This paper was presented at Symposium in honor of Professor Ugur Ersoy at Middle East Technical University, Ankara, Turkey, on July 1/2, 1999

SEISMIC RETROFITTING TECHNOLOGY FOR

REINFORCED CONCRETE BUILDINGS IN JAPAN

S. Otani* and T. Kaminosono** *Department of Architecture, Graduate School of Engineering, University of Tokyo, Tokyo, 113-8656, Japan **Building Research Institute, Ministry of Construction, Tsukuba, 305-0802, Japan Keywords: Earthquake damage, seismic diagnosis, seismic strengthening, carbon fiber, fiber reinforced plastics, reinforced concrete ABSTRACT: Development of seismic retrofitting technology for reinforced concrete buildings in Japan after the 1995 Hyogo-ken Nanbu earthquake was briefly reviewed. Significant damage was observed in buildings constructed before the enforcement of present Building Standard Law (revised in 1981). Consequently, a law to promote seismic strengthening of existing buildings was proclaimed in late 1995. The Ministry of Construction issued the guidelines for seismic diagnosis of existing buildings and for strengthening of deficient buildings. The Building Research Institute, Ministry of Construction, organized a three-year research program for the development of earthquake disaster reduction technology through the application of new technology in 1996. INTRODUCTION The earthquake resistant structural design technology has been improved with the development in earthquake engineering research. The structures built in more recent years can resist an earthquake motion with less damage. The 1995 Hyogo-ken Nanbu Earthquake, commonly known as the Kobe Earthquake Disaster, revealed the same with convincing statistics. Revision of Building Standard Law in 1981 The Building Standard Law was significantly revised in 1981 after an extensive study. Major points of the 1981 revision were listed below: (1) Design earthquake forces were specified (a) by story shear rather than horizontal floor forces, (b) as a function of the fundamental period of a structure and the dominant period of subsoil at the construction site. (2) Stresses in structural members must not exceed the allowable stresses of materials under design earthquake forces corresponding to a standard base shear coefficient Co of 0.20; the story drift angle under the design earthquake forces (Co=0.20) must not

exceed 1/200 of the story height (3) Story shear at the formation of collapse mechanisms under lateral loading must be larger than a value specified considering (a) story shear capacity corresponding to a standard base shear coefficient Co of 1.0, (b) reduction of the required story shear capacity by deformation capacity of yielding members, (c) amplification of the required story shear capacity by discontinuous stiffness distribution along the structural height, and (d) amplification of the required story shear capacity by eccentricity in plan between the centers of floor mass and stiffness. Japanese development of earthquake resistant building design can be found in Ref. 1. Damage statistics of reinforced concrete buildings in Kobe The Kinki Branch, Architectural Institute of Japan, investigated the damage of "all" existing reinforced concrete buildings (3,861 buildings in total) in a region of highest seismic intensity in Nada and Higashi-Nada wards, Kobe City (Ref. 2). The construction age was identified by (a) a plaque to commemorate the construction, (b) a plaque at the installation of lightning rods, (c) building records at fire stations, or (d) detailed aerial photographs taken in 1971 and 1984. Damage levels were grouped into six levels in the investigation, but this paper uses three integrated damage levels: i.e., (a) "operational damage" requiring no repair work for continuous use immediately after the earthquake, (b) "heavy damage" requiring significant repair and strengthening work for occupancy after the earthquake, and (c) "collapse." Among those 2,017 buildings constructed before the current Building Standard Law revised in 1981, 7.4 percent suffered "heavy damage" and 7.8 percent collapsed. Among those 1,844 buildings constructed in conformance with the current Building Standard Law, 4.0 percent suffered "heavy damage" and 2.1 percent collapsed. The 1981 revision of the Building Standard Law enhanced significantly the performance of reinforced concrete buildings against earthquake attack. The relation between damage level and the number of stories is shown in Tables 1 and 2 for buildings constructed before and after the enforcement of the 1981 Building Standard Law. The ratio of severer damage increases with the number of stories, especially for buildings taller than 5 stories. The percentage of the pre-1981 buildings, which remained operational after the earthquake, is 87.9 percent for 1- to 5-story buildings, 66.1 percent for 6- to 8-story buildings, and 46.0 percent for 9-story and higher buildings. The corresponding percentages of the post-1981 buildings are 97.1, 87.1 and 70.7 percent, respectively. The revision of the law improved the level of safety almost uniformly from low-rise to mid-rise buildings. It is obvious that the pre-1981 buildings need to be strengthened. If the target performance level of a building under an earthquake of intensity as strong as the Kobe Earthquake is selected to be "operational," it should be noted, however, that those requiring strengthening work are only 15 percent of the pre-1981 buildings. Therefore, a simple but efficient screening procedure is desired to identify such vulnerable structures by the seismic diagnosis out of the existing building stock. PROMOTION OF SEISMIC STRENGTHENING

