bonding consists of keysand anchors4 provided in the existing members at regular interval. The details of mechanicalbonding are provided in the next chapters on retrofitting.

REPLACEMENT AND JACKETING MATERIALS

In case of damaged structures, material in some parts of members is to be replaced by newmaterial. For strengthening existing members in deficient buildings, additional material includ-ing reinforcement is to be provided. The material used for replacement should have good bondwith existing material and it should be non-shrinking. A variety of strengthening and replace-ment materials is available.

ORDINARY PORTLAND CEMENT CONCRETE AND MORTAR

The advantage of using ordinary concrete and mortar is that these have similar thermal move-ment and appearance as the existing concrete. Further, these are cheap and do not require spe-cial skills for application. Generally, these consist of high early strength cement and an expan-sive component to compensate the shrinkage. The expansive component also results in goodbond. The common expansive agents used are aluminum powder, coke powder, anhydrous cal-cium sulfoaluminate and calcium oxide.

In case of concrete, use of higher strength (at least by 5 MPa) then the existing concrete isrecommended. Maximum size of coarse aggregate is limited to 20 mm for ease in pouring theconcrete through narrow spaces. To ease the compaction, workability is enhanced by usingsuper plastisizers. The surface of existing concrete is made as rough as possible and cleanedproperly. After placing the forms a final dusting should be done using compressed air to removedust from the surface.

Sometimes a special application of ordinary concrete-'preplaced concrete' is also used. In thismethod, the aggregate is first packed in the space to be concreted and the cement is applied inthe form of grout intrusion. This concrete has very little shrinkage but requires skill in applica-tion.

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Dry pack is another application method of ordinary concrete. In this method the concrete hasvery little water and has almost zero slump. The moisture is just sufficient to stick the materialtogether when molded into a ball by hand. The low water content results in reduced shrinkage,but makes compaction difficult and there are chances of voids being left.

Dry packs are available under several commercial names and usually consist of fine sand,superplasticizers and an expansive agent in appropriate proportion. This mixed with water at-tains very high strength in very short time. (e.g. 30 MPa in 24 hours and 70 MPa in 28 days).This high strength is a result of formation of a special silica calcium hydrate from the reactionof the cement with expansive agent. The expansive agent also results in no-shrinkage of thematerial. This material is very suitable for jacketing.

SHOTCRETE

Shotcrete or guniting has the same characteristics as ordinary concrete but it has smaller aggre-gate size and it is applied under pressure with low water content. It requires no framework andcan be applied on any surface including inclined and vertical surfaces and even on ceilings.This results in very good adhesion between old and new concrete and good compaction due toapplication under pressure. The low water-cement ratio results in high strength and low shrink-age. The permeability of shotcrete is also lower than that of ordinary concrete and results inbetter protection of steel against corrosion.

Shotcrete requires special equipment. Two types of equipment are used depending on dry orwet mix type of application. In dry mix application, the proportioned or pre-packaged cement-aggregate mixture is transferred to nozzle using highly compressed air. Water is introduced atnozzle under pressure (Fig 6). The mixture is impacted on the surface to be shotcreted. In wetmix type shotcrete, proportioned mixture of cement, aggregate, water and admixtures is dis-charged into a conventional concrete pump to a discharge nozzle. Compressed air is used toproject the material from nozzle.

Fig. 6 Apparatus for Shotcrete

272

Before application of shotcrete, damaged concrete is removed and the surface is thoroughlycleaned by sand blasting to remove all dirt and to expose the aggregate. Steel is cleaned on fullcircumference of bar to bare metal. Usually a welded wire mash is applied over the surface to beshotcreted and attached to the existing concrete through nailing. This wire mesh reduces theshrinkage and improves the bond between existing concrete and shotcrete. Sometimes, to im-prove the bond between old and new material, surface coatings, such as epoxy bonding agents,latex modified cement slurries or neat cement slurries, are also used.

In case of dry mix shotcrete, the water/cement ratio cannot be controlled quantitatively as it ismixed at nozzle and controlled visually by the operator. Therefore, the skill of the crew is veryimportant. The variation in density of shotcrete is more than that of normal concrete. Also, theshotcrete results in a rough surface.

POLYMER MODIFIED CONCRETE AND MORTAR

Polymers are long molecule hydrocarbons, built by combination of single units called mono-mers. The process is called polymerization. Small diameter particles of polymers emulsified inwater are called polymer latexes. These latexes form continuous film at drying. Adding poly-mer latexes to ordinary mortar and concrete is the most common method of making PolymerModified Mortar (PMM) and Polymer Modified Concrete (PMC). Cement hydration in PMM/PMC results in drying of latex and formation of the film of polymers. This film binds thecement hydrates together to from a monolithic network in which the polymer phase interpen-etrates throughout the cement hydrate phase. The resulting matrix binds the aggregate morestrongly and enhances the properties of mortar/concrete.

The polymer can also be mixed in the form of re-dispersible powder in the dry cement-aggre-gate mix. When water is added to this mixture, a process similar to that described above takesplace. Some polymers are water soluble. When added to mortar/concrete, these result in en-hanced workability but no increase in strength. In some liquid thermosetting resins, polymer-ization is initiated by water. These are also added to concrete/mortar to result in enhancementsimilar to that resulting from latex.

The PMM/PMC has better workability and water retention properties than ordinary concrete/mortar. This reduces the requirement of water curing considerably. Polymer modification doesnot result in any appreciable increase in compressive strength of concrete, but it results signifi-cant increase in tensile and bending strength of concrete.

The main advantage of PMM/PMC is its improved adhesion and bond with existing concreteand significantly reduced permeability. Reduced permeability results in reduced risk of corro-sion of reinforcing steel.

STEEL PLATE BONDING

Steel plates can be bonded to concrete members as external reinforcement to increase theirstrength. The plates are glued to the member surface by epoxies. This requires a careful prepa-ration of the member surface and application of epoxy layer. Steel plates can also be provided inthe form of jackets either by gluing to surface or by grouting. However, these jackets are notvery effective as these try to separate out from the members due to Poisson's effect, loosingconfinement.

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FIBRE REINFORCED PLASTICS (FRP)

Fibre-reinforced polymers/plastics is a recently developed material for strengthening of RC andmasonry structure. This is an advanced material and most of the development in its

application in structural retrofitting has taken place in the last two decades. It has been found tobe a replacement of steel plate bonding and is very effective in strengthening of columns byexterior wrapping. The main advantage of FRP is its high strength to weight ratio and highcorrosion resistance. FRP plates can be 2 to 10 times stronger than steel plates, while theirweight is just 20% of that of steel. However, at present, their cost is high.

FRP composites are formed by embedding a continuous fibre matrix in a resin matrix. The resinmatrix binds the fibre together and also provides bond between concrete and FRP. The com-monly used fibres are Carbon fibres, Glass fibres and Aramid fibres, and the commonly usedresins are polyester, vinyl ester and epoxy. FRP is named after the fibre used, e.g. Carbon FibreReinforced Polymer (CFRP), Glass Fibre Reinforced Polymer (GFRP), and Aramid Fibre Rein-forced Polymer (AFRP).

Table 1 Typical Properties of GFRP, CFRP and AFRP

Unidirectional advanced composite

materials

Fibre content (% by weight)

Density (kg/m3)

E (Long.) (GPa)

Tensile strength (MPa)

Glass fibre/polyester GFRP laminate Carbon/epoxy CFRP laminate Aramid/epoxy AFRP laminate

50-80

65-75

60-70

1600-2000

1600-1900

1050-1250

20-55

120-250

40-125

400-1800

1200-2250

1000-1800

The fibres are available in two forms (i) Unidirectional tow sheet, and (ii) Woven fabric. Theapplication of resin can be in-situ or in the form of prefabrication of FRP plates and othershapes by pultrusion. The in-situ application is by wet lay-up of a woven fabric or tow plateimmersed in resin. This method is more versatile as it can be used on any shape. On the otherhand, prefabrication results in better quality control. The manufacturers supply these materialsas a package and each brand has specific method of application, which is to be followed care-fully. Specialized firms have developed in India also, which take up the complete executionwork and supply of material. It is important to note the difference between the properties ofsteel and FRP and it should be understood that FRP cannot be treated as reinforcement in con-ventional RC design methods. Table 1 gives a typical range of properties for three types offibres. This range may change from one brand to another and with change in fibre content.

Figure 7 shows the qualitative stress-strain curves for mild steel, CFRP, AFRP and GFRP. It canbe seen that not only there is drastic difference in tensile strength and modulus of elasticity,unlike to mild steel, FRP is elastic right up to failure. This shows total lack of ductility in caseof FRP. This brittleness of FRP must be considered while predicting the behavior of retrofittedmembers. This brittleness does not allow the redistribution of stress in RC members and there-fore, the conventional design theories are not valid for FRP reinforced concrete members.

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Fig. 7 Stress-strain behaviour of FRP

REFERENCES

Mailvagnam, Noel P., 1992, Repair and Protection of Concrete Structures, CRC Press.

FEMA 308, 1992, Repair of Earthquake Damaged Concrete and Masonry Wall Buildings, Fed-eral Emergency Management Agency, Building Seismic Safety Council, Washington, D.C.

ACI Committee 224R, 1994, "Control of Cracking in Concrete Structures," ACI Manual ofConcrete Practice, Detroit, Michigan.

Penelis, George G., and Kappos, Andreas J., 1997, Earthquake Resistant Concrete Structures, E& FN Spon.

Bungey, J. H., 1989, The Testing of Concrete in Structures, Surrey University Press.IS 456-2000, Plain and Reinforce Concrete - Code of Practice, Bureau of Indian Standards,New Delhi.ACI 506-90, 1994, "Guide to Shotcrete," ACI Manual of Concrete Practice, Detroit, Michigan.ACI 506.2-90, 1994, "Specifications for Shotcrete," ACI Manual of Concrete Practice, Detroit,Michigan.ACI Committee 503R, 1994, "Use of Epoxy Compounds with Concrete," ACI Manual of Con-crete Practice, Detroit, Michigan.Hollaway, L.C., and Leeming, M.B., ed., 1999, Strengthening of Reinforced Concrete Struc-tures - using Externally Bonded FRP Composites in Structural and Civil Engineering, CRCPress.Teng, J.G, Chen, J.F., Smith, S.T., and Lam, L., 2002, FRP Strengthened RC Structures, JohnWiley & Sons, Ltd.

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QUALITY CONTROL ON CONSTRUCTIONAnand S. Arya

Professor Emeritus, Deptt. of Earthquake Engg., IIT Roorkee, RoorkeeNational Seismic Advisor, GoI-UNDP (DRM), New Delhi

INTRODUCTION

Good quality of construction work including Repair. Retrofitting and Reconstruction of thebuildings in the affected villages is very important to meet the following objectives.

i ) To achieve the strength of the building under. Normal Dead and Live Loads. Foundation.Walls, column,. floor and roof are the main structural elements to he taken cute of.

ii) To achieve adequate strength of the building for earthquake effects. The various detailsincluding plinth band. lintel band .and vertical reinforcing are specified in IS :4326 & 13828of 1993 and illustrated in the ''Guidelines for Reconstruction and New Construction ofHouses in Kachahh earthquake affected Areas of Gujarat" published by Gujarat State Di-saster Management Authority, May 2001

iii) To achieve durability of the building over long time so as to meet the seismic requirementsas and when the next earthquake strikes.

Every effort should therefore be made by all concerned to achieve the specified standards. Itshould also be realized that the constructions are and will be keenly watched by the prospectivebeneficiaries. Hence quality must be ensured by all building agencies in architectural and struc-tural designs as well as construction.

QUALITY OF STONE MASONRY WALLS

The strength of coursed rubble' (RCR) masonry under vertical as well as horizontal earthquakeloads depends upon the integrity of the wall cross-section and the bond between perpendicularwalls. To achieve high strength of rubble masonry the following measures must be ensured

Interlocking of Stones and Breaking Vertical Joints

To achieve the integral behaviour of the stonewall, interlocking of the stones in the cross-section as well as in the length of' the wall is necessary and "stacking bond' should not be doneas shown in Fig. 1 in which vertical joints occur continuously. The proper interlocking of stonesincluding breaking of vertical joints is shown in Figs. 2 and 3.

Chapter 18

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Fig. 1 Stacked stone wallpoor construction

Fig. 2 Laterlocking stone wall with`through' and `corner' stones

(Very good construction)

Provision of 'through' stones

Through' stones must he provided in the wall at horizontal intervals of 1.2 m and vertical intervalof 0.6 in (Fig. 2.3). Where such long stones are not available, concrete blocks, cast using 1:3:6 orM 10 concrete, having 150 x 150 mm cross section and length equal to the thickness of the wallmay be used instead. Such bonding elements are lo be provided whether the wall is 350- 380 mmthick using cement mortar or 400-450 mm thick using mud mortar.

Fig. 3 later locking stone wallwith `through' stone

Fig.4 Galvanized chicken wire mash (double layer)at corner and T-junction at window sill level

Provision of Long Stones at at all Wall Junctions

For ensuring good bond between perpendicular walls, long stones are to be provided at thecorners as well as T-junctions (Fig. 2,3). Such stones should be about 50 cm long in the case of350-380 mm thick walls and about 60 cm long in 400-450 mm thick walls. Where such stonesare not available, solid concrete blocks of 150 x 150 mm in section and 500 or 600 mm long(cast using 1:3:6 concrete) may be used instead.

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Guarding against Vertical Separation Between Perpendicular Wall

In seismic Zone V, District Kachchh. it will safeguard the walls against splitting at the verticaljoint between any two perpendicular walls if at the window side levels (that is, about mid waybetween the plinth and lintel hands) galvanized chicken wire mesh in double layer is embeddedin the 1:4 cement mortal joint as shown in Fig. 1.

Making and filling of Pockets around Vertical Bars

Correct method of creating a pocket around the vertical bar in the masonry and filling thepocket with micro-concrete is shown in Fig.5. After moving the 75 mm dia plastic pipe upwardout of the masonry, micro-cement (coarse aggregate below 10 mm) is to be filled in the hollowand tamped by mean of 10 mm dia rods.

Fig. 5 Method of filling micro-concrete around vertical bars in masonry

Bending of Bars and Casting of Seismic Bands

Figure 6 shows the bending of longitudinal burs with adequate overlaps and tying of cross linkswith them. This will ensure full continuity of the longitudinal reinforcement at the coners. It isalso suggested that stones may be cast in the band-concrete so that 1/4 to 1/3 of stone remainsprojecting outside the concrete. This arrangement will save. the amount of concrete as well as :Iwell as establish excellent bond and the wall masonry

Curing of Stone Wall Built using Cement Mortar

All newly constructed masonry should be covered with polythene sheets:to prevent fast dryingof mortar in the very dry conditions in Kachchh. Covering the walls with such sheets aftersprinkling water for caring will help preserving the moisture over longer periods thus savingconsumption of which is rather scarce in most parts of Kachchh and Saurashtra.

(a) (b)

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( c ) Cross section ( d ) Stones cast in concrete

Fig. 6 Seismic band at Plinth or Lintel level in stonewalls

QUALITY OF CONCRETE BLOCK WALL

'The strength and stability of the wall will depend on the strength and quality of the block, thequality of the mortar and construction of wall. The wall should be built truly vertical by fre-quent checking using a plumb.

The dry blocks should be welted before lying so that they do not suck the water from the mortar.Cement mortar should be freshly mixed and must be fully consumed within 60 minutes ofcement mixing with water to avoid setting of cement before laying.

After the wall portions are constructed, they shall be cured for a minimum period of 7 days byfrequent watering. Uncured mortar does not set and its proper strength is not achieved. (InManjil earthquake of 1990 in Iran, the main cause of destruction of brickwork constructedusing cement mortar was that the bricks were not sucked in water before laying and no curingwas done after construction). Needless lo say that proper bond should be maintained to breakthe vertical joints and the vertical joints between the blocks must be fully filled with mortar.

Note 1. Tests conducted in the Tehran laboratory on brick walls under lateral showed thatunsoaked and uncured brick walls failed at a lateral pressure of only 1/70 of that required forbreaking the fully cured walls built by soaking the bricks.

Note 2. To ensure filling of vertical joints between the blocks fully, mortar grout may be usedbefore starting the next coarse.

Quality of Concrete Blocks

The main wall material is concrete blocks of 300 x 200 x 150 mm nominal (290 x 200 x 140 mmactual size) laid in 1:6 cement-sand mortar. To obtain good quality concrete blocks of adequatestrength, 5.0 MPa (50 kg/sqcm at 28 days) and imperviousness as well as strength of wall, thefollowing points are to be implemented..

