EarthquakeTsai, Engineering Hsiao, Bruneau: and Engineering Overview Seismology of building damages in 921 Chi-Chi earthquake 9393 Volume 2, Number 1, March 2000, pp. 93–108 Overview of Building Damages in 921 Chi-Chi Earthquake 1) 2) 3) K.C. Tsai Chiang Pi Hsiao Michel Bruneau 1) Professor, Department of Civil Engineering, National Taiwan University, Taipei, Taiwan 10617, R.O.C. 2) Director, Architecture and Building Research Institute, Ministry of Interior, Taipei, Taiwan, R.O.C. 3) Professor, Department of Civil Engineering and Deputy Director, Multidisciplinary Center for Earthquake Engineering Research, SUNY at Buffalo, USA. ABSTRACT The 921 Chi-Chi earthquake caused approximately 9,000 buildings col- lapses or damages of various degrees. A large percentage of buildings that collapsed are un-reinforced clay block masonry cottages and non- engineered and one-to-three stories reinforced concrete frame structures constructed with brick in-fill partitions and exterior walls. Many collapsed buildings had a pedestrian corridor and open front at the ground floor. More than two dozen modern reinforced concrete moment resisting high- rise apartment buildings overturned or collapsed. The observed damages on reinforced concrete structures suggested a large effect of masonry and lightly reinforced concrete nonstructural components. The observed damages also suggest that only ductile details be used in all new construc- tions and in the repair of damaged structures, and buildings having “soft- story” characteristics should not be allowed in new constructions. INTRODUCTION with more than 6,000 others partially collapsed and countless others damaged On 1:47a.m. of September 21, 1999, a to various degrees. While it will take magnitude ML = 7.3 earthquake struck considerable effort and time to inventory the central region of Taiwan. Following that damage to a level of refinement that the 921 Chi-Chi earthquake, two after will allow formulation of a reliable critique shocks of ML = 6.8 occurred about 30 of current building codes and practices, hours and 127 hours after the main preliminary observations indicate that shock. Preliminary data indicates that buildings that collapsed typically exhibit- approximately 3,000 buildings totally ed non-ductile reinforcing details collapsed as a result of the earthquakes, compounded by detrimental building 94 Earthquake Engineering and Engineering Seismology, Vol. 2, No. 1 configurations. A large percentage of accounts for the majority of the complete buildings that collapsed due to the main building collapses near the epicenter due shock or strong aftershocks are old un- to severe ground shakings. Many school reinforced clay block masonry cottages buildings suffered extensive and severe and non-engineered and one-to-three damages. The “short-column” type of stories reinforced concrete frame struc- damage in these reinforced concrete tures constructed with brick in-fill structures is rather common in the partitions and exterior walls. Many direction parallel to the exterior corridor collapsed buildings had a pedestrian outside the classrooms where windows corridor and open front at the ground above half-height in-fill shortened the floor, and only one wall at the back of the effective length of almost all the columns building along the street direction (Photo 2). (Photo 1). This type of building damage Photo 1 Collapse of the typical building Photo 2 Shear failure of RC columns having pedestrian corridor due to the short column effect However, in the affected area, more (UBC) used in the United States, albeit than two dozen modern 10-to-20 stories generally one edition behind the latest apartment buildings overturned or published. Seismic force requirements collapsed. These were reinforced con- of building designs in Taiwan for the past crete moment resisting frames, most of 25 years are given in Table 1. The them constructed with cast-in-place seismic zonation maps in 1974, 1982 and 15cm thick exterior wall and 12cm thick 1997 editions of Taiwan Building Stan- partition walls (Photo 3). The observed dards (BTS, 1974, 1982 and 1997) are damages on reinforced concrete struc- given in Fig. 1 through Fig. 3, respectively. tures suggested a large effect of masonry In the Nanto county, where most of the and lightly reinforced concrete non- damage