The 1995 Hyogo-ken Nanbu Earthquake caused severe damage to buildings and killed more than 6,300 people, including death by indirect causes after the earthquake. A large number of people were killed in detached houses and apartment buildings. More than one-half of the killed were older than 60 years; approximately 80 percent of the death were caused by the collapse of buildings. The earthquake occurred early in the morning; if the earthquake had occurred during the working hours, the death tolls must have significantly increased in office buildings and stores. Figure 1 shows the building stock before and after 1981 (Ref. 3). Approximately 12 million residential houses and 22 million non-residential buildings of the current building stock in Japan were constructed before 1981. Japanese Diet (Congress), recognizing the urgent importance of improving seismic resistance of existing buildings, proclaimed a law to promote the seismic strengthening of existing buildings on October 27, 1995 (Ref. 4). The law was enforced on December 25, 1995. Specially designated buildings The law requires that the owner of a building (specially designated building) for use by a number of people must make efforts to perform the seismic diagnosis (examination of safety under a severe earthquake motion) of the structure. The law also requires that the owner must make efforts to strengthen the structure if needed. The specially designated building is defined as an existing building that does not satisfy earthquake resistant building requirements of the current Building Standard Law, and that has a size more than two-stories high and an area more than 1,000 m2. The use of the specially designated building includes schools, gymnasiums, hospitals and medical offices, theaters, assembly halls, exhibition halls, department stores, banks, offices, markets and stores, hotels, restaurants, rental apartment buildings, dormitories, nursery homes, public baths, factories, transportation stations, automobile garages, government offices and others. A number of visitors other than the owner may frequently use these buildings. Therefore, the owner is believed to be responsible for the safety of the people in the building. Government efforts The governor of a prefecture, mayor of a city or a town, and headman of a village (administrative agency) may give necessary guidance, advice and instruction to the owner of a specially designated building about the seismic diagnosis and strengthening of the building. The administrative agency can direct the owner of hospitals, theaters, assembly halls, exhibition halls and department stores of an area larger than 2,000 m2, to submit the report of seismic diagnosis and of the progress in seismic strengthening work. The administrative agency can send its officers to inspect the building and direct the owner to take necessary actions. The law requires that the Minister of Construction should publish the guidelines for seismic diagnosis of existing buildings and for seismic strengthening of deficient buildings. The seismic diagnosis should be performed for each story and in each principal direction of the structure considering the story shear and story-drift capacities. National government must make efforts to collect necessary technical information on seismic diagnosis and strengthening methods and disseminate the information to the public in demand. National and municipal governments must make efforts to deepen

people’s understanding of the necessity of earthquake strengthening through publicity activities and public education. Approval of seismic strengthening plan Those who intend to seismically retrofit an existing building must apply for the approval of the plan to the administrative agency concerned; the agency may approve the plan if it satisfies requirements of the Building Standard Law and the seismic diagnosis and strengthening guidelines. Some requirements of Building Standard Law may be waived to facilitate the seismic strengthening of a structure; e.g., the placement of structural walls for seismic strengthening may violate some fire proofing requirements of the Building Standard Law. Financial support In order to promote the seismic strengthening of existing buildings as an urgent policy, it is essential to provide financial backing measures. The interest on loan for the approved strengthening work may be set at a lower rate at the Housing Financial Corporation; the interest rate is to be decided by the cabinet order. National and municipal governments must assist the owner of a specially designated building to find financial loans and provide necessary information to promote the seismic diagnosis and strengthening works. GUIDELINES FOR SEISMIC DIAGNOSIS AND STRENGTHENING The Ministry of Construction issued the notification on the guidelines for seismic diagnosis and strengthening of a specially designated building on December 25, 1995 (Ref. 4). The notification covers the safety of structural members, roofing members and building facilities. The location, configuration, dimensions, fastening, corrosion, and material strengths must be carefully investigated at the site as well as on architectural and structural drawings. The seismic diagnosis technology was developed after the 1968 Tokachi-oki Earthquake which caused shear failure in short reinforced concrete columns especially in school buildings. The reinforced concrete construction had been accepted in Japanese society as fire resistant and earthquake resistant after the 1923 Kanto (Tokyo) Earthquake. The damage frightened the reinforced concrete researchers. An extensive series of tests were carried out in laboratories of government, universities and industry to investigate the method to improve shear strength. The requirements of lateral reinforcement in columns were tightened in 1971. The methodology to examine the seismic safety of existing buildings was developed by the Ministry of Education for school buildings, Ministry of Construction for government buildings, and construction companies for their client buildings. The standard for seismic diagnosis for reinforced concrete buildings was published by the Japan Building Disaster Prevention Association in 1977 (Ref. 5). The guidelines for seismic diagnosis and strengthening by the Ministry of Construction were based on this standard. Structural seismic index