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Mix-Design

Concrete mix should be well graded, with*a few 40 mm size aggregate, coarse sand of highfineness modulus and such quantity of fines so as to fill the voids. The proportions should bearrived at through a mix-design method or by trials, by using the locally available materials. Forthis purpose each block-making center should have a set of standard sieves and a weighingmachine for determining the Fineness Modulus values of the materials.

Note The following proportions used in Lalur District in HUDCO's earthquake rehabilitation program, gave 28-day strength of blocks at 9 N/ mm2 (90 kg/cm2).

Overall Mix: Cement I part to 15 parts of the fine + coarse aggregates, measured byvolume.The aggregates consisted of the following:Coarsesand(FM3.17)Stone dust below 6 mm(FM 3.67)Crushed grit 6 to 10 mm (FM 4.11)Crusiled aggregutes 12 to less than 40 mm .Hand broken aggregates 40 to less than 50m mm.

Testing of sand

The coarse sand should not contain more than 8% of the silt by weight. Silt content may betested by using graduated cylinder method, held stationary for minimum 12 hours. Record ofsilt content should be kept at block making/construction site.

Note. If silt content is found to be more than permissible, the cement should be increased bytrial to achieve the desired strength.

Bulking of moist sand has to be tested and the quantity of sand is to be adjusted to achievedesign mix. A record of bulking test may also be ~maintained.

Making of blocks

After ensuring the design mix by proper control of measurement of the materials, proper mixingin the mixer and compaction by vibrating machines are the key factors. The large stone pieceshave a tendency of either coming to the top or settling below the mix in the mixer. Uniformmixing should be ensured by adjusting the angle of the revolving mixer. Regarding compaction,it should be noted that 10 percent less compaction might reduce strength by 40 to 50 percent;hence adequate compaction must be ensured.

Note 1 In many rural und town areas, block a re being made by hand molding with uncon-trolled or without compaction in such cases even a concrete mix of 1:4:8.(ie:1 part of cement to12 parts of fine + coarse aggregates) may not give the desired minimum strength of.50kg/sqcm.Hence thorough compaction by vibrators or by adequate rodding using l;6 mm dia rods of 400mm length is absolutely necessary for strength as well economy in tile use of cement.

2. In order to increase the horizontal shear strength of cement block walls, the blocks maybe made with a 'frog' as in bricks on its face. For a 290 x 200 .x 140mm block, the 'frog ' may bemade 150 x l00 x 6 mm deep.

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Curing and transporting

The blocks should be cured for 7 days and dried for few hours before transporting to avoidexcessive breakage.

Control on Strength of Blocks

Each block-making Center of appreciable size should have a power-operated compression-test-ing machine. A small Center may have a hand-operated machine. The machine should be ofgood quality make, such as AIMIL.

It may be specified that from the daily output of each labour gang, 3 blocks should be selectedat random which should be tested after 7 days curing under compression testing machine. Forstandardization purpose, once a week, 3 additional blocks should be cast for testing after 28days of curing. The record of testing should be maintained in bound registers at each site andeach entry to be signed by the engineering staff of executing and supervising agencies.

Recording Test Results

The rest results obtained in accordance with the above guidelines are to be recorded in appro-priate tables. The test results and acceptability should be written with proper signatures. Forexample, the block test results may be recorded in tables with the headings shown in Table 1.

Table 1. - Test Results of Concrete Blocks (or Concrete Cubes)

Name of gang making block

Date of casting

Date of Testing

Comp.Strength, kN

Average strength kN

Streng N/mm2 (or kg/cm2

Remark (signature)

Block 1

Block 2

Block 3

The average of 3 blocks' comprehensive strength should be expressed in N/mm2 or in kg/cm2 orMpa Signature of technician of the builder and verification by construction supervisor or J.E. in-charge should be recorded against each test entry

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QUALITY OF BRICK WORK

Quality of Bricks

The bricks should be well burnt with red color, neither under-burnt not over-burnt. having aminimum compressive strength of 5.0 N/mm2 (50 kg/cm2), when tested flat. During testing thefrog may be filled with a mortar 1:4 and the flat surfaces smoothened either by grinding, rub-bing with carborandom stone or by applying a thin layer of plaster. The bricks should give aringing sound when struck with each other.

Bonding in Brick Work

In normal construction, English bond is used in brick work in India as shown in Fig. 7 for onebrick thick walls as normally used in one to two storeys constructed using cement mortar. The'bond' used in brick columns of size 1 x 1 upto 2 x 2 bricks is also shown in the figure. This willensure that the vertical mortar joints will be broken in every two consecutive courses.

(a) 1 x 1 Brick column (b) 1 x 1½ Brick column

(c) 1½ x 1½ Brick column (d) 2 x 2 Brick column

(e) One Brick Wall

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( f ) One Brick wall corner

Note : ½, ¼, ¾ and 1 indicate the thickness in brick lengths

Fig. 7 Bonding in Brick Walls and Column, Forming pocket for vertical bar

Brick Laying

Bricks being porous absorb water. It is therefore essential that the bricks are soaked in waterfully before laying on the cement mortar layer. Unsoaked bricks will suck water from the mortarand create hindrans in the setting of cement mortar. For achieving full strength of brickwork itis necessary that all vertical joints between the bricks must be fully filled with mortar. Onedefect in brickwork commonly seen at the sites is that the longitudinal joint between two bricksis not filled and left open. Another defect seen is that the bricks are laid upside down, that is, thefrog is on the under side. This does not allow development of proper shear key between thebrick courses since the frog remains unfilled. These defects should not be allowed. To ensurecomplete filling of all vertical joints it may be necessary to fill the joints with mortar groutbefore starting the next course.

If these precautions are taken in construction and proper curing of the brickwork is carried outfor a minimum period of 7 days, full strength of the brick work under vertical as well as lateralloading due to wind or earthquake will be fully achieved.

Note :- For proper curing of the walls reference may be made to para 2.7 above.

Vertical joint between Perpendicular Walls

For convenience of constructions, builders prefer to make a toothed joint between perpendicu-lar walls, which is many times left hollow and weak. To obtain full bond it is necessary to makea sloping, (that is stepped joint by making the corners first to a height of 600 mm and thenbuilding the wall in between them. Otherwise, the toothed joint should be made in both thewalls alternately in lifts of about 450 mm (See Fig. 8).

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Fig. 8 Alternating toothed joint in walls at corner and T-Junction

BENDING AND PLACING OF REINFORCEMENT

The following care must be taken for achieving high strength of the RCC and durability of thebar by avoiding/minimizing of corrosion:

a. The bars should be straight, not crooked, cut to required sizes and bent to proper shapes asper drawings.

b. The bars for the seismic bands should have a minimum cover of 25 mm below and abovethem. The concrete mix should be M 20 (1:11/2:3 nominal) to prevent corrosion.

c. To keep the vertical reinforcing bars at the corners and jointly properly vertical, an L-bendshould be provided at its bottom end and each bar should be held by a tripod of bamboos orother spare reinforcing bars till such time that the concrete filled in the pocket around thebar is fully set and capable of holding the bar in vertical position.

d. A minimum overlap of 600 mm for 12-mm bars, 500 mm for 10 mm and 400 mm for 8-mmdia should be provided.

e. The cover to any bar (main or distribution) should be kept 15 mm minimum and 20 mmmaximum in concrete slabs used as floor or roof. The cover in beams to the main barsshould not be less than 25 mm and to the stirrups not less than 15 mm. For achieving propercover, either cover blocks of 1:3 cement sand mortar of required thickness or PVC coverparts should be used.

REINFORCED CONCRETE LINTELS AND SLABS

Concrete Mix

The concrete mix shall be M 20 (1:11/2:3 nominal) using cement, coarse sand and crushed gritof less than 20 mm size. The slump should not exceed 10 cm and the concrete should be com-pacted by rodding using 16 mm bars of about 600 mm length. Use of vibrator will, of course, bebetter keeping the slump of about 50 mm.

When the mix is to be designed to give the characteristic strength of 20 MPa, the target strengthin the mix design should be 26.5 MPa on 150 mm cubes at the age of 28 days. For quality

284

control on the concrete mix during construction, regular sampling and testing of concrete using150mm cubes should be carried out and the concrete should give an average strength at 28 daysof 24 MPa, the individual cube strength lying between + 15% of the mean strength obtained.

Slope in Roofs

To prevent ponding of water on the roof and consequent leakage, the concrete roofs must be laidso as to have a minimum camber of 1/200 of span at the centre and a minimum slope of about Iin 60. That is, for a roof width of 4m, the camber may be kept as 20 mm and the height differ-ence between the opposite edges should be about 70 mm. It is further suggested that the roofslab be kept projecting beyond the wall with a minimum of 75 mm at the lower edge andprovided with a drip course.

Curing of Concrete

Exposed surface of concrete shall be kept continuously in a damp or wet condition by pondingor by covering with a layer of sacking, canvas. Hess Tan or similar materials and kept constantlywet for at least seven days from the date of placing concrete in case of ordinary Portland Ce-ment of 33 or 43 Grade. The period of curing shall not be less than 10 days for concrete exposedto dry and hot weather conditions. Impermeable membranes such as polyethylene sheeting cov-ering closely the concrete surfaces may also be used to provide effective barrier against evapo-ration.

Striking Formwork

In normal circumstances where ambient temperature does not fall below 15oC and where ordi-nary Portland cement is used and adequate curing is done, the minimum period for strikingformwork as given in Table 2 may be adopted.

Table 2. - Minimum Time for Striking Formwork

Type of Formwork Minimum Period Before Striking Formwork

a) Vertical formwork to columns, walls, beams 16~41 h

b) Soffit formwork to slabs (Props to be refixed immediately after removal of formwork)

3 days

c) Soffit formwork to beams (Props to be refixed immediately after removal of formwork)

7 days

d) Props to slabs: 1) Spanning up to 4.5 m 2) Spanning over 4.5 m

7 days 14 days

e) Props to beams and arches: 1) Spanning up to 6m 2) Spanning over 6 m

14 days 21 days

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CONCLUDING REMARKS

Experience during recent earthquakes in Uttarkashi (1991), Latur (1993) and Chamoli (1999)has shown that stone building in which the traditional practice of 'Through" and "Comer" stoneswas used, suffered only minimum damage. Also in other earthquakes in India and abroad, it hasbeen a definite observation that good quality in construction and maintenance of buildingsprevented their collapse whereas indifferent to bad construction practices resulted in catastrophiccollapses. This has been fully demonstrated in the recent Kachchh earthquake also where notonly stone or block wall construction but also the reinforced concrete frame buildings wereseverely destroyed even in seismic Intensities of MSK VII, hence the critical importance ofmaintaining good construction practices and quality of materials in all buildings used for hous-ing or community purposes.

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287

FIRE SAFETY OF BUILDINGS

Yogendra SinghAsstt. Professor, Department of Earthquake Engineering, IIT Roorkee, 247 667

GENERAL

Fire has been recognized as a potential hazard world over. Most of the countries have their fireprotection code and fire resistant design is mandatory. However, in our country fire resistantdesign has not gained much attention. Although, a large number of big fires have occurred inour country and a few collapses of buildings due to fire have also been reported, fire safety atdesign stage is not properly considered. In developed countries, there have been very big firesin beginning of industrialization. In London, the building act for fire protection came into beingas early as in 1189. Concept of fire containment came into focus after the great fire of Londonin 1666. Structural fire protection was seriously investigated in 18th century.

In structural fire protection, the first aim is to ensure the stability of the building. In addition tothis, it has the following main components:

(i) Preventing the initiation of fire(ii) Restricting the growth and spread of fire(iii) Containment of fire within specified boundaries -a compartment forming part of building

or the whole building(iv) Means of escape for the occupants of building.(v) Control of fire by automatic devices and by active fire fighting.

FIRE SAFETY MEASURESFire safety measures can be classified into two categories:

Passive Measures

These measures are part of the building system and are functional all the time. The passivemeasures include the following:

(a) Design and installation of energy sources away from combustible materials.(b) Reduction in the quantity and surface area of combustible material in the building(c) Compartmentation within a building and fire resistant design of compartment boundaries(d) Separation of buildings to avoid spread of fire(e) Provision and design of escape routes(f) Measures for smoke control and resave facilities(g) Structural Design of building components to withstand fire loads.

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Active Measures

These come into action on include the following:

(a) Fire detection and warring system(b) Sprinkler installation with automatic/manual control(c) Fire fighting

TERMINOLOGY

Historically, many terms have been used in literature to express the fire resistnace of buildingcomponents. There is a slight difference in meaning:

(a) Fire Proof: Indicates not only expected performance but also indicates that the materialsused are non-combustible.

(b) Fire Resistant: Indicates immunity to the effects of fire upto a required degree. 1932 BritishStandard on fires defines fire resistance as "that property by virtue of which an element ofa structure as a whole functions satisfactorily for a specified period whilst subjected to aprescribed heat influence and load". It does not apply to the individual material of con-struction, but to the complete element such as beam, column, wall or floor

(c) Fire Endurance: Adopted by ASTM Standard on fire - E119, has the same meaning as 'FireResistance'. It indicates the duration to which a member can sustain the effects of heating,subjected to the normal loads on the member and serving the normal function expectedfrom the member.

EFFECT OF FIRE ON BUILDING ELEMENTS

Fire results in increase of temperature in a building compartment. The rate of burning and temperature increase depends on the ventilation parameters, size and shape of compartment and surface area. With small ventilation openings, the burning rate depends on availability of air. Such fires are called ‘Ventilation Controlled Fires’.On the other hand, fires in compartments with large openings are controlled by fuel characteristics. These are called ‘fuel bed controlled fires’. Fig. 1 shows the relationship between the temperature and the ratio of fire load to ventilation. Fire load is expressed either in terms of weight of wood or celluloses based material, with a calorific values of 16-18 MJ/kg or in terms of heat units (MJ or MCal)/m2. Ventilation is expressed by opening factor tvv AHA / , where Av is window area, Hv is window height and At is total compartment surface area. Fig. 2 shows the heat balance of an enclosure. The heat is produced by combustion of the fuel and is lost to the walls and outside through the exhaust gases. The energy balance can be expressed as,

REWGAF QQQQQQ +++=+ (1)

Where, FQ is the heat produced by combustion

AQ is the heat content of the incoming air

GQ is the heat used in raising ambient gas temperature

WQ is the heat transferred to walls, floor and ceiling

EQ is the heat content of exhaust gages

RQ is the heat loss by radiation from the window

289

Fig. 1 Relationship between fire temperature and ratio of fire load to ventilation

Fig. 2 Heat balance for a compartment under fire

290

Fig. 3 Three phases of fire development

Fig. 4 Equal area concept of fire severity

291

There are three phases of fire in a chamber as shown in Fig. 3. The temperature is low in thegrowth period and increases very rapidly and becomes stable in the fully developed period ofire. Finally it decreases rapidly in the decay period of fire.

Fire Severity

Fire severity is expressed by its time-temperature curve. Fig. 4 shows a comparison of time-temperature curves of two fires. The standard furnace curve is used for five resistance testing ofbuilding components in furnace. A relationship of this curve with the actual fire is required toestimate the requirement of fire resistance test.

Fire severity is the aggregate measure of the heating conditions and their effect on structurewhen exposed to fire. To compare the severity of two fires an equal area concept is used inwhich the area under the fire course upto 1500C or 3000C is compared. Fig. 5 shows the relation-ship between the fire load and the fire resistance time required corresponding to the standard firecurve

Fig. 5 Relationship between fire load and fire resistance requirement

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Effect on Structural Components and Assemblies

Fire has two types of effects on structural components. Due to heating it results in loss ofstrength and deformations of components; and due to transfer of heat through walls and floors,ignition of combustible material on the other side takes place. Fig. 6 shows the loss of strengthand deflection of components with time, when subjected to fire. Initially, the loss of strength israther gradual, but it reduces very rapidly after some time. When the deflection of memberexceeds L/30, collapse is initialed due to instability.

Fig. 6 Loss of strength with time in fire

Fig. 7 change in bending moment in a continuous beam due to fire

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Flexural Members

Fig. 7 shows the bending moment diagram of a continuous beam, before and after fire, due torestraint of continuous beans at supports a negative moment is generated. This has a beneficialeffect as the strength loss at supports is not as rapid as at the center. If the heating continues,plastic hinge is formed at the center. If the restraint at the support is adequate, a continuousbeam can act as two cantilevers. It is also possible for the beam to act as a catenary. If the depthof beam is sufficient, it can also develop some arching action. The arching action is facilitatedby the thermal expansion of beam.

