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1.1 BASIC PRINCIPLES OF REINFORCED CONCRETE

Concrete and steel have widely differing basic properties. Plain concrete can safely resist compressive forces and is also good in durability and fire resistance but weak in tension. Steel is good in resisting tension, compression and shear but poor in durability and fire resistance. An ideal combination of both steel and concrete can overcome the deficiencies of the individual materials. The composite material christened as ‘Reinforced Concrete’ resists both compressive and tensile forces developed in flexural and compressive members under external loads. The coefficients of thermal expansion for steel and concrete are of the order of 10 × 10−6 per oC and 7 to 12 × 10−6 per oC respectively. These values are sufficiently close that problems with bond seldom arise from differential expansion between the two materials over normal temperature ranges. The composite action of this material in structural elements is attributed to the bond between concrete and steel reinforcements which ensures strain compatibility so that the external loads on the structural elements is shared by steel and concrete without disruption of the composite material.

The reinforcing steel imparts ductility to concrete which is a brittle material and this property is essential in structural elements to prevent explosive failures. The early 20th century witnessed significant improvements in the development and use of reinforced concrete mainly due to the production of good quality of concrete and steel with desirable surface characteristics to facilitate superior bond between the two materials. Reinforced concrete has established itself as an universally suitable composite material ideally suited for structural members like slabs, beams, columns, walls, footings, water tanks, retaining walls, stair cases, electric poles, pipes, piles and dams, pavements, marine structures, swimming pools, cooling towers, bunkers, silos, chimneys, tunnels, shells and folded plates.

The development of reliable design and construction procedures during the 20th century has paved the way for extensive use of reinforced concrete in the construction industry throughout the world. The amenability of concrete to be cast in various shapes and with attractive surface characteristics is a salient feature for preference of this material by architects and engineers in comparison with other materials. Reinforced concrete is a structural material with desirable properties like, mould ability, strength, elasticity, durability and impermeability, good resistance to static, fatigue and dynamic loads.

Reinforced concrete has established itself as an universally economical material mainly due to the sustaining efforts of various research workers during the last two centuries resulting in acceptance as a standard material codified for widespread use by the leading countries of the world.

1

General Features of Reinforced Concrete

C H A P T E R

1

2 ReinfoRced concRete StRuctuRal elementS

1.2 HISTORICAL EVOLUTION

The development in the field of reinforced concrete is attributed to the continuous research work done during the last 150 years. Soon after the invention of Portland cement by Joseph Aspdin in 1845, Joseph Monier1, a Parison gardener embedded iron and steel rods in concrete and made garden posts and tubs of reinforced concrete reinforced with an iron mesh heralding the birth of Reinforced Concrete in 1849. In 1853, French Engineer, Francois Coignet2 built a three storied building using reinforced concrete shown in Fig. 1.1. The detailing of steel reinforcements prevailing during the period of Coignet is shown in Fig. 1.2. The first pedestrian reinforced concrete foot bridge built by Joseph Monier in 1875 is outlined in Fig. 1.3. The first reinforced concrete sky scrapper shown in Fig. 1.4 was built by E.L. Ransome in 1903.

Joseph Lambot3, a French Engineer constructed a boat using concrete and iron rods and exhibited it in the Paris Exhibition in 1854 and applied for a patent in 1855 for reinforced concrete beams and columns containing four round iron rods. Willium B. Wilkinson4 of England took out a patent for reinforced Concrete floors in 1854. Several investigators like Talbot5, Taylor et al6, Turneaure7, Morsch8, have worked incessantly for more than half a century for the development of reinforced concrete. An excellent example of the application of reinforced concrete in bridge construction attempted by Hennebique in 1900 is shown in Fig. 1.5