The structural seismic index sI of a story is calculated by the following expression;

tes

os RZF

EI = (1)

where, oE : index which defines the seismic resistance (combination of strength and deformation capacities) of a story, Z : seismic zone factor (0.7 to 1.0 for four seismic zones in Japan), tR : vibration characteristic factor, and esF : structural configuration factor, representing the distribution of stiffness and mass in a story. The seismic zone factor, vibration characteristic factor, and structural configuration factor are the same as those prescribed in the Building Standard Law Enforcement Order. Vibration characteristic factor The vibration characteristic factor tR (Fig. 2) represents the shape of design earthquake spectrum for three types of soil and is defined by Eq. (2):

TTforTTR

TTTforTTR

TTforR

cc

t

ccc

t

ct

≤=

<≤−−=

<=

26.1

2}1{2.00.1

0.1

2 (2)

where, cT : dominant period of subsoil (=0.4 sec for stiff sand or gravel soil, =0.6 sec for other soil, and =0.8 sec for alluvium mainly consisting of organic or other soft soil); T : natural period of a building. The natural period T may be estimated by Eq. (3):

HT 02.0= (3) where, H : total height of a reinforced concrete building in m. Structural configuration factor Structural configuration factor esF considers the amplification of required story resistance due to an irregular distribution of stiffness along the height of a structure and also due to a large eccentricity of mass center with respect to the center of rigidity in a floor plan. The structural configuration factor is calculated as the product of factors sF and eF representing the irregularity in stiffness distribution along height and eccentricity in plan, respectively, as given in Eq. (4):

eses FFF = (4) The regularity in stiffness distribution along structural height is judged by the value of rigidity ratio sR , defined by Eq. (5) at each story:

s

ss r

rR = (5)

in which, sr : reciprocal of drift angle at a story under design earthquake forces corresponding to Co=0.2, sr : average value of sr ’s at all stories. Factor sF is 1.0 for

sR ≥ 0.6, 2.0 for sR =0.0, and is interpolated in the range 0.0< sR <0.6 (Fig. 3.a). Eccentricity ratio eR is defined by Eq. (6) as a ratio of eccentricity e between the

center of mass and the center of stiffness to the elastic radius er of stiffness in the story;

ee r

eR = (6)

Mass center is determined from the column axial forces under gravity loads. Stiffness center is determined for the lateral stiffness of vertical members; the lateral stiffness of a vertical member is defined as a ratio of the shear to the inter-story drift of the member under design earthquake forces corresponding to Co=0.2. The elastic radius exr in the x-direction in plan is defined as the square root of the ratio of the torsional resistance to the sum of lateral resistance (Eq. 7);

∑∑ ∑+=

x

yxex K

xKyKr

22

(7)

where, xK and yK : lateral stiffness of a vertical member at distance x and y in x and y directions from the stiffness center. Factor eF is 1.0 for eR ≤ 0.15, 1.5 for

eR ≥ 0.30, and interpolated in the range 0.15< eR <0.30 (Fig. 3.b). Earthquake resistance index Columns and structural walls of a story must be classified into three groups by their deformation capacity index kF corresponding to their failure mode as given in Table 3, and story shear kQ carried by each group k is calculated (see Fig. 4). The index oE may be taken as the larger value of Eqs. (8) and (9);

ii

uo AW

FQE = (8)

iio AW

FQFQFQE

233

222

211 )()()( ++

= (9)

where, uQ : sum of shear carrying capacities of all vertical members under lateral loading, F : index to represent deformation capacity of representative lateral load carrying members (columns and structural walls) in a story (Table 3), iW : sum of dead and live loads above a story under consideration, iA : factor representing vertical distribution of a seismic story shear coefficient ( ii WQ /= ), given by Eq. (10),

TTA i

ii 31

2)1(1+

−+= αα

(10)

where iQ : story shear at story i, 1/WWii=α , and iW : sum of dead and live loads above story i, and 1W : total dead and live loads of the building. Equation (9) considers the energy dissipation by less ductile group of members before the failure of a story by the most ductile group of members (Fig. 4). For example, ductile columns can support the structure after the failure of less ductile structural walls. If the failure of some columns leads to the collapse of a story, the index F in Eq. (8)

must be taken as the value corresponding to the deformation capacity of such columns and the story shear capacity uQ must be evaluated at the failure of these columns; furthermore, Eq. (9) should not be used. Lateral force capacity index An index q of structural lateral force resisting capacity is defined by Eq. (11);

tites

u

SARZWFQq = (11)