Compression Members

In a concrete column the outer layers are subjected higher temperature the core consequentlythe hotter outer layers are subjected to much higher compressive stresses compared to the coolercentral core. As soon as the outer material looses its strength, the load is transferred to centralcore.

Beam Column Frame

Fig. 8 shows the effect of temperature rise on a portal frame. Due to rigid joint conditions,columns are subjected to additional bending moments. However, this effect is beneficial for thebeam, as discussed above.

Fig. 8 Effect of heating on portal frame

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Fig. 9 Sequence of hinge formation in a multi-storey building

In a multi-storey building, if good compartmentalization is provided, then fire is likely to attackonly a part of the building. Fig. 9 shows the deformation of members in a multistory frame andthe sequence of formation of plastic hanger.

Masonry Elements

Clay bricks are made by firing of day at high temperatures. This imparts to the bricks an abilityto withstand high temperatures without much physical or chemical charge. Bricks are virtuallyinert to high temperatures. However, the brick walls fail due to their own thermal expansion orthe expansion of other elements.

MATERIAL PROPERTIES AT HIGH TEMPERATURES

To design the structural components for fire resistance, it is important to understand the mate-rial properties at higher temperatures. The rate of deterioration of material strength at increasedtemperature determines the fire resistances of the member.

Concrete

One of the effects of heating on concrete is to drive away the free moisture as soon as thetemperature increases beyond 1000C. The vapour migrates through capillaries to the outer sur-face and on the heated side it appears as steam and on the other side it condenses and appearsas 'weeping'. However, this loss of moisture does not have any significant effect on strength ofconcrete.

Steep temperature gradients exist in the concrete members subjected to fire. This results inspalling of concrete. This may result in damage to concrete members in early stage of fire.

295

Fig. 10 Effect of temperature on compressive strength of concrete

Fig. 10 shows the change in compressive strength of unstressed concrete with temperature,while Fig. 11 shows strength change and residual strength of stressed

Fig. 11 Effect of temperature on compressive strength of stressed concrete

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Fig. 12 Recovery of strength in fire affected concrete with time

concrete subjected to higher temperateness. Lower strength reduction is resulted in stressedconditions and the residual strength after cooling is much lower than that in the hot conditions.The loss of strength is affected by the type of aggregate. In lighter concretes, the loss of strengthis less. It is also affected by the cement-aggregate ratio. The loss of strength in concrete recov-ers to some extent with time as shown in Fig. 12.

Fig. 13 Effect of fire on modulus of elasticity of concrete

297

Fig. 14 Effect of fire on stress-strain of concrete

Modules of elasticity of concrete also shows a steady reduction with temperature as shown inFig. 13.

Most important is the effect of temperature on stress-strain relationship of concrete. Fig. 14shows the effect of temperature on stress-strain curves of concrete, when tested under con-trolled strain ratio. Increased temperature results in lower ultimate strengths but higher maxi-mum strains.

Fig. 15 stress-strain curves for Mild Steel at high temperatures

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Steel

For mild steel there is an increased in the ultimate strength at temperatures up to 300 0C, but itreduces after 300 0C. Fig. 15 shows the stress-strain curves for mild steel at different tempera-tures. The well defined yield point gradually disappears with increased temperature. Fig. 16shows the change in ultimate strength of different types of steels at higher temperatures. Highstrength steel looses strength at a slightly faster rate than mild steel. Table-1 summarises thestrength and modulus of elasticity of steel in different temperature ranges.

Table-1: Effect of temperature on properties of steel

Temperature range Elastic properties 20-300 0C 300-700 0C 700-900 0C

20fyfyT

30001

0T− 500

3009.0

0 −− T

200700

1.00 −− T

20EET

30001

0T− )900300(6110

3009.0 0

0

CT −−−

FIRE RESISTANCE EVALUATION AND DESIGN

Fire resistance of an element can be either obtained using a standard furnace test or it can becomputed analytically.

Standard Furnace Test

Most of the countries use standard furnace tests for fire resistance evaluation and have codes fortesting. An international specification ISO-834 is also available. In the test, prototype

Fig. 16 Effect of temperature on strength of different types of steel

299

construction is exposed to heating in a furnace, simulating the use of construction in the actualbuilding as far as possible. Fig. 17 shows the schematic arrangement of a fire resistance testfurnace. A standard temperature time curve is maintained in the furnace. Fig. 18 compares thestandard fire curves used in various countries. The fire resistance is expressed in units of timefor which various performance criteria one satisfied.

Analytical Methods

A number of analytical and computational techniques along with software packages are nowavailable to analysis the building structures subjected to fire. Three approaches have been usedfor analytical modeling of building components subjected to fire.

i. Empirical and code equationsii. Limit state and plastic analysisiii. Finite Element, Finite Difference and Boundary Elements methods.

Fig. 17 Furnace for fire resistance test

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Fig. 18 Standard Temperature-time curves used in various countries

REFERENCES

Andrew H. Buchanen, Structural Design for Fire Safety, John Wiley & Sons, Ltd., 2001.ASTM E-l 19-88, Standard Test Methods for Fire Tests of Building Construction and Materials.American Society for Testing and Materials, Philadelphia, 1988.BS 5588, Code of Practice for Fire Protection, British Standards Institution, BSI, London.Fire Resistance Design Manual, Gypsum Association, Evanston, 1984.

Harmathy, T.Z., "Ten Rules of Fire Endurance Rating," Fire Technology, 1(2), 1965, pp 93-102.Harmathy, T.Z., Thermal Performance of Concrete Masonry Walls and Fire, Special TechnicalPublication No. 464, ASTM, 1970.International Standards Organisation, Fire Resistance Tests - Elements of Building Construc-tion, ISO, 1977.Lie, T. T., (ed.) Structural Fire Protection, ASCE, 1992.

Lie, T. T., "A Procedure to Calculate Fire Resistance of Structural Members," Fire and Materi-als, 8(1), 1984.

Lie, T. T., Fire and Buildings. Applied Science, London, 1982.M. Y. H. Bangash, Prototype Building Structures, Thomas Telford, 1999.Malhotra, H. L., Design of Fire-Resisting Structures, Surrey University Press, London; Distrib-uted by Chapman and Hall, 1982.

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IMPROVING WIND/CYCLONE RESISTANCEOF BUILDINGS: GUIDELINES*

A.S. Arya, Prem Krishna and N.M. Bhandari

INTRODUCTION

The coastal areas of India receive a number of cyclonic wind storms practically every yearcausing devastation over large areas due to (i) high speed winds, which destroy traditionalhomes and uproot trees and electric line supports (ii) floods, caused by heavy rains, and (iii)storm surge waters, first flowing towards the land then receding back towards the sea, drowningpeople, destroying homes, agriculture, trees etc., whatever comes in the path of the flowingwaters. High speed wind storms on mainland also many times cause severe damage to build-ings, particularly light weight roofs, free standing boundary walls, etc. Horticultural crops suf-fer badly in both cases at sea coast and inland under high speed winds.

Although the main destruction during cyclones or high winds occurs in the traditional non-engineered buildings built using local clay, stone, Adobe or agro based materials, the engi-neered buildings having high sheeted roofs also suffer huge damage unless propriate precau-tions are taken in design as well as construction. Even in heavy constructions, substantial non-structural damage occurs to doors, windows, cladding wall panels, etc.

The aim of these guidelines is firstly, to briefly explain the action of wind on buildings and statethe general principles of planning and design; secondly, to bring out details to prevent the non-structural damage in the various buildings; thirdly, to deal with the safety aspects of traditionalnon-engineered buildings: and finally, to suggest retrofitting details which could be adopted inexisting buildings to minimise the damages under high winds. Suggestions are also included forsafety against storm surge.

SCOPE

These guidelines deal with the construction of wind/cyclone resistant buildings of both engi-neered and non-engineered types. The proposed measures are generally applicable to wind re-sistant construction, but have particularly been framed keeping in view the regions having windvelocity greater than or equal to 39 m/sec. Wind zoning map of India is given in IS: 875 (Part 3)-1987. The same has been redrawn for various States and Union Territories on 1:2.5 millionscale in the Vulnerability Atlas of India (1997)

Such additional issues or provisions which are specifically useful and/or required for cycloneaffected regions are highlighted.

* Published byBMTPC, Ministry of Housing & Urban Poverty Alleviation, Govt. of India, New Delhi

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To improve the wind/cyclone resistance of existing buildings, some retrofitting measures havealso been presented.

WIND PRESSURES ON BUILDINGS AND STORM SURGE HEIGHTS

Basic Wind Speed Zones

The macro-level wind speed zones of India have been formulated and published in IS: 875 (Part3) - 1987 titled "Indian Standard Code of Practice for Design Loads (other than earthquakes) forBuildings and Structures, Part 3, Wind Loads". There are six basic wind speeds `Vs' consideredfor zoning, namely 55, 50, 47, 44, 39 and 33 m/s. From wind damage viewpoint, these could bedescribed as follows:

55 m/s (198 km/h) - Very High Damage Risk Zone -A50 m/s (180 km/h) - Very High Damage Risk Zone - B47 m/s (169.2 km/h) - High Damage Risk Zone44 m/s (158.4 km/h) - Moderate Damage Risk Zone - A39 m/s (140.4 km/h) - Moderate Damage Risk Zone - B33 m/s (118.8 km/h) - Low Damage Risk Zone

The cyclone affected coastal areas of the country are classified in 50 and 55 m/s zones. Thebasic wind speeds are applicable to 10m height above mean ground level in an open terrain witha return period of 50 years.

Design Wind Speed and Pres-sures

The basic wind speed is re-duced or enhanced for designof buildings and structures dueto factors like (i) the risk levelof the structure measured interms of adopted return periodand life of structures (5,25,50or 100 years), (ii) terrainroughness deter-mined by thesurrounding buildings or treesand, height and size of thestructure, (iii) local topographylike hills, valleys, cliffs, orridges, etc. Thus general basicwind speed being the same ina given zone, structures in dif-ferent site conditions couldhave appreciable modificationand must be considered indetermining design wind velocity as per IS: 875 (Part 3)-1987.

Fig. 1. External wind pressure areas on building faces

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The design wind pressure at height z above ground level on a surface normal to the wind streamsis given by

2zV0006.0p =z , whe (1)

zV = design wind velocity, m/s

zp = design wind pressure, kN/m2

The value of wind pressure actually to be considered on various elements depends onacredynamics of flow around buildings the windward vertical faces being subjected to pressure,the leeward and lateral faces getting suction effects, and the sloping roofs getting pressures orsuction effects depending on the slope. The projecting window shades, roof projections at eavelevels are subjected to up-lift pressures several times the intensity of pz . These factors play animportant role in determining the vul-nerability of given building types in given wind speedzones. For example, Fig.1 shows the various cladding areas of a building, which will havedifferent pressure coefficients.

Figures 2 and 3 show typical effects of openings in the walls for a given angle of attack of windas indicated; only one large opening in a wall will cause very large internal pressure say ± 0.7 pz.which combined with external suction will increase the wind effects on cladding and theirconnections immensely. A building with all windows and doors locked will have zero or verysmall internal suction or pressure <0.20 pz . If a room has openings distributed in all walls or atleast in opposite walls and the overall porosity is less than 5%, the passage of air will cause onlylow internal pressure say only 0.2 pz . Effects of wind uplift on roof projections can also be seenin Fig. 2 and 3. For a design speed of 50 m/s, the basic pressure will be 1.5 kN/m2 and the designpressures could be obtained by multiplying with the coefficients given in Fig. 2 and 3 for thespecimen cases shown. For other dimensions of length, width and height and direction of wind,reference may be made to I.S: 875 Part 3-1987.

Areas of high local suction (negative pressure concentration) frequently occurring near theedges of walls and roofs are separately shown in the code. Coefficients for local effects shouldonly be used for calculation of forces on these local areas affecting roof sheeting, the glasspanels, individual cladding units including their fixtures, they should not be used for calculat-ing force on entire structural elements such as roofs, walls or the structure as a whole

Coastal Areas

The coastal areas are subjected to severe wind storms and cyclonic storms. It is known that incertain events, the wind gusts could appreciably exceed the specified basic wind speeds (by asmuch as 40 to 55%). But for design of structures (except those considered very important) theabove macro-level zoning stated in 3.1 is considered as sufficient.

The frequency of occurrence of cyclones on the different portions of the coast has been differ-ent. Even for the same design wind speed in some areas, the risk of damage per year will behigher in areas subjected to more frequently cyclones.

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Fig. 2. Structural Load Coefficients, Internal and External Pressures (Openings on windward side or large permeability case, internal pressure coefficient taken as 0.7;h ≥ w/2, L ≤ 1.5w)

Fig. 3. Structural Load Coefficients, Internal and External Pressures (Openings in opposite walls or low permeability case, internal pressure coefficient taken as 0.2; h ≥ w/2, L ≤ 1.5w)

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Storm Surge

Besides the very high velocity winds, the coastal areas suffer from the onslaught of sea waterover the coast due to storm surge generated by cyclones. A storm surge is the sudden abnormalrise in sea level caused by the cyclone. The surge is generated due to interaction of air, sea andland. The sea water flows across the coast as well as inland and then recedes back to the sea.Huge loss of life and property takes place in the process. The height of the storm surge is evenhigher during the period of high tides. Scientists from India Meterological Department haveestimated the probable maximum heights of storm surge in various sections of the sea coast.The estimated probable maximum storm surge heights are shown in the relevant state's windhazard maps in the Vulnerability Atlas of India. The area affected due to storm surge will bemore in flat terrain than in steeply rising terrain.

TYPES OF DAMAGE DURING CYCLONES

The wind pressures and suction effects on flat objects could be sufficient to lift them off and flyaway from their place of rest unless adequately tied down to substantial supports. Table 1 showsthe aerofoil effects of some wind speeds.

As a consequence of the wind pressures/suctions acting on elements obstructing the passage ofwind the following types of damage are commonly seen to occur during high wind speeds:

i uprooting of trees which disrupt transportation and relief supply missionii failures in many cantilever structures such as sign posts, electric poles, and

transmission line towersiii damage to improperly attached windows or window framesiv damage to roof projections, chajjas and sunshades

Table 1: Aerofoil Effect of Wind

Wind Speed m/sec Typical Movement

5-10 10-15 15-20 25-30 30-35 35-40 40-45 45-50 50-55 55-60 60-65 65-70 70-75 75-80

Lose aluminum sheets fly Lose galvanized iron sheets fly Lose fiber cement sheets fly Lose concrete and clay tiles fly Roof sheets fixed to battens fly Small aircrafts take off speed Roof tiles nailed to battens fly Garden walls blow over Unreinforced brick walls fail Major damage from flying debris 75 mm thick concrete slabs fly 100 mm thick concrete slabs fly 120 mm thick concrete slabs fl y 150 mm thick concrete slabs fly

306

v failure of improperly attached or constructed parapets,vi overturning failures of compound walls of various types;vii failure of weakly built walls and consequent failure of roofs and roof covering;viii failure of roofing elements and walls along the gable ends particularly due to high internal

pressures;ix failure of large industrial buildings with light weight roof coverings and long/tall walls due

to combination of internal and external pressures;x brittle failure of asbestos- cement (AC) sheeting of the roofs of Industrial sheds; failure of

AC sheets is generally along eaves, ridges, and gable ends);xi punching and blowing off of corrugated iron roofing sheets attached to steel trussesxii though a thatch roof commonly employed in rural construction lacks durability, it provides

greater permeability and attracts less forces of wind compared to an impermeable mem-brane.

PLANNING ASPECTS

Site Selection

i. Though cyclonic storms al-ways approach from the direc-tion of the sea towards thecoast, the wind velocity anddirection relative to a buildingremain random. In non-cy-clonic region where the pre-dominant strong wind direc-tion is well established, thearea behind a mound or ahillock should be preferred toprovide for natural shielding(Fig.4). Similarly a row of treesplanted upwind will act as ashield (Fig.5). The influence ofsuch a shield will be over a lim-ited distance, generally from 8to 10 times the height of thetrees.

ii. In hilly regions, construction along ridges should be avoided since they experience an ac-centuation of wind velocity whereas valleys experience lower speeds in general as shownin Fig .6.

iii. Cyclonic windstorms commonly generate storm tides leading to coastal inundation. In cy-clonic regions, close to the coast a site above the likely

Fig. 4. Shielding of house by hillock

No shielding from highwind due to absence of

barriers

Shielding from highwind by of barriers

307

inundation level should be given preference. In case of non availability of high elevation naturalground, construction should be done on stilts with no masonry or bracings upto maximum surgelevel, or raised earthen mounds as shown in Fig. 7 to avoid flooding/inundation.