Fig. 1.1. First Reinforced Concrete Building Built by Francois Coignet in 1853

A

B

C

E

D

Coignet Beam and Slab Beam Supporting Floor Slab Centering

GeneRal featuReS of ReinfoRced concRete 3

Section ofCoignet Column

Plan of Coignet Column Coignet Pipe at Grinjur

Coignet Beam ReinforcementConsisting of Group of Links

Fig. 1.2. Reinforced Concrete Detailing—Coignet System 1855–1860

Fig. 1.3. Chazelet Pedestrian Foot Bridge built by Joseph Monier in 1875


Fig. 1.4. First Reinforced Concrete Sky Scrapper (1903) built by E.L. Ransome

Fig. 1.5. Typical Reinforced Concrete Bridge Designed by F. Hannebique (1899-1900)

The earliest text book in English entitled “Principles of Reinforced Concrete Construction” authored by Frederick E. Turneaure and E.R. Maurer of the University of Wisconsisn was published by John Wiley & Sons, New York in 1907. The cover page is shown in Fig. 1.6.


P R I N C I P L E S

OF

REINFORCED CONCRETE

CONSTRUCTION

By

AND

F. E. TURNEAUREDew of the collage of engineering, University of

E. R. MAURERProfessor of

F I R S T E D I T I O N

FIRST THOUSAND

NEW YORK

JOHN WILEY & SONS

London: CHAPMAN & HALL, Limited1907

LIBRARYOF THE

UNIVERSITY

OF

Fig. 1.6. First Book on Principles of Reinforced Concrete Construction (1907)

During the first few decades of the 20th century, several books on reinforced concrete were authored by engineering scientists, the prominent among them are Reinforced concrete construction by Adams & Mathews9 in 1911, Fundamental Principles of Reinforced Concrete by George Hool10 in 1912, Reinforced Concrete Design, Theory & Practice in two volumes by Faber & Bowie11 during 1912–1920. Added to this, Concrete Engineers Hand Book by Hool12 in 1918 and Concrete Designers Manual by Whitney et al13 in 1921 provide valuable information on reinforced Concrete published during the early period of 20th century.

The quality of cements produced gradually improved during the early decades of the 20th century with the development of mass production of good quality cement. The development of prestressed


concrete by Freeyssinet in 1950, paved the way for production of high quality and high strength cements coupled with vibration techniques for compacting concrete resulting in higher strength and durability for hardened concrete. At present Ordinary Portland cement of various strengths designated as C-33, C-43, & C-53 are available for use in different types of structures. Different types of cements with specific properties have been developed for use in the construction of highways, marine structures, multi-storey buildings & industrial structures. The last few decades of the 20th century witnessed phenomenal developments in the field of high strength cements.

Significant developments during the early part of 20th century resulted in improved quality of concrete and steel. Cement was mass-produced with quality control & improved methods of proportioning concrete mixes resulted in concrete of desired compressive strength ranging from 15 N/mm2 to 60 N/mm2. Phenomenal developments in the field of concrete technology has paved the way for production of high strength, ultra high strength, high performance14 and Nano concretes15 having a compressive strength in the range of 120 to 200 N/mm2.

The Prussian regulations comprising the complete set of design rules of reinforced concrete appeared in 1907. While the French commission on reinforced concrete had formulated the design rules in 1906, professional societies like the American Concrete Institute (ACI) and the American Society of Civil Engineers (ASCE) introduced the first joint code on reinforced concrete in 1909.

The first major application of reinforced concrete was in bridges mainly due to the economy in comparison with steel bridges. The elastic method of design was firmly established & widely used during this period, The rebuilding of bridges & buildings during the post war periods resulted in establishing reinforced concrete as an economical structural material for use in different types of structures. However, the inadequacy of the elastic or working load design in predicting the ultimate loads of a structure paved the way for the ultimate load theories & design based on ultimate loads computed by applying load factors to the working loads.

Various Investigators like Emperger (1936) Whitney (1937) Jenson (1943) Chambaud (1949) Hognestad16 (1951) and Evans17 developed the ultimate load design based on different types of stress blocks. Reinforced concrete structures designed solely on the basis of ultimate load theory resulted in slender structural elements and their serviceability characteristics (deflections & cracks) under working loads were not within the codified acceptable limits.