where, tS : minimum base shear coefficient 0.30 required for reinforced concrete construction in the Building Standard Law Enforcement Order. Evaluation of structural safety The structural safety of a story is judged by the structural seismic index sI and the index q of structural lateral force resisting capacity as shown in Table 4. The structure may be considered to be safe when the structural seismic indices sI of every story are greater than 0.6 and the indices q of every story are greater than 1.0. Roofing materials should not fall off by the vibration during an earthquake. Chimneys and water tanks on the roof should have sufficient strength. Water supply and drainage facilities should be provided with sufficient strength for safety. Guidelines for seismic strengthening The existing building, judged to be “likely to collapse” during a strong earthquake by the seismic diagnosis guidelines, must be strengthened to the level “unlikely to collapse” at an earliest opportunity. Structural members must be placed regularly to effectively resist earthquake forces. If the retrofit work is carried out in several phases due to a financial limitation or a continued use of the building, the placement of structural walls must be carefully planned to avoid the eccentricity in stiffness in plan during the retrofit work. The foundation must be carefully examined to avoid overstress by the addition of dead loads associated with the retrofit work. The earthquake resistance can be improved by adding lateral load resistance and/or by improving deformation capacity of a structure. The lateral load resistance can be typically increased by placement of structural walls or steel bracing. The deformation capacity of structural members can be increased by shifting brittle failure mode to ductile failure mode; e.g., a column may be jacketed by steel plates or wrapped by FRP (fiber reinforced plastics) sheets to increase shear resistance, or a captured column may be separated from adjacent spandrel walls to lengthen deformable height of the column. Energy dissipating devices or base isolation devices may be installed to reduce the deformation response of a structure. INTEGRATED NATIONAL RESEARCH PROJECT (1995 - 1998) The Ministry of Construction initiated an Integrated National Research Project (1995-1998) for Development of New Technology for Earthquake Disaster Reduction in

Large Cities. As a part of the project, the Building Research Institute, Ministry of Construction, organized a research program for the development of earthquake disaster reduction methodology through the application of new technology for a period of April 1996 to March 1999. The Housing and Urban Development Corporation, construction companies and material producing companies participated in the research program. University researchers were invited to participate in the program. The research has been carried out in three subject areas; i.e., (a) development of methods to strengthen structural members, (b) development of methods to control structural response, and (c) development of methods to strengthen structural foundation. A strong emphasis was placed on the development of methodology for the retrofitting work without interrupting the use of a building or without significantly disturbing the resident by noise or vibration. The final products of the research program will be published soon. The work (Ref. 6) is briefly introduced below. Strengthening of reinforced concrete members The 1995 Hyogo-ken Nanbu Earthquake revealed the weakness of reinforced concrete columns designed in accordance with the Building Standard Law before 1971. Failure of reinforced concrete columns was primarily in shear attributable to the lack of lateral reinforcement. The failure of buildings took place in the form of concentrated damage in the soft story due to shear failure of columns. Two types of reinforcement are normally provided to enhance the shear capacity of reinforced concrete columns; (a) jacketing by steel plates and (b) wrapping by fiber reinforced plastics (FRP). The use of FRP sheets has merit of easy construction work and of light material weight. The methods for practical application of FRP sheets were studied in the research program. The strengthening work of reinforced concrete columns normally requires the removal of mortar and other finishing materials (tiles) from the concrete surface to effectively confine the concrete. The work causes noise, vibration and dust and the building may not be occupied during the retrofit work. In many occasions, this additional preparation work lengthens the duration of work and increases the cost. Therefore, the development of retrofit work without removing existing finishing materials is highly desired. The guidelines for retrofit design and construction work were drafted for the strengthening of reinforced concrete members using FRP sheets. The research results are introduced in a separate section. A full-scale test was carried out to study the effectiveness of strengthening of a reinforced concrete frame by attaching precast concrete panels using prestressing rods. Research on response control devices The response of a structure may be reduced by placing base isolation devices at the base, and installing tuned mass dampers or energy dissipating devices in the structure. Different types of energy dissipating devices such as visco-elastic fluid damper, visco-elastic solid damper, hysteretic energy dissipating damper and friction damper have been used to reduce the earthquake response, hence limiting the brittle failure of reinforced concrete columns in buildings. Active vibration control is not considered in this research program.