Planforms & Orientation

i. For individual buildings, a circular or polygonal plan shape is preferred over rectangular orsquare plans, but from the view point of functional efficiency, often a rectangular plan iscommonly used. Where most prevalent wind direction is known, a building should be sooriented, where feasible, that its smallest fecade faces the wind

ii. A symmetrical building with a compact plan-form is more stable than an asymmetricalbuilding with a zig-zag plan, having empty pockets as the latter is more prone to wind/cyclone related damage (See Fig. 8)

iii. In case of construction of group of buildings with a row type or cluster arrangement (seeFig.9) can be followed in preference to row type. However, in certain cases both may giverise to adverse wind pressure due to tunnel action and studies need to be conducted to lookinto this aspect

Roof Architecture

i. The overall effect of wind on a pitched roof building and the critical location are shown inFig. 10 to 13. It is seen that roof projections and wall not roof corners experience highsuction. According places where typical failures begin are shown in Fig. 10. Therefore, theroof projections should be kept to a minimum, say not exceeding 500 mm, or else, the largerprojections be tied down adequately (Fig. 11).

Buildings in valleys protected from highwind velocities

Buildings in ridge attracting high windvelocities

Fig. 6 Appropriate location of buildings in hilly terrains

Construction at ground level riskof innundation

If natural elevation not available constructionon stilts or artificially raised earth mounds

Fig. 7 Construction on raised ground / stilts to prevent innundation

308

Asymmetric building with empty pocketsare more vulnerable to damage

Symmetric buildings are more stable

Fig. 8. Desirable orientation and plan form for reducing wind damage

Zig-zag planning avoids windtunnel evvects

Row planning creates wind tunneleffects

Fig. 9. Group planning of buildings

Fig. 10 a. Roofing sheets Fig. 10 c. Reeper lifts from the rather

Fig.10b Roofing sheets lift at the end Fig.10d Holding down of rafter to wall inadequate Fig.10 Types of roof damages due to wind

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Large overhangs get lifted upand broken smooth finish on

walls undesirable

Avoid large overhangs/use the/openings in overhangs, rough

finished walls desirable

Fig. 11. Overhangs

Fig. 12 a Gable ended roof get high up lift Fig.12 b Hip roof get lower uplift

Fig. 12 c Pyramidal roof gelowest up liftFig. 12 Effects of roof architecture on up

lift forces

Fig. 13 Unclosed openings on windwardside creates high positive pressure

inside aiding up lift

Note: For rain protection, a minimum roof projection of 500 mm is desirable. Tying down.will be very advantageous.

ii. For the purpose of reducing wind forces on the roof, a hipped or pyramidal roof is prefer-able to the gable type roof (see Fig. 12).

iii. In areas of high wind or those located in regions of high cyclonic activity, light weight (Glor AC sheet) low pitch roofs either should be avoided or strongly held down to purlins andrafters. Pitched roofs with slopes in the range 22-30° will not only reduce suction in roofsbut would also facilitate quick drainage of storm water.

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Wall Openings

Openings in general are areas of weakness and stress concentration, but needed essentially forlighting and ventilation. The following norms are recommended in respect of openings.

i. Openings in load bearing walls should not be within a distance of h/6 from inner comer forthe purpose of providing lateral support to cross walls, where h is the storey height uptoeave level.

ii. Openings just below roof level be avoided except that two small vents without shutter shouldbe provided in opposite walls to prevent suffocation in case room gets filled with water andpeople may try to climb up on lofts or pegs.

iii. Since the failure of any door or window on wind-ward side may lead to adverse upliftpressures under roof (see Fig.2, 3 and 13), the openings should have strong closing/lockingarrangement and lass/wooden panels be securely fixed (Fig. 14).

Large and thin unprotectedglass area in windows

Small and thick /wired glass protectedwith guard bars /tapes / wooden

battensFig. 14. Shutters and windows

Glass Panelling

a. One of the most damaging effect of strong winds or cyclones is the extensive breakage ofglass panes caused by high local wind pressure or impact of flying objects in air. The largesize door or window glass pane may shatter because they are too thin to resist the local windpressures. A broken glass pane of a windward side opening increases internal pressuresabnormally and may lead to a chain of events including a roof failure.

b. The way to reduce this problem is to provide well designed glass panels. In cyclonic re-gions where the exposure to high wind and gustiness is sustained, it is recommended that indesigning glass panels any relief by way of increase in permissible stresses on account ofthe consideration of wind load be not allowed.

c. Further, recourse may be taken to reduce the panel size to smaller dimensions. Also glasspanes can be strengthened by pasting thin plastic film or paper strips (Fig 15). This willhelp in holding the debris of glass panes from flying in case of breakage. It will alsointroduce some damping in the glass panels and reduce their vibrations.

d. Further, to prevent damage to the glass panels from flying wind borne missiles, a metallicfabric/mesh be provided outside the large panels.

e. The locking arrangement of shutters should be sturdy and the door or window frames besecurely fixed in the walls using hold fasts (Fig.16) so as to resist the local wind pressures.

311

Fig. 15 Protection ofglass panes

Fig. 16 Adequate anchorage of doorand window frames with hold fasts

Foundations

Buildings usually have shallow foundation for sandy soil and deep foundations for expansiveclayey soils. All shallow foundations should be designed as per IS: 1904-1978. It is desirablethat information about soil type be obtained and estimate of safe bearing capacity made fromthe available records of past constructions in the area or by proper soil investigation.

In addition the following parameters need to be properly accounted for in the design offoundation.

i. Effect of Surge or Flooding - Invariably a cyclonic storm is accompanied by torrential rainand tidal surge (in coastal areas) resulting into flooding of the low lying areas. The flurry oftidal surge diminishes as it travels on shore, which can extend even upto 10 to 15 km.Flooding causes saturation of soil and thus significantly affects the safe bearing capacity ofthe soil. Also the likelihood of any scour due to receding tidal surge needs to be taken intoaccount while deciding on the depth of foundation, and the protection works around a raisedground used for locating cyclone shelters or other buildings.

ii. Building on Stilts - Where a building is constructed on stilts it is necessary that stilts areproperly braced in both the principal directions. This will provide stability to completebuilding under lateral loads. Knee braces will be preferable to full diagonal bracing so asnot to obstruct the passage of floating debris during storm surge. The pressure loading onstilts is considerably different from buildings on ground.

iii. Building in Hilly Region - In case of hilly regions where construction is made after cuttingterraces on the hill slopes, it is essential that for the stability of slopes, a minimum edgedistance of the foundation from any terrace be kept 1.5 times the depth of foundation (Fig.17)and foundation should rest on the natural firm strata. Further proper drainage of the area beensured allowing surface water to flow unobstructed.

Fig. 17 Recommended edge distance of foundations in hilly regions

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DESIGN PROCEDURE FOR WIND RESISTANT BUILDINGS

The following procedure may be followed to design a building that will be resistant to damagesduring high winds/cyclones.

Fix Design Data

a. Identify the national wind zone in which the building is situated. This can be seen fromwind code (IS: 875 Part 3-1987) or the Vulnerability Atlas of India (1997).

b. Corresponding to the zone, fix the basic design wind speed, Vb which can be treated asconstant upto the height of 10m.

c. Choose the risk co-efficient or the importance factor k1 for the building, as for examplegiven below:

Building type Coefficient )1(1k

i. Ordinary residential building 1.0ii. Important building (e.g. hospital; 1.08

police station; telecommunication,school, community and religiousbuildings; cyclone shelters, etc.)

d. Choose appropriate value of K2 corresponding to building height, type of terrain and size ofbuilding structure, as per IS:875 (Part 3), 1987. For buildings upto 10m height and cat-egory-A, which will cover the majority of housing, the values are:

Terrain Coefficient (2)2k

i. Flat sea-coastal area 1.05ii. Level open ground 1.00iii. Built-up suburban area 0.91iv. Built-up city area 0.80

e. The factor k3 depends upon the topography of the area and its location above sea level. Itaccounts for the acceleration of wind near crest of cliffs or along ridge lines and deceleration in valleys etc.

Determine the wind forces

a. Determine the design wind velocity Vz and normal design pressure pz

Vz = Vb k1 k2 k3 (2)

pz = 0.0006 Vz kN/m2 for Vz in m/s

b. Corresponding to the building dimensions (length, height, width), the shape in plan andelevation, the roof type and its slopes as well as projections beyond the waits, determine thecoefficients for loads on all walls, roofs and projections(2) as for example shown inFigs. 1-3, taking into consideration the internal pressures based on size and location of

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openings. Hence calculate the wind loads on the various elements normal to their surface.c. Decide on the lines of resistance which will indicate the bracing requirements in the planes

of roof slopes, at eave level in horizontal plane, and in the plane of walls. Determine theloads generated on the following connections:

- roof cladding to purlins,- purlins to rafters/trusses,- rafters/trusses to wall elements,- between long and cross walls,- walls to footings.

Design the elements and their connections

a. Load effects shall be determined considering all critical combinations of dead load, live loadand wind load. In the design of elements, stress reversal under wind suctions should be givendue consideration. Members or flanges which are usually in tension under dead and liveloads may be subjected to compression under dead load and wind, requiring consideration ofbuckling resistance in their design.

b. .Even thin reinforced concrete slabs, say 75 to 100 mm thick, may be subjected to upliftunder wind speeds of 60 m/s and larger requiring holding down by anchors at the edges, andreinforcement on top face! As a guide, there should be extra dead load (like insulation,weathering course etc.) on such roofs to increase the effective weight to about 375 kg/m2.

c. Since cyclonic wind could blow from any direction, building must have wind resistancealong both the axes.

d. Resistance to corrosion is a definite requirement in cyclone prone sea coastal areas. Paint-ing of steel structures by corrosion- resistant paints must be adopted. In rein-forced con-crete construction, a mix of M20 grade with increased cover to the reinforcement has to beadopted. Low water cement ratio with densification by means of vibrators will minimisecorrosion. In important structures, epoxy coating of reinforcing bars should be considered.The external surface should be treated with water proofing paints.

e. All dynamically sensitive structures such as chimney stacks, specially shaped water tanks,transmission line towers, etc. should be designed following the dynamic design proceduresgiven in various IS codes.

f. The minimum dimensions of electrical poles and their foundations can be chosen to achievetheir fundamental frequency above 1.25 Hz so as to avoid large amplitude vibrations, andconsequent structural failure.

It may be emphasised that good quality of design and construction is the single factor ensuringsafety as well as durability in the cyclone hazard prone areas. Hence all building materials, andbuilding techniques must follow the applicable Indian Standard Specifications.

Design Considerations for Roofs

Depending upon the construction material used and the geometrical aspects, the roofs can bebroadly classified into two main types:

a. Flat roofs of various typesb. Pitched roofs with various covering materials

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Their design considerations are stated here-below:

Flat Roofs

a. Flat roofs may consist of (i) R.C. slabs, (ii) wooden or R.C. joists, inverted T-irons placedclosely spaced and carrying brick tiles, stone slabs or reeds with clay topping, and (iii)prefabricated R.C. elements of various designs placed side by side. Whereas R.C. slabs arerigid in their own planes, the other types will require their integration through diagonalbracing or topping R.C. screed (structural deck concrete).

b. Structural deck concrete of grade not leaner than M15 (M20 in cyclone areas) shall beprovided over precast components to act monolithic with them (Fig 18). Wherever, deckconcrete is to be provided, the top surface of the components shall be finished rough. Ce-ment slurry with 0.5 kg of cement per sq.m. of the surface area shall be applied over thecomponents immediately before laying the deck concrete and the concrete shall be com-pacted using plate vibrators. The minimum thickness of deck concrete shall be 35 to 40 mmreinforced with 6 mm dia bars @ 150 mm apart bothways and anchored into the roof bandor tie beam placed all round.

c. In view of large uplift forces, particularly if wind speed could exceed 55 m/s, the total Roofweight should preferably be kept about 375 kg/m'. Lighter roofs should be designed for nethogging forces and properly held down to supporting beams/walls, etc.

d. Ferrocement (F.C.) is an emerging construction material having many advantages. Aferrocement roof will have a reduced dead weigh compared to an R.C. roof, though it willnot be so light as an AC or Gl roof. Furthermore ferrocement has finer and well-distributedcracks as compared to RC hence better corrosion resistance. This new material could beused for flat or sloping roofs provided that the ferro-cement sheets are adequately an-choredto the supporting walls/beams against the wind-uplift forces.

Fig. 18 Provision of reinforcement in structural deck concrete

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Pitched Roofs

a. The main load bearing structural members are timber or steel trusses, purlins, and bracings.The cladding may be of Gl or AC sheeting, tiles, timber planks or prefabricated R.C. orFerrocement elements. It will be preferable to use sheeting with adequate fixtures than tilesin cyclone areas.

b. The different design requirements for pitched roofs are as follows:Analysis and design of pitched roof is carried out as per provisions of relevant codes ofpractice i.e. IS: 800-1984 for steel trusses and IS: 883-1970 for timber trusses. Underhighvelocity wind along the ridge of pitched roofs, the suction forces may exceed the dead loadof the roof appreciably, causing compression in the bottom chord and stress reversals in alltruss members in general. Buckling consideration in all members of roof trusses which arenormally under tension, therefore, assumes significance. Therefore, the main ties of rooftrusses may requires lateral bracing and strutting against their buckling in lateral direction.

Note: Since the probable maximum wind velocities in coastal areas exceeds the designvelocities specified in the wind code, and the time duration for which a building is exposed tohigh wind velocities is much larger than in a 'passing' storm, very important buildings andstructures may be designed for such probable velocities (see Vulnerability Atlas of India)

Fig. 19 a1 J bolt - cyclone connection forroof cladding to purlins

Fig. 19 a2 U bolt - cyclone connection forroof cladding to purlines

Fig. 19 b Fixing of corrugatedsheeting to purlines with bolts

Fig. 19 c Using reinforcing bands inhigh suction zones

Fig. 19 Cyclone resistant connection details

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c. .Connections for cladding - The corners and roof edges are zones of higher local windsuctions (see Fig. 1 to 3) and the connections of cladding/sheeting to the truss need to bedesigned for the increased forces as evidenced from past damage surveys, this is a vulner-able zone. The local pressure co-efficient given in IS: 875 --Part 3 may be used for design of connections such vulnerable areas. Further, failure at anyone of these locations could lead progressively to complete roof failure. Hence, particularlyin the cyclone affected zones, a reduced spacing of bolts 3/4 of that admissible as per IS:800 is recommended. For normal connections J bolts may be used but for cyclone resistantconnections U-bolts are recommended as shown in Figs. 19 a1 and 19 a2. As an alternativeto the use of U-bolts, a strap may be used at least along the edges to fix the cladding with thepurlins as shown in Fig. 19(b) to avoid punching through the sheet. Properly connectedM.S. flat can be used as reinforcing band in high suction zones as shown in Fig. 19 (c)In residential buildings in some areas, roof cladding may comprise of earthen tiles. Be-cause of lower dead weight, these may be unable to resist the uplifting force and thusexperience heavy damage, particularly during cyclones. Anchoring of roof tiles into a R.C.strap beam along the edges is recommended for improved cyclone resistance.

d. Anchoring of roof framing to wall/posts - The proper connection of roof framing to thevertical load resisting elements i.e. wall or post, is equally important for overall stability ofthe roof. Care is particularly needed while connecting roof trusses to R.C. columns or ma-sonry walls in cyclonic regions, by providing properly designed anchor bolts and base plates.Typical connection of wooden framing to wooden post is shown in Fig. 20 through cyclonebolt or metal straps. The anchoring of roof framing to masonry wall should be accom-plished through anchor bolts properly embedded into concrete cores. The weight of partici-pating masonry at an angle of half horizontal to 1 vertical as shown in Fig. 21 should bemore than the total uplift at the support. In case of large uplift forces, the anchoring bars canbe taken down to the foundation level with a structural layout that could en-sure the partici-pation of filler and cross walls in resisting the uplift.

e. Bracing -Adequate diagonal or knee bracing should be provided both at the rafter level andthe eaves level in a pitched roof (see Fig.22). The purlins should be properly anchored atthe gable end. It is desirable that at least two bays, one at each end, be braced both inhorizontal and vertical plane to provide adequate wind resistance. Where number of bays ismore than 5, use additional bracing in every

fourth bay.

In order to reduce wind induced flutter/vibration of the roof in cyclonic regions, it is recom-mended that all members of the truss and the bracings be connected at the ends by at least tworivets/bolts or welds. Further the cross bracing members be welded/connected at the crossingsto reduce vibrations.