The ultimate load method of design ensures the safety of the structures against the collapse limit state only and as such does not give any information about the behaviour of the structure at service loads and the range between service and collapse loads. The inadequacy of the ultimate load method in not ensuring the serviceability of the structure resulted in the development of Limit State design18.

The deficiencies in elastic and ultimate load design resulted in the evolution of limit state design philosophy19,20, first incorporated in the Russian code in 1955. Basically, limit state design is a method of designing structures based on a statistical concept of safety and the associated statistical probability of failure. Limit state design is based on the concept of probability and comprises the application of the method of statistics to the variations that occur in practice in the loads acting on the structure and the strength of the materials.

The Limit state design overcomes the inadequacies of the working stress and ultimate load methods and ensures the safety of the structure against excessive deflections and cracking under service loads and also provides for the desirable load factor against failure. Hence, the British Code21, American Code22, Australian Code23, German Code24, Canadian Code25 and the Indian Standard Code26 have adopted the limit State design concepts.


1.3 STRUCTURAL DESIGN PHILOSOPHY

The Philosophy of structural design incorporated in the various national and International codes specifies that any design should comply with the following essential requirements. 1. Structures designed should satisfy the criterion of desirable ultimate strength, in flexure, shear,

compression, tension and torsion developed under a given system of loads and their combinations. In addition, the stresses developed in the structure under the given system of loads should be within the safe permissible limits under service loads.

2. The structure designed should satisfy the criterion of serviceability, which limits the deflections and cracking to be within acceptable limits. The structure should also have adequate durability and impermeability, resistance to acids, corrosion, frost etc.

3. The structure should have adequate stability against overturning, sliding, buckling, and vibration under the action of loads.

A satisfactory structural design should ensure the three basic criteria of strength, serviceability and stability. In addition, the structural designer should also consider aesthetics and economy. The structural designer and the architect should co-ordinate so that the structures designed are not only aesthetically superior, but also strong enough to safely sustain the designed loads without any distress during the life time of the structure.

1.4 STRUCTURAL APPLICATIONS OF REINFORCED CONCRETE

Reinforced concrete is a well established construction material often preferred to steel mainly due to its universal adaptability, versatility, coupled with resistance to fire and corrosion resulting in negligible maintenance costs. Development of better quality cements during the last few decades has resulted in stronger, durable and specific types of concretes suited for different types of structures and environmental conditions.

Structural elements of Reinforced concrete are preferred in the construction of floor and roof slabs, columns and beams in residential and commercial and industrial structures. The present trend is to adopt reinforced concrete for bridges of small, medium and long spans resulting in aesthetically superior and economical structures in comparison with steel bridges. Typical use of reinforced concrete in earth retaining structures includes retaining walls for earthen embankments and abutments and piers for bridges.

In water retaining structures reinforced concrete is ideally suited for ground and overhead tanks and hydraulic structures like gravity and arch dams. The material is widely used for the construction of large domes for water tanks, pipes, poles, sports stadiums and conference halls.

For covering large areas like conference halls where column free space is an essential requirement, reinforced concrete grid floors comprising of beams and slabs are generally preferred. In spatial structures like aircraft hangers, factories and godowns, reinforced concrete thin shells are invariably used for economy.

Reinforced concrete folded plate construction has been used for industrial structures where large column free space is required under the roof. In coastal areas where corrosion is imminent due to humid environment, reinforced concrete is ideally suited for the construction of marine structures like wharfs, quay walls, watchtowers, and lighthouses. For warehouses in coastal areas, reinforced concrete trusses are preferred to steel trusses.


Power transmission structures like electric poles have completely replaced steel poles. Tall T.V. transmission towers are invariably constructed using reinforced concrete. Multi-storey reinforced concrete buildings are routinely adopted for both residential & office complexes. For heavy-duty floors in factories, reinforced concrete is ideally suited due to its resistance to wear and tear and improved durability.