Effectiveness of the response control devices was confirmed by a series of one-story one-bay full-scale reinforced concrete frame tests on the earthquake simulator at the Disaster Prevention Research Center, Science and Technology Agency, Tsukuba (Fig. 5). Every frame survived an intense earthquake motion without serious damage when an energy dissipation device was installed in the specimen. The frame failed under the same intensity earthquake motion when the energy dissipation device was removed from the specimen in the next test run. The method to estimate necessary amount of energy dissipating devices is studied by a series of nonlinear earthquake response analyses of reinforced concrete frames, varying the amount of energy dissipating devices placed in a weak story. The method is studied to reflect the results in the seismic diagnosis method. Research on foundation strengthening Most Japanese cities were developed on alluvium plains. Except for light residential houses, reinforced concrete buildings are normally supported on foundation piles. The failure of foundation piles was reported after the 1995 Hyogo-ken Nanbu Earthquake. The cost of damage investigation of foundation as well as the repair work of damaged foundation is very expensive. Therefore, it is normally desired to provide the foundation structure with higher force resistance, which also requires higher construction cost. In an 11-story structure in Kobe, the failure of pile foundation was believed to reduce the earthquake ground motion input to the structure and limited the damage in the super-structure. The damage statistics indicate that the damage of a super-structure was smaller if the foundation was severely damaged. It is necessary to examine the overall seismic performance of the structure including its foundation. The following subjects were studied in the research program; (1) The effect of reducing interaction between the structure and foundation was studied on the earthquake response of a super structure for three cases; i.e., (a) fixed connection, (b) friction connection and (c) flexible connection between the structural base and pile top. The placement of sliding elements could reduce the response of a super-structure and also reduced bending moment and shear at the top of piles. (2) The effect of foundation strengthening was studied. The foundation was modeled by finite elements, a pile by a lineal element, and a super-structure by an elastic frame.

When the surface layer (4 m deep) of soft soil under a structure was improved and stiffened, bending moment in piles was reduced, but shear increased at the boundary of the improved and old soil layers.

When the top part of a pile was reinforced by steel plates, the additional stiffness at the pile top attracted larger bending moment.

When additional piles were placed along the periphery of a structure, rocking response of the structure was reduced and bending moment and shear in a pile were reduced.

When the surface soil was confined by the use of sheet pile walls along the periphery of a structure, bending moment and shear in the piles under exterior columns were reduced, but those of piles under interior columns were not influenced.

(3) The seismic diagnosis guidelines for foundation structures were developed. The desired performance of foundation structure must be examined in relation with that of

the super-structure. STRENGTHENIG OF RC COLUMNS BY FRP SHEETS Short plastic fibers were used to reinforce cement products in the early 1980s in civil engineering and building construction, or to replace the metal fiber in the fiber reinforced concrete. Fiber reinforced plastic (FRP) sheets and plastic fiber strands were used to strengthen or repair reinforced concrete columns from the latter part of the 1980s. The use of FRP sheets and carbon fiber strands increased after the 1995 Kobe Earthquake Disaster to strengthen the existing reinforced concrete structures. The merit is light and easy for construction work compared with the steel plate jacketing on reinforced concrete members. As a part of the research program for the development of earthquake disaster reduction methodology through the application of new technology, organized by Building Research Institute, the application of FRP sheets has been extensively studied for the strengthening of existing reinforced concrete members (Ref. 6). Material properties Different FRP sheets have been developed for practical use in the strengthening work of existing reinforced concrete columns; e.g., FRP sheets are made of carbon fiber (5 to 8 µ m diameter), aramid fiber (approximately 12 µ m diameter), glass fiber, vinyl-plastic fiber and others. Carbon FRP sheet, normally used for strengthening work, must have tensile strength larger than 2,900 MPa or 3,400 MPa, and the elastic modulus (in tension) larger than 235 GPa. Aramid FRP sheet must have tensile strength larger than 2,060 MPa and the elastic modulus larger than 120 GPa, or tensile strength larger than 2,350 MPa and elastic modulus larger than 80 GPa. The FRP sheet fails in a brittle manner after reaching its strength. FRP sheets are pasted by adhesive on a reinforced concrete member after repairing and smoothing the concrete surface followed by coating by primer of epoxy-family. The ends of an FRP sheet are bonded by adhesive over more than 200 mm. Epoxy resin or methyl methacryl resin is normally used as adhesive for FRP sheets. These sheets are light and flexible, hence they are easy for use in construction work. FRP sheet can be easily cut by a pair of scissors or a cutter at the construction site. The corner of a reinforced concrete section must be rounded to a radius more than 20 mm for carbon fiber sheets and 10 mm for aramid fiber sheets to prevent brittle fracture of FRP sheets at the corner. Effect on shear strength The shear strength of a reinforced concrete member is observed to increase with lateral reinforcement using FRP sheets. The failure mode of a member can be shifted from diagonal tension shear failure to shear compression failure or to flexural yielding by the FRP sheet reinforcement, improving the deformation capability. The design tensile strength of FRP sheets is either a stress at 1.0 percent tensile strain or two-third the tensile strength, whichever is smaller. The shear strength suV (N) of a member after

strengthening by FRP sheets may be estimated by Eq. (12);

dbANpp

dVM

fpV wfwfwyw

ctsu )