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Fig. 20 a Bolting Fig. 20 b Connecting roof frame to wall frame

Fig. 20 Connection of roof framing to wall framing

Masonry Walls of Good Design

General

These are usually made from rectangularised masonry units (with crushing strength not less than5.0 MPa) bonded in cement/cement lime mortar (not leaner than 1:6 cement-and and 1:2:9Cement-Lime-Sand). The commonly used masonry units are bricks, stones or concrete blocks.The stability of walls under lateral wind loads depends on their hickness,

height and distance between transverse supports. Less height, larger thickness and less spanmake them more stable

Fig. 21 Anchoring of roof framing in masonry

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External Walls

All external walls or wall panels must be designed to resist the out of plane wind pressuresadequately. The mortar used will determine the permissible tensile bending stress as per IS:1905-1987. In case of walls of large halls or industrial buildings (more than 8 m long) adequatelateral restraint in the form of buttresses/piers should be provided.

In case of cellular plans with cast-in-situ R.C. slab, the lateral load due to wind is usuallyresisted by all walls lying parallel to the lateral force direction in proportion of their stiffness(by shear wall action). The walls are designed for their share of vertical and lateral load. Theprovisions of IS: 1905-1987 need to be complied with for the safety of the walls.

Fig. 22 a Bracing in planes of rafters Fig. 22 b Eaves level knee bracing

Fig. 22 Typical roof bracings for industrial buildings

Strengthening of Walls Against High Winds/Cyclones

For high wind and cyclone prone areas ( )ms50Vb ≥ , it is found necessary to reinforce the wallsby means of reinforced concrete bands (equivalent to those required in masonry buildings inseismic zone V vide IS: 4326-1993) as given below to be provided at the door-window lintellevel, eaves level of pitched roofs, below flexible flat roofs, and top of external gable walls. Thestrengthening methods suggested herein need further research using probable maximum windspeeds in cyclone prone sea coast areas, but are in the right direction and may be adopted for thetime being.

a. Lintel band is a band provided at lintel level on all load bearing internal, external longitudi-nal and cross walls.

b. Roof band or eave band is a band provided immediately below the roof or floors. Such aband need not be provided underneath reinforced concrete or brick-work slabs resting onbearing walls, provided that the slabs are continuous over the intermediate wall.

c. Gable band is a band provided at the top of gable masonry below the purlins. This bandshall be made continuous with the roof band at the eaves level.

d. Section and Reinforcement of Band. See Table 2. The band shall be made of reinforcedconcrete of grade not leaner than M15 or reinforced brickwork in cement mortar not leanerthan 1:3, and shall cover the width of end walls, fully or at least 3/4 of the wall thickness.

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See Fig. 23 for details of reinforcement placing and bending.e.. It is advisable that in wind velocity 50 m/s or higher velocity areas vertical reinforcing bars

are provided, against uplift, between the foundation and roof band and between eave bandand gable band, as follows:

- From foundation through Lintel One bar of 12 mm dia. H.S.D.Band into Roof/Eave Bands at each corner and junction of walls

- Between Eave and Gable Band One bar of 12 mm dia. H.S.D.under ridgeand at every 2 m apart in between.

Fig. 23 a Section of band with 2 bars Fig. 23 b Section of band with 4 bars

Fig. 23 c Structural plan at corner junction Fig. 23 d Section plan at T junction of walls

Fig. 23 Reinforcement and bending detail in R.C. band

Table 2 : Recommended Longitudinal Steel in R.C. Bands(High Strength Deformed Bars, Fe415)

Span Design Wind Speed, m/s > 55 50-55 44-49 33-44 M No. of

Bars Dia. mm

No. of Bars

Dia. mm

No. of Bars

Dia. mm

No. of Bars

Dia. mm

5 or less 6 7 8

2 2 4 4

10 12 10 12

2 2 2 4

8 10 12 10

2 2 2 2

8 8 10 12

Nil Nil 2 2

- - 8 10

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Notes:

1. Span of wall will be the distance between centre lines of its cross walls or buttresses. Forspans greater than 8 m, it will be desirable to insert pilasters or buttresses to reduce thespan.

2. Width of R. C. band is assumed same as the thickness of the wall. Wall thickness shall be200 mm minimum. A clear cover of 20 mm from face of wall will be maintained.

3. The vertical thickness of R.C. band be kept 75 mm minimum, where two longitudinal barsare specified, one on each face: and 150 mm, where four bars are specified.

4. Concrete mix shall be of grade M20 of IS 456: 1978 (or 1:1.5:3 by volume) in cyclone areasand MI 5 in others.

5. The longitudinal steel bars shall be held in position by steel links or stirrups 6 mm diaspaced at 150 mm apart.

Openings

Openings in walls create stress concentrations and are thus points of weaknesses. Yet these areunavoidable. In general, large openings close to the corners, or, too many openings should beavoided. The total width of openings in a load bearing or shearing wall should not exceed 50%of the length of the wall. For taking the full advantage of return wall in the form of participatingeffective flange width for providing lateral load resistance, no opening should be located withina distance of 6 times the wall thickness or one twelfth of the storey height which ever ii less,from the cross wall. The openings should be in a regular pattern to permit a smoother stressflow.

Framed Buildings

As an alternative to vertical load bearing walls, reinforced concrete, steel, or timber framingcan be used. In R.C. constructions, the frame comprises of rigidly connected beams and col-umns or posts. In steel and timber constructions, complete structural framing should be ad-equately braced both in the vertical and the horizontal planes. Stipulations for cyclonic regionsas made in the foregoing section 5.5 dealing with walls are applicable to the cladding wallpanels also The recommended guidelines for the design of frames are as follows:

a. Loading - The different loads and load combinations to be considered for the design are asper IS: 875 (Part 1 to 4). The dead loads, superimposed loads, wind and snow loads to beconsidered are given in parts 1, 2, 3 and 4 respectively.

b. Cladding - For enclosing the space it is necessary that cladding be provided, firmly securedto columns or posts, on all the external faces and where partitioning is required. It is usualto have masonry wall panels as cladding in buildings with R.C. framing. The design ofpanel wall shall be carried out for out of plane local wind pressures as per IS: 1905-1987.

In industrial buildings corrugated galvanized iron/asbestos cement (CGI/AC) sheet claddingmaybe used for side covering. The design should be carried out for local wind pressures. Properattention be paid to connections specially near corners and roof edge where local pressures/suctions are very high.

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Alternatively, timber planks if available may be used for panelling particularly with timber posts.The planks and their connections with end posts shall be designed as per IS: 883-1970.

Note: In cyclone affected areas CGI may either be avoided or designed for the pressures andsuctions caused by cyclonic winds. AC sheeting will be vulnerable to missile impacts, hencepreferably avoided.

c. Bracing -Adequate diagonal bracings with strong end connections shall be provided insteel/timber framing in both the horizontal and vertical planes to improve their lateral loadresistance. In industrial buildings employing steel framing, at least the two end bays shallbe braced in the vertical and horizontal plane as per Pig.22. In timber framing it is normalpractice to brace each bay as well as to provide bracing in horizontal plane as shown inFig.24 so that complete structure is integrated.

d. Anchoring - The frame columns and shear wall where used shall be properly anchored in tothe foundation against uplift forces, as found necessary. For R.C. frames, usually a mono-lithic footing is provided which provides due stability against uplift. In case of steel fram-ing too, column posts are properly tied to steel/concrete footing through anchor bolts. Fortimber posts usually cross pieces are nailed at bottom end of the post and buried into theground to provide necessary anchorage (Fig.-25).

Special care is necessary for corrosion resistance of connectors used below the ground, particu-larly in cyclonic regions.

Fig. 24 Wind bracings in timber frames Fig. 25 Anchoring of wooden post usingcross pieces

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Floors

Floors usually carry no wind loads unless the building is constructed on stilts (in a cyclonicregion). The design is carried out for vertical loads only. For a building on stilts, flow of windunderneath the floor is possible, thereby causing wind forces (both uplift and suction type). Theforces as calculated using design wind pressures should be considered.

NON-ENGINEERED CONSTRUCTION

All constructions though using the conventional building materials but made intuitively with-out carrying out a proper structural design and or constructed without adequate control at site,with respect to both materials used and construction practices employed, may generally betermed as non-engineered construction. All constructions in low strength masonry or clay mud,and similar other forms of biomass, will fall under the category of non-engineered construction.The cyclone resistance of non- engineered construction may be improved by following suitableguidelines as given here below.

Roof Covering

i. In case of thatched roof it should be properly tied down to wooden framing underneath byusing organic or nylon ropes in diagonal pattern as shown in Fig 26 (a). The spacing of ropeshould be kept 450 mm or less so as to hold down the thatch length. For connecting thewooden members, use of non corrodible fixtures should be made. If non-metallic elementsare used, these may need frequent replacement (See Fig. 26 b).

ii. Projection of roof to be minimised, say not more than 500 mm in high wind/cyclone areas,and, larger projections be properly tied. (see Fig.11).

iii. In case of roof tiles, the overlap joint along the edges should be provided in cement mortar.In cyclones areas, tiled roofs should be provided with restraining concrete bands at a spac-ing not exceeding 1.2 m, and connected to rafters as shown in Fig. 26 c & d. As alternativeto the bands, a cement mortar screed, reinforced with galvanised chicken mesh, may be laidover the entire tiled roof.

iv. A through and through tie of bamboo or timber, instead of m.s. flat, be provided along theedges of sheeted roofs, in addition to intermediate ties for long roofs (see Fig. 19c).

v. After a cyclone warning is received, all the lighter roofs should preferably be held down bya rope net and properly anchored to ground (see. Fig. 27).

Low Strength Masonry Construction

General

Two types of construction are included herein, namely:

a. brick construction using weak mortar, such as clay mud or lime-sandb Random rubble and half-dressed stone masonry construction using different mortars such

as clay mud or lime-sand.

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Raise the ground to provideda platform

Fig. 26 a Gable type roof house Fig. 26 b Conical roof house

Fig. 26 c Wooden member connection details

Fig. 26 d Connection ofconcrete strip to rafter

Fig. 26 Have a secure roof jointing Fig. 27 Rope tie-backs for weak structures

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These constructions should not be permitted for important buildings in cyclone areas and shouldpreferably be avoided for ordinary buildings. Where used, the following precautions should betaken:

a. It will be useful to provide damp-proof course at plinth level to stop the rise of pore waterinto the superstructure.

b. Precautions should be taken to keep the rain water away from soaking into the wall so thatthe mortar is not softened due to wetness. An effective way is to take out roof projectionsbeyond the walls by about 500 mm.

c. Use of a water-proof plaster on outside face of walls will enhance the life of the buildingand maintain its strength at the time of cyclone or high wind as well (see 7.4).

d. Free standing boundary walls should be checked against overturning under the action ofdesign wind pressures allowing for a factor of safety of 1.2.

Brick work in Weak Mortars

a. The fired bricks should have a compressive strength not less than 3.5 MPa. Strength ofbricks and wall thickness should be selected based on the total building height.

b. The mortar should be lime-sand (1:3) or clay mud of good quality.c. The minimum wall thickness should be one brick in one storey construction and one brick

in top storey and 1.5 brick in bottom storeys of upto three storey construction. It should alsonot be less than 1/16 of the length of wall between two consecutive perpendicular walls orbuttresses.

Stone Masonry (Random Rubble or Half-Dressed)

a. The mortar should be lime-sand (1:3) or clay mud of good quality.b. The wall thickness 'f should not be larger than 450 mm. Preferably it should be about 350

mm, and the stones on the inner and outer wythes should be interlocked with each other.c. The masonry should preferably be brought to courses at not more than 600 mm lift.d. Through' stones of full length equal to wall thickness should be used in every 600 mm lift at

not more than 1.2 m apart horizontally. If full length stones are not available, stones in pairseach of about 3/4 of the wall thickness may be used in place of one full length stone so as toprovide an overlap between them.

e. In place of 'through' stones, 'bonding elements' of steel bars 8 to 10 mm dia bent to S-shapeor as hooked links may be used with a cover of 25 mm from each face of the wall (see Fig.28). Alternatively, wood bars of 38 mm x 38 mm cross section or concrete bars of 50 mm x50 mm section with an 8 mm dia rod placed centrally may be used in place of 'through'stones. The wood should be well treated with preservative so that it is durable againstweathering and insect action.

f. Use of 'bonding' elements of adequate length should also be made at corners and junctionsof walls to break the vertical joints and provide bonding between perpendicular walls.

g. Height of the stone masonry walls (random rubble or half-dressed) should be restricted to 2storeys in lime-sand mortar and one storey when clay mud mortar is used, the storey heightto be kept 3.0 m maximum, and span of walls between cross walls to be limited to 5.0 m.

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h. If walls longer than 5 m are needed, buttresses may be used at intermediate points notfarther apart than 4.0 m. The size of the buttress be kept of uniform thickness. Top widthshould be equal to the thickness of main wall, and the base width equal to one sixth of wallheight.

Openings in Bearing Walls

a. Door and window openings in walls reduce their lateral load resistance and hence shouldpreferably, be small and more centrally located. The total width of all openings should notexceed one-third of total length of a wall.

b. Openings in any storey shall preferably have their top at the same level so that a continuousband could be provided over them including the lintels throughout the building.

Strengthening Arrangements for High Wind Resistance

a. R.C. Bands. The walls should be reinforced with reinforced concrete bands as specified in6.5.3.

b. Wooden Band. As an alternative to reinforced concrete band, the band could be providedusing wood beams of two parallel pieces with cross elements as shown in Fig.29.

Fig. 28 a Sectional plan of wall Fig. 28 d Cross-section of wall

Fig. 28 Through stone and bond elements

Fig. 29 Wooden band for low strength masonry earthen buildings

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Earthen Buildings

General

a. For the safety of earthen houses, appropriate precautions must be taken against the actionsof rain and flood waters and high winds. Minimum precautions are recommended herein.

b. Whereas dry clay block is hard and strong in compression and shear, water penetration willmake it soft and weak, the reduction in strength could be as high as 80 percent. Hence, oncebuilt, ingress of moisture in the watts must be prevented by roof projection and water proofplastering.

`c. The following recommendations are low-cost and do not include the use of stabilizers,which are rather costly though effective in increasing the strength and water-resistance ofthe clay units or walls. Where feasible lime-stabilized compacted clay blocks or cement-stabilized sandy soil blocks may be used with compatible stronger mortars.

Construction of Earthen WallsEarthen walls may be constructed in the following four ways.

a. Hand-formed in layers using mud-lumps to form walls.b. Built by using sun-dried blocks or adobe which may be cut from hardened soil, or formed in

moulds, or moulded and compacted and laid in courses using clay mud as mortar.c. Built by using rammed earth in which moist soil is filled between wall forms and com-

pacted manually or mechanicallyd. Constructed using wood, bamboo or cane structure encased in clay mud, or wood, bamboo,

cane or ikra mesh enclosures plastered with mud.Whereas systems (a), (b), (c) depend on the strength of earthen walls for stability, the sys-tem (d) behaves like wood frame.

Recommendations for Cyclone Areas

a. The height of the earthen building should be restricted to one storey only in cyclone areaand to two storeys in other zones. Important building should not be constructed with earthenwalls.

b. The length of a wall, between two consecutive walls at right angles to it, should not begreater than 10 times the wall thickness t nor greater than 64 t2/h where h is the height ofwall.

c. When a longer wall is required, the walls should be strengthened by intermediate verticalbuttresses.

d. The height of wall should not be greater than 8 times its thickness.e. The width of an opening should not be greater than 1.20 m.f. The distance between an outside corner and the opening should be not less than 1.20 m.g. The sum of the widths of openings in a wall should not exceed one third the total wall

length in cyclone areas and 40 percent in other areas.h. he bearing length (embedment) of lintels on each side of an opening should not be less than

300 mm.i. Hand-formed walls should preferably be made tapering upwards keeping the

minimumthickness 300 mm at top and increasing it with a batter of 1:12 at bottom.

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j. The footing should preferably be built by using stone or fired brick laid with lime mortar.Alternatively, it may be made in lean cement concrete with plums (cement sand gravel:stones as 1:4:6:10) or without plums as 1:5:10. Lime could be used in place of cement inthe ratio lime: sand :gravel as 1:4: 8.

k. Plinth Masonry. The wall above foundation up to plinth level should preferably be con-structed using stone or burnt bricks laid in cement or lime mortar. Clay mud mortar may beused only as a last resort.

The height of plinth should be above the flood water line or a minimum of 300 mm aboveground level. It will be preferable to use a waterproofing layer in the form of waterproof mud(see 7.4) or heavy black polythene sheet at the plinth level before starting the construction ofsuperstructure wall. If adobe itself is used for plinth construction, the outside face of plinthshould be protected against damage by water by suitable facia or plaster. A water drain shouldbe made slightly away from the wall to save it from seepage.