Reinforced concrete in conjunction with prestressing is preferred to steel for pressure vessel construction in atomic power generation structures due to the superior radiation absorption characteristics of high strength and high density concrete. Reinforced concrete piles, both precast and cast in sites have been in use for foundations of structures of different types likes bridges and buildings. Reinforced concrete is widely used in transportation structures like pavements for highways and airport runways. Fibre reinforced concrete with thin structural elements is widely used for precast slabs, heavy duty factory floors to provide superior resistance to wear and tear.

Continuous research in the field of concrete has resulted in the development specific cements suitable for the production of ultra high strength concrete, high performance concrete and nano concrete with superior structural properties. The Twentieth century has witnessed reinforced concrete as a revolutionary material suitable for the construction of most simple to complex structures. With significant improvements in the quality of cement and steel, reinforced concrete will continue to find new applications and widespread use in the 21st century.

1.5 BASIC REINFORCED CONCRETE STRUCTURAL ELEMENTS & SYSTEMS

Reinforced concrete structural systems are basically assemblages of different types structural elements developed to perform the function of resisting various types of forces. The primary structural systems can be classified as 1. One dimensional elements comprising of beams, columns, arches etc. 2. Two dimensional elements such as, slabs, plates, grids, shells etc. 3. Three dimensional elements like pipes, domes, dams, retaining walls, Chimneys, Silos etc.

The most common types of structural systems are grouped as below:

1. One Way Slab Systems

Slabs supported on walls or beams at the opposite edges and transmitting the dead and live loads to the supports are considered as one way slabs. A typical one way slab is shown in Figure 1.7. The slabs are subjected to maximum bending at centre of span along the shorter span direction and maximum shear at the support sections under loads.

Slab

Support

Fig. 1.7. One Way Slab


2. Cantilever Slab Systems

Cantilevered slabs are commonly used at entrances of buildings, balconies and for rain water protection over windows and ventilators. The slabs cantilever out from a beam over the supports. A typical cantilever slab system is shown in Fig. 1.8.

WallSlab

Lintel beam

Fig. 1.8. Cantilever Slab

3. Two Way Slab Systems

Figure 1.9 shows a typical two-way slab floor or roof system commonly used in buildings. In this type, the slab rests on all the four sides near the edges and the loads are transmitted to all the supports. The slab is subjected to flexure in the two principal directions and shear forces are maximum near the supports.

Slab

x x

y

y

Section-xx

Sec

tion-y

y

Fig. 1.9. Two Way Slab

4. Continuous Slab Floor Systems

In this type, the slab is continuous over number of supports. The supports may be beams or walls spaced generally at regular intervals as shown in Fig. 1.10. These floor systems are commonly adopted to cover large spaces without using columns at short intervals.

Slab

Beam

Fig. 1.10. Continuous Slab


5. Beam and Slab Floor Systems

To cover large spaces like marriage halls and offices, it is common to adopt beam and slab floor and roof systems using main and secondary beams at regular intervals. The secondary beams rest on main beams in between the columns. The main beams are of larger depth between the main supports. A typical beam and slab floor system is shown in Fig. 1.11.

y

y

x xSlab

Beam

Slab

BeamSection-xx

Sec

tion-y

y

Fig. 1.11. Beam and Slab Floor

6. Coffered or Grid Floor Systems

Grid floor system comprising beams spaced at short intervals, spanning in perpendicular directions with a thin slab cast integrally with the beams to form the floor. Fig. 1.12 shows a typical grid floor system. Grid floors are employed to cover large areas and provide column free space between the end supports. If the area to be covered is large, the beams are prestressed in the transverse directions to reduce excessive deflections towards the centre of span.

yy

x

x

Section-yy

Sectio

n-x

x

Beams

Beams

Slab

Fig. 1.12. Grid Floor System

c hapter 1 general features of reinforced · pdf fileimprovements in the development and use...

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