87(1.085.0

)'18()100(053.011

23.0

++++

= σσ

(12) where, tp : tensile reinforcement ratio of longitudinal reinforcement, cf ' : concrete strength of a column (MPa), VM / : design shear span (design moment divided by design shear) (mm), d : effective depth of section (mm), b: width of section (mm),

1wp : shear reinforcement ratio of column, 1wyσ : yield stress of lateral reinforcement (MPa), wfp : lateral reinforcement ratio of FRP sheets, wfσ : design strength of FRP sheets (MPa), AN / : average axial stress (MPa). The equation is commonly known as Arakawa empirical equation and commonly used to estimate shear strength of reinforced concrete members. The calculated and observed shear strengths from 61 specimens failing in shear are compared in Fig. 6 (Ref. 7). The design equation is generally conservative, but shear strength of some specimens reinforced by carbon FRP sheets was lower than the estimated. The resistance at bond splitting failure has also been observed to increase with FRP sheet reinforcement. A damaged reinforced concrete member must be repaired by injecting epoxy resins into cracks before the application of FRP sheet reinforcement. When plain bars are used as longitudinal reinforcement in a reinforced concrete member, the shear resistance may not increase with the amount of FRP sheet reinforcement attributable to early loss of bond between the longitudinal reinforcement and the concrete. The rate of ductility enhancement with the amount of FRP sheets is observed to be less for a member subjected to high axial forces. Effect of existing surface finishing Six reinforced concrete column specimens of a 300 x 300 mm cross section and of a 900 mm clear height were tested with and without carbon FRP sheet reinforcement under lateral load reversals to study the effect of mortar cover on the shear strength (Ref. 8). Major variables in the test were the existence of mortar cover, the mortar strength, and the separation between concrete and mortar finishing. The specimens were designed to fail in shear without carbon fiber sheet reinforcement. Twelve 13-mm diameter high-strength deformed bars of a yield stress of 792 N/mm2 were used as longitudinal reinforcement, and 6-mm diameter deformed bars of a yield stress of 360 N/mm2 were used as lateral ties at 160 mm on centers. Concrete cover to the center of longitudinal reinforcement was 45 mm, and the thickness of mortar finishing was 20 mm. Compressive strength of concrete ranged from 30 to 35 N/mm2. Compressive strength of low-strength mortar varied from 15 to 17 N/mm2, and that of high-strength mortar was 33 N/mm2. The tensile strength of carbon FRP sheets was 4,490 N/mm2. Specimen S-RC is the reference specimen without carbon sheet reinforcement and

without mortar finishing. Specimen S-B was reinforced with carbon fiber sheet wrapping, but without mortar finishing. Figure 7 shows the effect of carbon fiber sheet reinforcement on the behavior of columns. Specimen S-RC failed abruptly in shear at a member drift angle of 0.01 rad after developing diagonal cracks. Specimen S-B also failed in shear (shear compression mode), but at a lateral load level approximately 20 percent higher. The failure of Specimen S-B took place in more gradual manner developing larger strain in carbon fiber sheets at mid-height of the specimen. Carbon fiber sheets were separated from the concrete at a member drift angle of 0.03 rad. Mortar finishing (20 mm thick) was left on concrete surface in Specimens S-B-LMB (low-strength mortar) and S-B-HMB (high-strength mortar comparable to concrete) under carbon fiber sheet reinforcement. Figure 8 compares the lateral load-member drift angle relations of specimens without mortar finishing (Specimen S-B), with weak mortar finishing (Specimen S-B-LMB) and with strong mortar finishing (Specimen S-B-HMB). The existence of mortar finishes increased shear resistance by 15 to 30 percent partly because the cross sectional area was increased by the mortar. In other words, the mortar finishing can be left on the concrete during the carbon fiber sheet reinforcement as long as the mortar is undamaged and tightly cemented on the concrete surface. In old buildings, mortar finishing may be separated from concrete face. The behavior of two specimens S-B-LMN and S-B-LMB was compared using weak mortar finishing on the concrete. Mortar finishing was separated from the concrete surface in Specimen S-B-LMN while mortar finishing was placed on the concrete in Specimen S-B-LMB. No noticeable difference was observed in shear strength with and without separation using comparable strength mortars (Fig. 9). However, the resistance after attaining maximum resistance was smaller in specimens with separation between mortar and concrete. The shear resistance of a reinforced concrete column, strengthened by carbon fiber sheet wrapping with or without mortar finishing, can be conservatively evaluated for the concrete section and for lateral reinforcement of original shear reinforcement and additional carbon fiber sheet reinforcement. Effect of adjacent structural walls A reinforced concrete column is often connected with a structural wall in the orthogonal direction. In such a case, the strengthening of a column by FRP sheets requires the separation of the orthogonal walls from the column. The noise and dust during the work disturbs the occupants. Two reinforced concrete column specimens of a 300 x 300 mm cross section and of a 900 mm clear height having structural walls in the orthogonal direction were tested under lateral load reversals to study the effect of special anchorage devices at the ends of carbon fiber sheets (Ref. 8). The column cross section and reinforcement are the same as those of Specimen S-RC in the previous section. The cross section and anchorage devices are shown in Fig. 10; the location of the transverse walls was varied in the two specimens. The carbon fiber sheets were pasted on the concrete surface by epoxy resin and to the L-shaped steel angles (12 mm thick) by epoxy mortar. The bolt size (16 mm diameter) was determined so that entire tensile stress of carbon fiber sheets can be carried by the bolts.