Strengthening of Earthen Buildings Against High Wind/Cyclones

a. Collar Beam or Horizontal Band. Two horizontal continuous reinforcing and binding beamsor bands should be placed, one coinciding with lintels of door and window openings, andthe other just below the roof in all walls in cyclone areas. Where the height of wall is notmore than 2.5 m, the lintel band may be omitted. Also only the band be-low the roof may beused in other zones. Proper connection of ties placed at right angles at the corners andjunctions of walls should be ensured. The bands could be in the! following forms:

Unfinished rough cut or sawn (70 x 150 mm in section) lumber in single pieces diagonal members provided for bracing at corners (see Fia.30a)

Unfinished rough cut or sawn (50 x 100 mm or 7' x 70 mm in section) lumber two pieces inparallel with halved joints at corners and junctions of wall placed in parallel (See Fig.30b)

Fig. 30 a Band with single timber and diagonal brace at corner

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Fig. 30 b Band with two timbers in parallel

Fig 30 Wooden band in walls at lintel and roof levels

In each case, the lengthening joint in the elements shall be made using iron-straps with sufficientnails/screws to ensure the strength of the original lumber at the joint.

Pilasters and Buttresses

Where pilasters or buttresses are used, at corner or T-junctions,.the collar beam should cover thebuttresses as well, as shown in Fig. 31. Use of diagonal struts al corners will further stiffen thecollar beam.

Earthen Constructions with Wood or Cane Structures

The scheme of earthen construction using structural framework of wood or cane, as shown inFig.32 consists of vertical posts and horizontal blocking members of wood or large diametercanes or bamboo the panels being filled with cane, bamboo or some kind of reed matting plas-tered over both sides with mud.

Fig. 31 Roof band on pillastered walls

Where pilasters or buttresses are usedat corner or T junctions, the collarbeam should caver the buttresses aswell. Used of diagonal struts at cornerswill further stiffen the collar beam.

Fig. 32 Earthen on site construction with cane,bamboo or wooden structure

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Plastering and Painting

The purpose of plastering and painting is to give protection and durability to the low strengthmasonry walls (6.5) and earthen walls (7.3) and thatch roof, in addition to obvious aestheticreasons.

a. In dry areas, plastering based on natural additives could be formed in two layers. The firstone of about 12 to 15 mm, is a mixture of mud and straw (1:1 in volume), plus a naturaladditive like cowdung used to increase the moisture resistance of the mud, thus preventingthe occurrence of fissures during the drying process. The second and last layer is made withfine mud which when dried, should be rubbed with small, hard, rounded pebbles.

b. In cyclone prone and other wet areas, the walls should be covered with waterproof mudplaster. To obtain this, the following procedure may be followed:

"Cut-back should be prepared by mixing bitumen 80/100 grade and kerosene oil in the ratio5:1. For 1.8 kg cutback, 1.5 kg bitumen is melted and is poured in a container having 300millilitres kerosene oil, with constant stirring, till complete mixing. This mixture can nowbe mixed with 30 litres of mud mortar to make it both, water repellent and fire resistant.

c. For improving water and fire resistance of thatch roof, the water proof plaster may be ap-plied on top surfaces of the thatch, 20 to 25 mm thick, and allowed to dry. It may then becoated twice with a wet mixture of cowdung and waterproof plaster in the ratio of 1:1, andallowed to dry again.

d. The exterior of walls after plastering and thatch roof after treatment as explained above maybe suitably painted using a water-insoluble paint or washed with water solutions of lime orcement or gypsum.

Framed Houses

GeneralThe following guidelines should be used in building framed houses:

a. The main framing should be made with timber posts, bamboos or hollow pipes, and, ensur-ing proper connections of post with eaves level beam and rafters, (see Fig.33).

b. Frames should be properly braced in both horizontal and vertical planes using knee bracesor using cross ties (Fig. 24)

Fig. 33 Wind bracing of frame

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Foundation

a. The drainage around the building be improved to prevent water collection tor the durability ofwalls and foundations.

b. All posts be properly anchored into the ground or reinforced cement footing. Alternatively,the posts with cross members connected at the tower end be embedded in ground (seeFig.34) by a minimum depth of 750 mm.

c. Walls be raised from a well compacted lean concrete bed or well compacted ground, from aminimum depth of 450 mm below the ground level (see Fig.35).

Softer the ground, deeper the posts shouldbe to withstand wind force

Fig. 34 Proper footings for timber post

Fig. 35 a Shallow foundation overloose soil

Fig 35 b Adequate depth of foundationsto reach natural firm soil or pile founda-

tions more desirable

RETROFITTING OF EXISTING BUILDINGS

For all the existing structures not having adequate cyclone resistance, appropriate to the zone inwhich located, retrofitting measures are advocated to reduce the risk of damage or failure.Some measures along with approximate cost as a proportion of the cost of the building are givenin Table 3 for preliminary guidance. These measures are based on the lessons learnt from thepost-cyclone damage surveys conducted in the past. While the recommendations can be more

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specific for engineered constructions, for non-engineered constructions the recommendationswould depend upon building typology, construction material and practices prevalent in the re-gion. The retrofitting guidelines will generally arise from the guidelines already given for newconstructions. Some measures are indicated here below. It is however recommended that forcyclone affected zones of the country, the retrofit measures be evolved through a detailed studybased upon building typology.

Engineered Constructions

In engineered constructions, the maximum wind forces should be evaluated as per the windcode and various elements checked for the worst combination of dead and live loads to identifythe points of weakness requiring retrofitting. Some points for special attention are indicated infollowing paras:

Roof

a. In case of light roofs (AC or CGI sheeting) connections near the edges should be strength-ened by providing additional U bolts. M.S. flat ties may be provided to hold down the roofin cyclonic regions. J-bolts if used earlier may be replaced by U-bolts.

b. All projections in roofs be properly checked for strength against uplift and tied down iffound necessary, particularly, if longer than 500 mm, (see Fig. 11).

c. All metallic connectors for different components of roof should preferably be of non-corro-sive material, or else must be painted and checked before each cyclone season and doubtfulones be replaced immediately.

d. There must be proper bracings (i) in the plane of rafters, in plan at eaves level, and, in thevertical plane of columns along both axes of the building in sufficient number of panelsdetermined by recalculation (see Fig. 22).

e. Flat roofs may be integrated to behave as horizontal diaphragms and either weighted downby dead weights or held down against uplift forces.

Framed Buildings

a. In case of a framed structure, the total system requires to be properly braced. If existinglateral strength or bracing is inadequate, braces be provided to improve the overall stability.

b. All roof trusses be properly connected to posts. Particularly in a cyclonic region this shouldbe done with the help of anchor bolts or metallic straps.

c. Undesirable openings in the walls specially near the comers or edges be closed perma-nently to improve the lateral support to the cross walls particularly in a cyclonic region.

Load Bearing Walls

a. Buttresses be provided to improve the lateral load resistance of long walls, achieving crosswall spacing to less than 5m, thus reducing the unsupported lengths.

b. The exterior perimeter may be belted all round by using ferro-cement plating in the span-drel wall portion between lintel and eave/roof levels.

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Table 3:Retrofitting Measures for Buildings and Structures to Increase Cyclonic Resistance

S. No. Type Retrofit/ Maintenance Measures Approximate cost as a proportion of cost of building

1. Non Engineered Building Thatched House

• Provisions of metal and nails at joints

• Holding down coir ropes • Replacement of worn out

fibre ropes

Retrofit – 4.5% Maintenance – 1%

2. Tiled Building • Concrete strips • Holding down rods • Metal straps for connection

to trusses • Provision of eaves holding

down angle/ metal strap • Maintenance replacement of

broken tiles, worn out bolts, metal straps, etc,

• R.C.C. holding down rafters

Retrofit – 8% Maintenance – 1%

3. Compound Wall Checking the available capacity and detailing retrofit measures consisting of reinforced concrete bends to obtain the required strength

Additional cost varies in the range of 25 to 60% of new construction satisfying the design requirements. Retrofitting cost + existing structure cost approximately equals the cost of new construction.

4. Lamp Masts • Provision of a foundation block and extending it upto a certain height above ground level to ensure natural frequency is greater than 1.5 Hz.

• Underground cables to reduce load on lamp mast/failure of masts by falling branches of tress.

Cost of individual lamp mast with foundation will be increased by 40 to 50%

5. Water Tanks Ferro-cement/Other Lightweight Tanks

Provision of holding down/preventing sliding etc.

Marginal

Glass Panelling

a. The size of large glass panes be reduced by adding battens at appropriate spacing.Large glass panes be strengthened by fixing adhesive tapes, along and parallel to diagonals, at100-150 mm spacings prior to each cyclone season. Alternatively, thin plastic film be pasted onboth faces of the panes to prevent shattering.

b. Protective cover in the form of mesh or iron grill be provided to prevent breakage ofglass panels by flying missiles.

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Door and Window Shutters

The locking arrangements for door and window shutters be strengthened to prevent opening ofdoors/windows during cyclone/gust causing failure of glass panels as well as adverse suctionon roofs.

Foundations

a. While checking the safety of a foundation, an allowance should be made for likely submer-gence of the foundation in a cyclone region by appropriately reducing the safe bearingcapacity of soil.

b. Proper drainage around the building should be provided to prevent pooling of water in itsvicinity.

c. The plinth should be protected against erosion by using pitching of suitable type.

Non Engineered Constructions

a. In case of thatched roof it should be properly tied to timber framing on underside. Use ofmetallic/synthetic connectors is desirable. Use of water proof mud plaster may be made tomake it leak proof.

b. In case of tiled roofs, the overlaps be jointed through use of cement mortar to provide morestability.

c. While relaying of roofs, its slope be changed to about 20 to 30° to reduce the wind suctionon roof and thus reducing the damage potential. At the same time, eave level wooden bandshould be introduced on top of walls (Fig. 29-31).

d. The wooden frame where used should be properly braced in both horizontal and verticalplanes by using knee braces or ties.

e. All mud walls have a limited life after which they need to be rebuilt and the suggestedstrengthening by bamboo mesh placed at the middle can be affected only then. However,for the existing walls such mesh may be provided on the inner face and the wall replastered.

f. For greater durability of wall against rain and water etc., external face of wall upto 1.0 to1.5 m height above plinth level should be covered with burnt clay tiles laid in cementmortar of 1:6 mix.

g. The roof rafters be properly tied to posts using metallic strap connectors.h. All openings very close to wall edges be closed. All asymmetric non-closable openings be

filled up to eliminate any unfavourable roof pressure from within. Two small vents in oppo-site walls close to the roof may be left open.

i. If the foundations of the posts are not made heavy enough to prevent uprooting of thebuilding, it is advisable that before the cyclone season a protective net be provided on theroof and securely tied to the ground to prevent flying away of roof/building.

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PROPOSED AMENDMENT IN TOWN & COUNTRYPLANNING LEGISLATIONS, LAND USE ZONING REGU-

LATIONS, DEVELOPMENT CONTROLREGULATIONS & BUILDING BYELAWS

Anand S. AryaProfessor Emeritus, Deptt. of Earthquake Engg., I.I.T. Roorkee

National Seismic Advisor, GoI-UNDP (DRM) New Delhi

INTRODUCTION

Realizing that much of destruction during the earthquakes, namely Latur Maharashtra Earth-quake 1993, Jabalpur Earthquake of 1997, Chamoli Earthquake of 1999 and the major earth-quake in Kutch Gujarat in 2001, has been due to the buildings constructed without earthquakesafety measures as specified in Indian Standard Building Codes, the Ministry of Home AffairsGOI, appointed an Expert Group (consisting of a Senior Town Planner, five Architects and sixStructural Engineers) to study the existing Municipal Byelaw's etc. and propose model Byelawsand regulations to be incorporated in the various legal documents for saving the constructionsfrom earthquake and other hazards. The Expert Group studied at the town and country planninglegislations, development control regulations as well as building byelaws adopted in severalstates in the past. The Expert group submitted its report in two volumes, Volume I dealing withtown and country planning legislation, land use zoning regulations, development control regu-lations, and building byelaws in model form which could be adopted by the States and the Citiesby incorporating them in their existing documents. Volume II consisted of all the documentsstudied by the Expert Group. This chapter presents the same in abridged form for ready refer-ence. The reader is encouraged to go through Volume I of this report available in hard as well assoft copy from the NDM division of Ministry of Home Affairs, Government of India, NorthBlock, Central Secretariate New Delhi.

AMENDMENT IN MODEL TOWN & COUNTRY PLANNING ACT, 1960

Definition Under Section 2

(a) Natural Hazard: The probability of occurrence, within a specific period of time in a givenarea, of a potentially damaging natural phenomenon.

(b) Natural Hazard Prone Areas: Areas likely to have (i) moderate to very high damage riskzone of earthquakes, OR (ii) moderate to very high damage risk of cyclones OR (Hi) significantflood flow or inundation, OR (iv) landslide potential or proneness, OR (v) one or more of thesehazards.

Chapter 21

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Note:

Moderate to very high damage risk zones of earthquakes are as shown in Seismic Zones III, IVand V specified in 15:1893; moderate to very high damage risk zones of cyclones are thoseareas along the sea coast of India prone to having wind velocities of 39 m/s or more as specifiedin IS:875(Part 3) and flood prone areas in river plains (unprotected and protected) are indicatedin the Flood Atlas of India prepared by the Central Water Commission, besides, other areas canbe flooded under conditions of heavy intensity rains, inundation in depressions, back flow indrains, inadequate drainage, etc. as identified through local surveys in the Development Plan ofthe area and landslide prone areas as identified by State Government/Local surveys.

(c) Natural Disaster: A serious disruption of the functioning of a society, causing widespreadhuman, material or environmental losses caused due to earthquake, cyclone, flood or landslidewhich exceeds the ability of the affected society to cope using only its own resources.

(d) Mitigation: Measures taken in advance of a disaster aimed at decreasing or eliminating itsimpact on society and on environment including preparedness and prevention.

State Planning Board

Section 4: Functions and Power of the Board

4(2) (a): direct the preparation of Development plans keeping in view the natural hazard prone-ness of the area by Local Planning Authorities

Section 11 Functions and Power of Local Planning Authorities

11 a) an Existing Land Use Map indicating hazard proneness of the area;11 b) an Interim Development Plan keeping in view the Regulations for Land Use Zoning for Natural Hazard Prone Areas;11 c) a Comprehensive Development Plan keeping in view the Regulations for Land Use Zoning for Natural Hazard Prone Areas;

Section 18 Interim Development Plans

18(2)(a) indicate broadly the manner in which the planning authority proposes that land in sucharea should be used Keeping in view the natural hazard proneness of the area;

Section 19 Comprehensive Development Plan

19(2) The Comprehensive Development Plan keeping in view the natural hazard proneness ofthe area shall-

Section 20 Development Plan

Prepared prior to the application of this Act to be deemed Development Plan under this Act;If any local authority has been declared as a planning authority for a planning area & the localauthority has prepared a development plan for the planning area before the application of thisAct to that area, the development plan already prepared may be deemed to be a developmentplan under Section 18 or Section 19 of this Act. However, when such plans are implemented

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due care should be taken while formulating the projects based on such plans to follow theRegulations pertaining to Land Use Zoning and necessary protection measures prescribed bythe Regulations.

Section 29 Prohibition of Development without payment of Development Charges and withoutPermission

29(2): Any person or body (excluding a department of Central or State Government or localauthority) intending to carry out any development on any land shall make an application inwriting to the planning authority for permission in such form and containing such particularsand accompanied by such documents and plans as may be prescribed by the rules or the regula-tions including Development Control, Building Regulation/Byelaws for Natural Hazard ProneAreas.Provided that in the case of a department of Central or State Government or local author-ity (where the local authority is not also the planning authority) intending to carry out anydevelopment on any land, the concerned department or authority, as the case may be, shallnotify in writing to the planning authority of its intension to do so, giving full particulars thereofand accompanied by such documents and plans "complying with development control, buildingregulations/bye-laws for natural hazard prone areas" as may be prescribed by the State govern-ment from time to time

Section 73 Power to make Regulations

73(e) any other matter which has to be or may be prescribed by rules under Section 72(1),Development Control and Building Regulations/Byelawsfor Natural Hazard Prone Areas;

73(f) any other matter which has to be or may be prescribed by regulation including Regulationfor Land Use Zoning for Natural Hazard Prone Areas.

a. Referred & Amendments suggested by identifying the relevant clauses in:b. Model Regional and Town Planning and Development Laws 1985c. Model Urban and Regional Planning and Development Law (Revised)

(Part if UDPFI Guidelines)

RECOMMENDATIONS FOR LANDUSE ZONING REGULATIONS

The regulations for Land Use Zoning for Natural Hazard Prone Areas are to be notified undersection

1. u/s 73(f) of Model Town & Country Planning Act, 1960; OR2. u/s 143(f) of Model Regional and Town Planning and Development Law; OR3. u/s 181(f) of Model Urban & Regional Planning and Development Law

(Revised) of UDPFI Guidelines as may be applicable in the respective States under the existingprovisions of Town & Country Planning Legislation as and when Master Plan/ DevelopmentPlan of different cities/ town/ areas are formulated. However, these zoning regulations are to beimplemented through the provisions of Development Control Regulations/ Building Bye-Laws,wherever the Master Plan are not in existence or not formulated.

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Classification of urban land uses

It is based upon the requirements of the various plans. For example, a perspective plan, which isa policy document, need not show many details of a specific land use and may only show themain use which could be, say, residential or commercial. In the case of a development plan,which is a comprehensive plan indicating use of each parcel of land, there is a need to showmore details of a specific land use. It has to indicate for the land designated as, say, commercial,the further details as to which land is for retail commercial, or for wholesale trade or for godowns.In the case of layouts of projects of a shopping centre further details shall be necessary, indicat-ing which block of retail commercial is for, say, cloth or electronics or vegetables. There couldbe three levels in land use classification shown under:

Level I For Perspective PlansLevel II For Development PlansLevel III For Layouts of Projects/Schemes LAND USE ZONING

Objectives of Land Use Zoning

1) The main purpose of the land use zoning is to provide regulations for development of aparticular area to serve the desired purpose efficiently and to preserve its character. It alsoprovides for the kind of buildings to be constructed. Zoning regulations are legal tools forguiding the use of land and protection of public health, welfare and safety.

2) Such regulations also include provisions for the use of premises /property and limitationsupon shape, size and type of buildings that are constructed or occupy the land. Further, theseprovide both horizontal as well as vertical use of land.

These regulations also improve the quality of life in urban centres For instance in flood zones,the land use may be parks, playground & gardens while restricting any building activity in suchvulnerable areas.

3) Life line structures should also be protected likewise while either proposing land uses orotherwise.4) Zoning protects residential areas from harmful invasions of other uses like indus-trial use and commercial use. It does not prohibit use of lands and buildings that are lawfullyestablished prior to coming into effect of such zoning regulations. If such uses are contrary toregulations in a particular 'use zone' and are not to be allowed, such uses are designated as 'non-conforming uses'. These are to be gradually eliminated without inflicting unreasonable hard-ship on the property owners/users.

4) The suggested list of uses/activities for various use zones should be comprehensive, keepingin mind the local and special characteristics of various sizes of settlements (large, medium andsmall). Depending upon the specific situation this list could be further enhanced or reduced, asthe case may be.

State Perspective Plan/Regional Plan

Development Plan (Master Plan/Zonal Development Plan)

While formulating Perspective Plan/Regional Plan, Development Plan (Master Plan/Zonal De-

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velopment Plan) for any notified area, the proposals should indicate, Natural hazard prone areaswith the type and extent of likely hazards.

Areas not Covered Under Master Plan

1) In such areas where there are no Master Plans or Development Plans, general guidelines &recommendations on natural disaster mitigation should be issued to the various local bodies,Municipalities and Town Area Committees and Panchayats to enable them to take these intoconsideration while siting various projects and deciding on construction of buildings etc.

2) Technical help may be required by some of the local bodies in implementation of the recom-mendations and for interpretation of the guidelines.

Earthquake Prone Areas

1) Macro Seismic Zones III, IV & V

2) Area liable to liquefaction have greater risk.

3) Those hilly areas which are identified to have poor slope stability conditions and wherelandslides could be triggered by earthquake or where due to prior saturated conditions, mudflow could be initiated by earthquakes and where avalanches could be triggered by earthquakewill be specially risk prone.

4) Special risky areas have to be determined specifically for the planning area under consider-ation through special studies to be carried out by geologists and geo-technical engineers.

Cyclone prone areas

1) Those areas likely to be subjected to heavy rain induced floods or to flooding by sea-waterunder the conditions of storm surge, are specially risky.

2) Areas under those where special risk have to be identified by special contour survey of theplanning area under consideration and study of the past flooding and storm surge history of thearea. Survey of India or locally appointed survey teams, and by reference to the Central WaterCommission, Government of India and the department of the State or U.T dealing with thefloods.

Flood prone areas

1) These are in river plains (unprotected and protected by bunds) are indicated in the FloodAtlas of India prepared by the Central Water Commission and reproduced on larger scale in thestate wise maps in the Vulnerability Atlas of India.

2) Besides, other areas can be flooded under conditions of heavy intensity rains, inundation indepressions, backflow in drains, inadequate drainage, failure of protection works, etc. Thesehave to be identified through local contour survey and study of the flood history of the planningarea (Survey of India or local survey teams, and by reference to the Central Water Commissionand the departments of the state or U.T dealing with the floods).

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Land Slide Prone Areas

1) The susceptibility of the various areas to landslide varies from very low to very high. Land-slide zoning naturally requires mapping on large scale. Normally medium scale of 1:25000 is atleast chosen.

2) In preparation of the landslide zone map, two types of factors are considered important aslisted here below:

i) Geological/Topographic Factors/Parameters

- Lithology, Geological Structures/Lineaments, Slope-dip (bedding, joint) relation,- Geomorphology, Drainage, Slope angle, slope aspect and slope morphology,- Land use, Soil texture and depth, Rock weathering

ii) Triggering Factors

Rainfall, Earthquake, Anthropogeny

Alternatives

a) Leaving the area unprotected. In this case it will be necessary to specify Land Use Zoning forvarious development purposes as recommended.

b) Using protection methods for the areas as a whole or in the construction of buildings, struc-tures & infrastructure facilities to cater for the hazard intensities likely in the planning area.

c) It will be appropriate to prioritise buildings, structures & infrastructures in terms of theirimportance from the point of view of impact of damage on the socio-economic structure of thesociety as recommended under Regulation no. 6.

In regard to Land Use Zoning, different types of buildings and utility services are groupedunder three priorities as indicated below.

Priority 1. Defence installation, industries, public utilities, life line structures like hospi-tals, electricity installations, water supply, telephone exchange, aerodromes and railway sta-tions; commercial centres, libraries, other buildings or installations with contents of high eco-nomic value.

Priority 2. Public and Semi Public institutions, Government offices, and residential areas.

Priority 3. Parks, play grounds, wood lands, gardens, green belts, and recreational areas.

i. Installations and Buildings of Priority 1 to be located above the levels corresponding to a100 year flood or the maximum observed flood levels whichever higher.

ii. Buildings of Priority 2 to be located outside the 25 year flood or a 10 year rainfall contour,provided that the buildings if constructed between the 10 and 25 yearcontours should haveeither high plinth level above 25 year flood mark or constructed on columns or stilts,with ground area left for the unimportant uses;

iii. Activities of Priority 3 viz. play grounds, gardens and parks etc. can be located in areasvulnerable to frequent floods.

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In order to ensure environmentally sound development of hill towns, the following restrictionsand conditions may be proposed for future activities.

1) An integrated development plan may be prepared taking into consideration environmentaland other relevant factors including ecologically sensitive areas, hazard prone areas, drainagechannels, steep slopes and fertile land.

2) Water bodies including underground water bodies in water scarces areas should be protected.

3) Where cutting of hill slope in an area causes ecological damage and slope instability inadjacent areas, such cuttings shall not be undertaken unless appropriate measures are taken toavoid or prevent such damages.

4) No construction should be ordinarily undertaken in areas having slope above 30° or areaswhich fall in landslide hazard zones or areas falling on the spring lines and first order streamsidentified by the State Government on the basis of available scientific evidence.

5) Construction may be permitted in areas with slope between 10° to 30° or spring rechargeareas or old landslide zones with such restrictions as the competent authority may decide.

Open Spaces

Out of the open spaces ear-marked as district parks, neighborhood parks and local parks in thedevelopment plan, zonal plans and local plans, suitable and approachable parks/ open spacesshould be identified for the use during the emergency to provide shelter and relief caused by anatural hazard. Such pockets should be clearly marked on the city maps.

AMENDMENT IN DEVELOPMENT CONTROL REGULATIONS

This part deals with the development control rules and general building requirements to ensurehealth and safety of the public. The regulations for Land Use Zoning in Hazard Prone Areas areto be taken into consideration while formulating the Development Plan and Area Plan under theTown Planning and Urban Development Act.

Every person who gives notice under relevant section of the Act shall furnish all information informs and format prescribed herein and as may be amended from time to time by the CompetentAuthority. The following particulars and documents shall also be submitted along with theapplication.

1) The forms, plans, sections and descriptions to be furnished under these Development ControlRegulations shall all be signed by each of the following persons:

- A person making application for development permission under relevant section of the Act.- A person who has prepared the plans and sections with descriptions who may be Architect on Record or Engineer on Record.- A person who is responsible for the structural design of the construction i.e. a Structural Engineer on Record.- A Construction Engineer on Record who is to look after the day-today supervision of the construction.- A Developer, Promoter

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2) A person who is engaged either to prepare plan or to prepare a structural design and structuralreport or to supervise the building shall give an undertaking:-

Certificate in the prescribed Form No.l by the "Owner, Developer, Structural Engineer on Recordand Architect on Record"; Form No.2 by the "Architect on Record"/ "Engineer on Record"; andForm No. 3 by the "Structural Engineer on Record; Form No. 4 by the Construction Engineer onRecord" as prescribed in Appendix B.

No land shall be used as a site for the construction of building-

i) If the site is found to be liable to liquefaction by the Competent Authority under the earth-quake intensity of the area, except where appropriate protection measures are taken.

ii) If the Competent Authority finds that the proposed development falls in the area liable tostorm surge during cyclone, except where protection measures are adopted to prevent stormsurge damage.

iii) In hilly terrain, the site plan should include location of land slide prone areas, if any, on ornear the site, detected during reconnaissance. The Authority in such case shall cause to ensurethat the site is away from such land slide prone areas.

iv) The site plan on a sloping site may also include proposals for diversion of the natural flow ofwater coming from uphill side of the building away from the foundation.

Grant or Refusal of the Permission for Development

On receipt of the application for Development Permission, the Competent Authority after mak-ing such inquiry and clearance from such an expert whenever considered necessary for thesafety of building, as it thinks fit may communicate its decisions granting with or without con-dition including condition of submission of detailed working drawing/ structural drawing alongwith soil investigation report before the commencement of the work or refusing permission tothe applicant as per the provisions of the Act.

The Competent Authority, however, may consider to grant exemption for submission of work-ing drawing, structural drawing and soil investigation report in case the Competent Authority issatisfied that in the area where the proposed construction is to be taken, similar types of struc-ture and soil investigation reports are already available on record and such request is from anindividual owner/developer, having plot of not more than 500 sq mt. in size and for a maximum3 storeyed residential building.

If the local site conditions do not require any soil testing or if a soil testing indicates that nospecial structural design is required, a small building having upto ground + 2 floors, having loadbearing structure, may be constructed.If the proposed small house is to be constructed with loadbearing type masonry construction technique, where no structural design is involved, no certifi-cate from a Structural Engineer on Record will be required (to be attached with Form No.2).However, a Structural Design Basis Report (Form No. 6), has to be submitted, duly filledin.Notwithstanding anything stated in the above regulations it shall be incumbent on everyperson whose plans have been approved to submit revised (amended) plans for any structural

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deviations he proposes to make during the course of construction of his building work and theprocedure laid down for plans or other documents here to before shall apply to all such Revised(amended) plans.

RECOMMENDATIONS FOR AMENDMENT IN BUILDING BYELAWSList of BIS Codes to be Complied with

For General Structural SafetyFor Cyclone/Wind Storm ProtectionFor Earthquake ProtectionFor Protection of Landslide HazardA list is given in Annexure 2.

In compliance of the design with the above Indian Standard, the Structural Engineer on Recordwill submit a structural design basis report in the Proforma attached herewith (Annexure 3)covering the essential safety requirements specified in the Standard. The"Structural DesignBasis

Report (SDBR)"consists of four partsPart-1 - General Information/ DataPart-2 - Load Bearing Masonry BuildingsPart-3 - Reinforced Concrete BuildingsPart-4- Steel Buildings

This report is to accompany the application for Building Development Permission.

Structural Design Review Panel

- The Competent Authority shall create a Structural Design Review Panel (SDRP) consistingof senior SER's and SDAR's whose task will be to review and certify the design prepared bySER or SDAR whenever referred by the competent authority.

- The Reviewing Agency shall submit addendum to the certificate or a new certificate in caseof subsequent changes in structural design.

- Table-1 gives requirements of SDRP for different seismic zones namely III, IV and V andfor structures of different complexities

- In seismic Zone II, buildings & structures greater than 40m in height will require proofchecking by SDRP as per detail at si. no.03 of Table 1.

Supervision

All construction except load bearing buildings upto 3 storeys shall be carried out under thesupervision of the Construction Engineer on Record (CER) or construction Management Agencyon Record (CMAR) for various seismic zones.

Certification of Structural Safety in Construction

CER/CMAR shall give a certificate of structural safety of construction as per proforma given inForm-13 at the time of completion.

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Inspection

All the construction higher than 7 storeys, public building & special structures shall be carriedout under quality inspection program prepared and implemented under the Quality Auditor onRecord (QAR) or Quality Auditor Agency on Record (QAAR) in Seismic Zones IV & V

Certification of Safety in Quality of Construction

Quality inspection to be carried on the site shall be worked out by QAR/QAAR in consultationwith the owner, builder, CER/CMAR. QAR/QAAR shall give a certificate of quality control asper proforma given in Form - 15

Other Issues

Vol I of the Expert Group also covers the following issues:

a). Structural Requirements Of Low Cost Housingb). Inspection

- General Requirements- Record of Construction Progress- Issue of Occupancy Certificate

c). Protective Measures In Natural Hazard Prone Areasd). Registration Of Professionalse). Appointment Of Professionalsf). Protection Against Hazardg). Registration, Qualifications And Duties Of Professionalsh). General Duties And Responsibilities Applicable To All Professionals

Certificates

1) Certificate Of Undertaking For Hazard Safety Requirement2) Certificate Of Undertaking Of Architect On Record/Engineer On Record3) Certificate Of Undertaking Of Structural Engineer On Record (Ser)4) Certificate Of Undertaking Of The Construction Engineer On Record5) Development Permission6) Structural Design Basis Report7) Progress Certificate8) Progress Certificate - First Storey9) Progress Certificate - Middle Storey In Case Of High-Rise Building10) Progress Certificate - Last Storey11) Completion Report12) Building Completion Certificate By Architect On Record13) Building Completion Certificate By Construction Engineer on Record14) Building Completion Certificate By Structural Engineer On record15) Model Proforma For Technical Audit Report16) Structural Inspection Report

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Annexure - 1Documents Referred - 1

(i)Town and Country Planning Legislation

a. Model Town & Country Planning Act 1960b. Model Regional and Town Planning and Development Laws 1985c. Model Urban and Regional Planning and Development Law (Revised) (Part if UDPFI Guide

lines)d. Legislation on Earthquake Safety in the State of Uttaranchal.

Document Referred - 2

Land use Zoning, Development Control and Building Regulationsa. Land use Zoning and Protection of Buildings of Essential Services Guidelines for Disaster

Preventions (document prepared by BMTPC/ADPC)b. Review of Current State Legislation on Earthquake Safety in the State of Uttaranchal - a

study conducted by BMTPC-ADPC.c. Development Control Rules, Master PlanRegulations & Building Bye-laws in the local bodies of Uttaranchal - a Study conducted byBMTPC-ADPC.d. Development Control Regulations of Ahmedabad Urban DevelopmentAuthority (AUDA)

e. Development Control Regulations of Mumbaif. Development Control Regulations of Puneg. Development Control Regulations of Delhih. Draft National Building Code - Part 2 pertaining to administration, and Part 4 per

taining to fire & life safety.