Figure 11 compares the lateral load and member drift angle relations of Specimen S-B-CW with transverse walls at the center and Specimen S-B without transverse walls. Maximum lateral load for the two specimens was attained at a member drift angle of 0.01 to 0.015 rad. Appreciable increase in shear resistance was observed in Specimen S-B-EW compared with that in Specimen S-B partially attributable to the contribution of the transverse walls to the shear resistance of the column. Figure 12 compares the response of Specimen S-B-EW with transverse walls at the column face and Specimen S-B without transverse walls. Specimen S-B-EW developed lateral load resistance higher than Specimen S-B. However, the increase in shear resistance over the square column specimen was smaller in Specimen S-B-EW than that in Specimen S-B-CW. Strain in carbon fiber sheets reached as large as 0.005 at maximum lateral load in Specimens S-B-CW and S-B-EW, while the strain was 0.004 in Specimen S-B. The study confirms the usefulness of strengthening method using special anchorage devices for the carbon fiber sheets. SUMMARY Development of seismic retrofitting technology for reinforced concrete buildings in Japan after the 1995 Hyogo-ken Nanbu earthquake was briefly introduced. Significant damage was observed in buildings constructed using old technology. It is important to examine the earthquake resistance of existing buildings and strengthen deficient existing buildings to mitigate earthquake disaster. It is desired to develop earthquake disaster reduction technology through the application of new technology. ACKNOWLEDGEMENT The authors express sincere gratitude to the work carried out by researchers in the research program for development of earthquake disaster reduction methodology through the application of new technology, organized by Building Research Institute.

REFERENCES 1. Otani, S., “A Brief History of Japanese Seismic Design Requirements,” Concrete International, ACI, Vol. 17, No. 12, December 1995, pp. 46-53. 2. Reinforced Concrete Committee, Damage Investigation Report on Concrete Buildings, the 1995 Hyogo-ken Nanbu Earthquake (in Japanese), Kinki Branch, Architectural Institute of Japan, 1966, 245 pp. 3. National Network for Promotion of Seismic Retrofit, Activities and Future Directions, Japan Building Disaster Prevention Association, 1995, 8 pp. 4. Japan Building Disaster Prevention Association and Building Center of Japan, Law for Promotion of Seismic Strengthening of Existing Buildings and Related Commentary (in Japanese), January 1996, 91 pp. 5. Aoyama, H. and S. Otani, “Recent Japanese Developments in Earthquake Resistant Design of Reinforced Concrete Buildings,” Significant Developments in Engineering Practice and Research, A Tribute to Chester P. Siess, Publication SP-72, American Concrete Institute, 1981, pp. 49 - 76. 6. Building Research Institute, Reports of Research Program for Development of Earthquake Disaster Reduction Methodology through Application of New Technology (in Japanese), Strength and Ductility Committee, Response Control Committee and Foundation and Geotechnical Committee, 1997, 1998, and 1999. 7. Fujii, Shigeru, “Evaluation of Seismic Strengthening Technology by Fiber Reinforced Plastic Sheets (in Japanese),” Proceedings, Symposium on Application of Fiber Reinforced Plastics to Concrete Structures, Architectural Institute of Japan, December 1998, pp. 67 - 78. 8. Fukuyama, H., H. Suzuki and H. Nakamura, “Seismic Retrofit of RC Columns by Fiber Sheet Wrapping without Removal of Finishing Mortar and Side Wall Concrete,” submitted for presentation at 1999 ACI Fall Convention, FRPRCS-4, October 1999.