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Annexure - 2

- IS: 456:2000 "Code of Practice for Plain and Reinforced Concrete- IS: 800-1984 "Code of Practice for General Construction in Steel- IS:801-1975 "Code of Practice for Use of Cold Formal Light Gauge Steel Structural Mem-

bers in General Building Construction- IS:875 ( Part 2):1987Design loads ( other than Eq.) for buildings &structures Part2 Imposed

Loads- IS:875 ( Part 3):1987Design loads ( other than Eq.) for buildings and structures Part 3 Wind

LoadsIS:875 ( Part 4):1987Design loads ( other than Eq.) for buildings and structures Part4 Snow LoadsIS:875 ( Part 5):1987Design loads ( other than Eq.) for buildings and struc-tures Part 5 Special loads and load combination

- IS:883:1966 "Code of Practice for Design of Structural Timber in Building- IS:1904:1987 "Code of Practice for Structural Safety of Buildings: Foundation"- 1S:1905:1987 "Code of Practice for Structural Safety of Buildings: Masonry- IS 2911 (Part 1): Section 1: 1979 "Code of Practice for Design and Construction of Pile

Foundation Section 1Part 1: Section 2 Based Cast-in-situ PilesPart 1: Section 3 Driven Precast Concrete PilesPart 1: Section 4 Based precast Concrete PilesPart 2: Timber PilesPart 3: Under Reamed PilesPart 4: Load Test on Piles

IS: 875 (3)-1987 "Code of Practice for Design Loads (other than Earthquake) for Buildings andStructures, Part 3, Wind Loads"

Guidelines (Based on IS 875 (3)-1987) for improving the CyclonicResistance of Low rise houses and other building- IS:1893-2002 "Criteria for Earthquake Resistant Design of Structures (Fifth Revision)"- 1S:13920-1993 "Ductile Detailing of Reinforced Concrete Structures subjected to Seismic

Forces - Code of Practice"- 1S:4326-1993 "Earthquake Resistant Design & Construction of Buildings - Code of Practice

(Second Revision)"- 1S:13828-1993 "Improving Earthquake Resistance of Low Strength Masonry Buildings -

Guidelines"- 1S:13827-1993 "Improving Earthquake Resistance of Earthen Buildings- Guidelines",- 1S:13935-1993 "Repair and Seismic Strengthening of Buildings - Guidelines"IS:14458 (Part

1): 1998 Guidelines for retaining wall for hill area: Part 1 Selection of type of wall.- IS:14458 (Part 2): 1997 Guidelines for retaining wall for hill area: Part 2 Design of retaining/

breast walls- IS:14458 (Part 3): 1998 Guidelines for retaining wall for hill area: Part 3 Construction of dry

stone walls- IS:14496 (Part 2): 1998 Guidelines for preparation of landslide - Hazard zonation maps in

mountainous terrains.

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Annexure - 3

STRUCTURAL DESIGN BASIS REPORT

1. This report to accompany the application for Building Development Permission.2. In case information on items 3, 10, 17, 18 and 19 can not be given at this time, it should be

submitted at least one week before commencement of construction.

Part 1 General DataS.No. Description Information Notes1 Address of the building

- Name of the building- Plot number- Subplot number- TPS scheme

a. Nameb. Number

- Locality/Township- District

2 Name of owner3 Name of Builder on record4 Name of Architect/Engineer

on record5 Name of Structural engineer

on record6 Use of the building7 Number of storeys above ground

level (including storeys to beadded later, if any)

8 Number of basements belowground level

9 Type of structure- Load bearing walls- R.C.C frame- R.C.C frame and Shear walls- Steel frame

10 Soil data- Type of soil- Design safe bearing capacity IS: 1893 Cl. 6.3.5.2IS: 1904

11 Dead loads (unit weight adopted)- Earth- Water- Brick masonry

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- Plain cement concrete- Reinforced cement concrete- Floor finish- Other fill materials- Piazza floor fill and landscape IS: 875 Part 1

12 Imposed (live) loads- Piazza floor accessible to Fire Tender- Piazza Floor not accessible to Fire Tender♥ - Floor loads♦ - Roof loads IS: 875 Part 2

13 Cyclone / Wind- Speed- Design pressure intensity IS: 875 Part 3

14 Seismic zone IS:1893 2002)15 Importance factor IS:1893 (2002)

Table 616 Seismic zone factor(Z) IS:1893 Table 217 Response reduction factor IS: 1893 Table-718 Fundamental natural period- approximate IS: 1893 Cl. 7.619 Design horizontal acceleration spectrum IS: 1893 Cl. 6.4.2

value (Ah)20 ♠ Expansion / Separation Joints

♥ Enclose small scale plans of each floor on A4 sheets ♦ Incase terrace garden is provided, indicate additional fill load and live load ♠ Indicate on a small scale plan on A4 sheet

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Bldg

Part 2 Load bearing masonry buildings

1

Building category

IS:4326 Cl. 7 read with IS: 1893

II III IV V Ordinary

B C D E

Important

C D E E

2 Basement Provided 3 Number of floors including Ground

Floor (all floors including stepped floors in hill slopes)

4 Type of wall masonry 5 Type and mix of Mortar IS:4326 Cl. 8.1.2 6 Re: size and position of openings

(See note No.1) • Minimum distance (b5) • Ratio (b1+b2+b3)/l1 or

(b6+b7)/l2 • Minimum pier width

between consequent opening (b4) • Vertical distance (h3) • Ratio of wall height to

thickness4 • Ratio of wall length

between cross wall to thickness

IS:4326 Table 4, Fig.7

7 Horizontal seismic band • at plinth level • at window sill level • at lintel level • at ceiling level • at eave level of sloping roof • at top of gable walls • at top of ridge walls

P 1 1 1 1 1 1 1

IP 1 1 1 1 1 1 1

NA 1 1 1 1 1 1 1

(see note no.2) IS:4326 Cl. 8.4.6 IS:4326 Cl. 8.3 IS:4326 Cl. 8.4.2 IS:4326 Cl. 8.4.3 IS:4326 Cl. 8.4.3 IS:4326 Cl. 8.4.4

8 Vertical reinforcing bar • at corners and T junction of

walls • at jambs of doors and

window openings

1 1

1 1

1 1

IS:4326 Cl. 8.4.8 IS:4326 Cl. 8.4.9

S.No. Description Information Notes

zone

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351

Part 3 Reinforced concrete framed buildings

1 Type of Building i Regular frames i Regular frames with Shear walls i Irregular frames i Irregular frames with shear walls i Soft storey

IS: 1893 Cl. 7.1

2 Number of basements 3 Number of floors including ground floor 4

Horizontal floor system i Beams and slabs i Waffles i Ribbed Floor i Flat slab with drops i Flat plate without drops

5 Soil data i Type of soil i Recommended type of foundation - Independent footings - Raft - Piles i Recommended bearing capacity of soil i Recommended, type, length, diameter and load capacity of piles i Depth of water table i Chemical analysis of ground water i Chemical analysis of soil

IS: 1498

6 Foundations

i Depth below ground level i Type

§ Independent § Interconnected § Raft § Piles

7 System of interconnecting foundations i Plinth beams i Foundation beams

IS: 1893 Cl. 7.12.1

8 Grades of concrete used in different parts of building

9 Method of analysis used 10 Computer software used 11 Torsion included IS: 1893 Cl. 7.9

Slno

Description Information Notes

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12 Base shear a. Based on approximate fundamental

period b. Based on dynamic analysis c. Ratio of a/b

IS: 1893 Cl. 7.5.3

13 Distribution of seismic forces along the height of the building

IS:1893 Cl. 7.7 (provide sketch)

14 The column of soft ground storey specially designed

IS:1893 Cl. 7.10

15 Clear minimum cover provided in • Footing • Column • Beams • Slabs • Walls

IS: 456 Cl. 26.4

16

Ductile detailing of RC frame • Type of reinforcement used • Minimum dimension of beams • Minimum dimension of columns • Minimum percentage of reinforcement

of beams at any cross section • Maximum percentage of reinforcement

at any section of beam • Spacing of transverse reinforcement in

2-d length of beams near the ends • Ratio of capacity of beams in shear to

capacity of beams in flexure • Maximum percentage of reinforcement

in column • Confining stirrups near ends of

columns and in beam-column joints a. Diameter b. Spacing • Ratio of shear capacity of columns to

maximum seismic shear in the storey

IS: 456 Cl. 5.6 IS:13920 Cl. 6.1 IS:13920Cl. 7.1.2 IS: 456 Cl. 26.5.1.1(a) IS:13920 Cl. 6.2.1 IS: 456 Cl. 26.5.1.1(b) IS:13920 Cl. 6.2.2 IS: 13920 Cl. 6.3.5 IS: 456 Cl. 26.5.3.1 IS: 13920 Cl. 7.4

General Notes 1. A certificate to the effect that this report will be completed and submitted at least one month before commencement of

Construction shall be submitted with the application for Building Development Permission. 2. In addition to the completed report following additional information shall be submitted, at the latest, one month

before commencement of Construction 2.1 Foundations

2.1.1 Incase raft foundation has been adopted indicate K value used for analysis of the raft 2.1.2 Incase pile foundations have been used give full particulars of the piles, type, dia, length, capacity 2.1.3 Incase of high water table indicate system of countering water pressure, and indicate the existing water table,

and that assumed to design foundations. 2.2 Idealization for Earthquake analysis 2.2.1 Incase of a composite system of shear walls and rigid frames, give distribution of base shear in the two systems on the

basis of analysis, and that used for design of each system. 2.2.2 Indicate the idealization of frames and shear walls adopted in the analysis with the help of sketches. 2.3 Submit framing plans of each floor 2.4 Incase of basements, indicate the system used to contain earth pressures

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Part 4 Buildings in Structural Steel

1 Adopted method of Design O Simple IS: 800; Cl. 3.4.4O Semi-rigid IS: 800; Cl. 3.4.5O Rigid IS: 800; Cl. 3.4.6

2 Design based on O Elastic analysis IS: 800; Section-9O Plastic analysis SP: 6 (6)

3 Floor Construction O CompositeO Non-compositeO Boarded

4 Roof Construction O CompositeO Non-compositeO MetalO Any other

5 Horizontal force resisting O Frames Note: Seismic forcesystem adopted O Braced frames As per IS: 1893Would

O Frames & shear depend on systemwalls

6 Slenderness ratios maintained Members defined in IS: 800; Cl. 3.7Table 3.1, IS: 800

7 Member deflection limited to Beams, Rafters IS: 800; Cl. 3.13Crane GirdersPurlinsTop of Columns

8 Structural members O Encased in IS: 800; Section-10Concrete

O Not encased

9 Proposed material O General weld-able IS: 2062O High strength IS: 8500O Cold formed IS: 801, 811O Tubular IS: 806

10 Minimum metal thickness O Hot rolled sections IS: 800, Cl. 3.8Specified for corrosion O Cold formed sections Cl. 3.8.1 to Cl. 3.8.4protection O Tubes Cl. 3.8.5

Cl. 3.8.511 Structural connections O Rivets IS: 800; Section-8

O C T Bolts IS: 1929,2155,1149

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O S H F G Bolts IS: 6639, 1367O Black Bolts IS: 3757, 4000O Welding- IS: 1363, 1367

Field IS: 816, 814, 1395,Shop 7280, 3613, 6419(Specify welding 6560, 813, 9595type proposed)

O Composite

12 Minimum Fire rating O Rating —— hours IS: 1641, 1642, 1643Proposed, with method O Method proposed-

- In tumescentPainting- Spraying- Quilting- Fire retardantboarding

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ESSENTIAL DETAILS IN STRUCTURAL DRAWINGS

Anand S. AryaProfessor Emeritus, Deptt. of Earthquake Engg., I.I.T. Roorkee

National Seismic Advisor, GoI-UNDP (DRM) New Delhi

INTRODUCTION

The essential structural details required in the drawings of reinforced concrete multi-storeybuildings are briefly outlined below for the guidance of the practicing architects, the structuralengineers and the municipal engineers. All the details may not be covered in this brief notewhich may have been learnt through the course of this training programme and may be added bythe trainers and the trainee engineers.

ARCHITECTURAL DRAWINGS

As the structural safety under earthquake depends to a large extent on the configuration adoptedby the architect, the following items may be considered most important in the architecturaldrawings:-

(i) Proper load path from the roof to the foundation without discontinuity in the columns/shearwalls. That is, use of floating columns on cantilever beams is undesirable.

(ii) Discontinuity in the provision of infill wall panels whether in the ground storey or in anyintermediate storey. Wherever this has to be adopted special design of the columns in suchstoreys will require larger size and other continuity details.

(iii) To achieve equal seismic resistance along both longitudinal and transverse direction of thebuilding, the larger dimension of the column should be oriented along both axes to the e x -tent of about 50% each. Alternatively, shear walls will need to be provided along bothaxes of the building, preferably on the periphery of the building.

(iv) Buildings should either be founded on raft or individual column footings must be connected along both axes of the building using substantial beams at foundation or plinth level.

(v) Water tanks, elevator machine rooms, parapets etc., that is, elements projecting above theroof will need to be supported by substantial RC columns.

(vi) The RC columns should have minimum dimension of the cross section not less than 300mm.

STRUCTURAL DRAWINGS

Normally the structural drawings are prepared in the following sequence:-ST-00 :- General instructions and detailing of reinforcement.

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356

This drawing covers general notes, abbreviations used, concrete mix to be adopted, cover to beadopted on various elements like slab, beam, column & foundation, overlap length, camber to beadopted in slab & beam, construction details like overlapping positions, beam column junctiondetails, sketched showing RC footing, column detailing upto roof, shape of stirrups to be used inbeam & column etc. (see dwg. ST-00 attached herewith)

ST-01 :- Column & Footing details

This gives the dimensions of all column & footing as well as the stirrup details to be adopted inthe column (see dwg. ST-01 attached herewith).

ST-02 :- Plinth beam details

This shows plan of plinth beams and reinforcing details of the various plinth beams, their longitu-dinal as well as transverse sections (see dwg. ST-02 attached herewith).

ST-03 :- First Floor Reinforcement details (Roof of ground storey)

This shows the reinforcement of the slab and beams including longitudinal & transverse sections(see dwg. ST-03 attached herewith).

ST-04,5,6………n :- Details of each floor and roofST-n+1:- Other element detailsStructural drawings of other elements like staircases, staircase mumty, elevator machine rooms,over head water tanks, cantilever balconies etc. to be furnished.

Note:- The checking engineer should verify the anchorage and continuity of reinforcementthrough all beam column junctions, ensuring that the reinforcement of beams & columns includ-ing their anchorages can indeed be contained in the joint with enough space for concreting andconcrete compaction. In this process the beam bars should not be required to be kinked to enterthe connections. A bar with a kink will not remain effective to resist the tension which thereinforcement is supposed to resist under vertical or lateral load.

357

DO'S AND DON'TS

BEFORE AN EARTHQUAKE

While constructing new buildings, follow building codes and other sound practices to minimizeearthquake disaster. Build on firm ground or go right up to the bed rock level when layingfoundations. Avoid filled up areas for construction as far as possible.

Place large and heavy objects at either ground level or in lower shelves of storage almirahs etc.

Do not stack glass or crystal-ware, as slight shaking will topple it.

Anchor overhead lighting fixtures appropriately to check their fall in shaking.

Provide strong support to gas and power appliances.

Teach responsible members of your family how to turn off electricity, gas, and water at mainswitches and valves. (Check with your utilities office for instructions).

Provide for responsible members of your family to receive basic first aid instructions becausemedical facilities may be overloaded immediately after a severe earthquake. call your local redcross or civil defense authorities for information about such training.

DURING AN EARTHQUAKE

Remain calm and think through the consequences of any action you plan to take. try to calm andreassure others.

Encourage others to follow your example and do not run outside in panic.

If indoors, watch for falling plasters, bricks/stones light fixtures, high book cases, shelves andothers cabinets, which might slide or topple.

Stay away from glass windows, mirrors, chimneys and other projecting parts of the building.

If in dander, get under a table, desk or bed in a corner away from the window.

If outside, avoid being close to high buildings, walls, power poles and other objects that couldfall.

If possible move to an open area from hazards.

If in an automobile, stop at a safe place.

AFTER AN EARTHQUAKE

Check for injuries, do not attempt to move seriously injured persons unless they are in immedi-ate dander of further injury.

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Check for fires

Wear shoes in all areas near debris and broken glass

Check service lines and appliances for damage. do not use matches or lighters until it has beenestablished that there are no gas leaks.

Draw moderate quantity of water in case service is disrupted. do not draw large quantity as thiscould interfere with fire fighting operation

Do not eat or drink anything from open containers, specially near shattered glass

Be prepared for additional earthquake shocks called 'aftershocks'. although most of these aremuch smaller than the main shock some may be large enough to cause additional damage.

Respond to requests for help from civil, defence, fire services, police and home guards

Do not crowd damaged areas unless help has been requested. cooperate with the public safetyofficials.

Do not spread rumors, they often do great harm following disasters.

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