Table 1: Damage of Buildings before 1981

No. Stories

OperationalDamage

Heavy Damage

Collapse

Total

1 20(90.9) 1( 4.5) 1( 4.5) 22(100) 2 215(92.7) 9( 3.9) 8( 3.4) 232(100) 3 532(93.0) 17( 3.0) 23( 4.0) 572(100) 4 524(85.8) 41( 6.7) 46( 7.5) 611(100) 5 269(79.6) 29( 8.6) 40(11.8) 338(100) 6 59(75.6) 10(12.8) 9(11.5) 78(100) 7 49(58.3) 16(19.0) 19(22.6) 84(100) 8 19(63.3) 7(23.3) 4(13.3) 30(100) 9 3(33.3) 4(44.4) 2(22.2) 9(100) 10 20(48.8) 15(36.6) 6(14.6) 41(100)

Total 1710(84.8) 149( 7.4) 158( 7.8) 2017(100) ( ): Ratio to the number of buildings of the same height(%)

Table 2: Damage of Buildings after 1981

No. Stories

OperationalDamage

Heavy Damage

Collapse

Total

1 8( 100) 0( 0.0) 0( 0.0) 8(100) 2 85(98.8) 0( 0.0) 1( 1.2) 86(100) 3 460(98.1) 2( 0.4) 7( 1.5) 469(100) 4 508(95.7) 9( 1.7) 14( 2.6) 531(100) 5 333(97.4) 5( 1.5) 4( 1.2) 342(100) 6 135(91.8) 9( 6.1) 3( 2.0) 147(100) 7 90(86.5) 12(11.5) 2( 1.9) 104(100) 8 44(75.9) 11(19.0) 3( 5.2) 58(100) 9 19(73.1) 7(26.9) 0( 0.0) 26(100) 10 51(69.9) 18(24.7) 4( 5.5) 73(100)

Total 1733(94.0) 73(4.0) 38(2.1) 1844(100) ( ): Ratio to the number of buildings of the same height(%)

Table 3: Deformation Capacity Index F of Members and Failure Mode

Failure mode of Columns and Walls Value Highly ductile columns without fear of shear failure 3.2 Columns connected to girders yielding in flexure 3.0 Ductile columns unlikely to fail in shear 2.2 Columns connected to girders likely to fail in shear 1.5 Less ductile columns, but unlikely to fail in shear 1.3 Columns likely to fail in shear 1.0 Brittle columns likely to fail in shear 0.8 Structural walls rotating at the base under lateral loading 3.0 Highly ductile structural walls without fear of shear failure 2.0 Structural walls likely to fail in shear 1.0

Table 4: Evaluation of Structural Safety under Strong Earthquake Motion

Structural Seismic Index sIand Index q

Structural Safety

3.0<sI or 5.0<q Likely to collapse others Possible to collapse

6.0≥sI and 0.1≥q Unlikely to collapse

Fig. 1 Construction Age of Japanese Building Stock (Ref. 4)

Fig. 2: Vibration Characteristic Factor Rt

Medium Soil

Vib

ratio

n C

hara

cter

istic

Fac

tor

Natural Period, sec

Hard Soil

Soft Soil

(a) Discontinuity in Stiffness along Height (b) Eccentricity in Plan

Fig. 3: Increase in Design Story Shear for Irregularity in the 1981 Cabinet Order

F1 F2 F3

Index F

Qu

Q3

Stor

y Sh

ear

Q1

Q2

Failure of Group 1 Members

Failure of Group 2 Members

Failure of Group 3 Members

Fig. 4: Sequence of Member Failures and Index F

0.5

1.0

1.5

0.0 0.3 0.6 1.0

Rs

2.0 Fs Fe

0.0 0.15 0.30

0.5

1.0

1.5

Re

Shaking Table

DeviceFull-scaleRC Frame

Test Floor

Mass of220 ton

Flexible Rod representingStructural Stiffness

Fig. 5 Test of Full-scale Reinforced Concrete Frames with Energy Dissipating Device

Calculated Average Shear Stress, MPa

Obs

erve

d Av

erag

e Sh

ear S

tress

, MPa

Carbon FRP sheet and HoopAramid FRP sheet and hoopsCarbon FRP sheetAramid FRP sheet

Fig. 6 Reliability of Equation (12) for Shear Strength of RC Members

Reinforced by FRP Sheets

Fig. 7 Effect of Carbon Fiber Sheet Reinforcement on Behavior of R/C Columns

Fig. 8 Effect of Mortar Finishing on Shear Strength of Strengthened RC Columns

Fig. 9 Effect of Mortar Separation from Concrete Surface on Behavior of RC Columns

Fig. 10 Specimens with Transverse Walls and Carbon Fiber Sheet Reinforcement

Fig. 11 Effect of Anchorage of Carbon Fiber Sheets on RC Column Behavior

(Transverse Walls at Column Center)

Fig. 12 Effect of Anchorage of Carbon Fiber Sheets on RC Column Behavior (Transverse Walls at Column Face)