occurred, the specified peak- structural components. These building ground-acceleration to consider for were typically designed following re- design was 0.23g (for the 475 years quirements for moment resisting frames return-period earthquake), which trans- identical to the Uniform Building Code lates into a design coefficient of Tsai, Hsiao, Bruneau: Overview of building damages in 921 Chi-Chi earthquake 95 approximately 0.11g for short period Many of them appear to have tall floor and structures. Incidentally, that coefficient open plaza features in the ground level. was 0.05g from 1974 to 1982, and 0.08g Commonly encountered were failed from 1982 to 1996, before the higher columns with widely spaced stirrups aforementioned value was adopted in (unconfined plastic hinge zone), splices 1997. Note that Nanto was located in with inadequate development length or Taiwan’s Seismic Zone 2, and that design located in the hinge region, light or force 22% and 43% higher were mandated inexistent joint transverse reinforcement, in Seismic Zones 1A and 1B, respectively stirrups with 90°, etc. Strong-beam (Fig. 3). Alternatively, the code permits weak-columns systems apparently the use of a slightly larger seismic force to common, might have resulted in numer- size members with some relaxation on the ous story collapses following excessive ductile detailing of the reinforcement, column damage. This was particularly following what is prescribed in the UBC. frequent in the first story of buildings, as Surprisingly, many of the buildings that the larger openings and higher story collapsed were engineered and construct- height there translate into a lower struc- ed in the last decade. However, none tural stiffness and strength, leading to exhibited evidence of ductile detailing. “soft-story” mechanisms (photo 4). Photo 3 Collapse of a apartment build- Photo 4 Collapse of a high-rise apart- ing in Fengyuan ment building in Dali Table 1 Seismic Force Requirements in Taiwan Year Seismic Base Shear Remarks Z = 1.25, 1.0, 0.75 1974 Vw = Z K C W K = 0.67, 0.8, 1.0, 1.33 Cmax = 0.10, W = D + 0.25 L Z = 1.0, 0.8, 0.6 K = 0.67, 0.8, 1.0, 1.33 1982 Vw = Z K C I W I = 1.0, 1.25, 1.5 Cmax = 0.15, W = D Z = 0.33, 0.28, 0.23, 0.18 Z I C W I = 1.0, 1.25, 1.5 1997 V = Cmax = 2.5, W = D 1.4 a y Fu ay = 1.2 (WSD), ay = 1.5 (USD) Fu » 2.9, 2.5, 2.1 96 Earthquake Engineering and Engineering Seismology, Vol. 2, No. 1 Fig. 1 Seismic zonation map in 1974 Taiwan Building Codes Fig. 2 Seismic zonation map in 1982 Taiwan Building Codes Tsai, Hsiao, Bruneau: Overview of building damages in 921 Chi-Chi earthquake 97 1997 Fig. 3 Seismic zonation map in 1997 Taiwan Building Codes BUILDING DAMAGE STATISTIC Island-wide BY GEOGRAPHIC REGION The 921 earthquake has resulted in a tremendous amount of damages in the Overview of Building Damage central part of Taiwan. Figure 4 indi- Statistic cates that approximately 90% of the 8,773 damages are concentrated in Shortly after the occurrence of the Nantou County and its adjacent Taichung 921 earthquake, the National Center for County. Over 300 damaged buildings Research on Earthquake Engineering are also observed in the Miaoli County, as (NCREE), the Architecture and Building well as in the Taipei County. The detail Research Institute (ABRI) and the private organizations had quickly formed the damages of Nantou County, Taichung reconnaissance team to investigate the County, and Taipei County are described island-wide building damages. There below. were 8,773 structural damages recorded Nantou County using the Building Damage Survey Form [1]. Each damage report was filed for Nantou County was severely impacted either an individual structure or a con- by this earthquake due to its epicenter secutively connected building block; location and the surface faulting. More therefore, the number of the actual than 4,500 structures, which equivalent damages had exceeded the reporting to 53% of the island-wide damages, were figure noted above. surveyed in Nantou. Figure 5 illustrates 98 Earthquake Engineering and Engineering Seismology, Vol. 2, No. 1 that Nantou City, Tsautuen Village, and Taichung County Jungliau Village are the top three most Being a highly populated county adja- damaged areas in this county; and their cent to Nantou, Taichung County also damaged building figures are 937, 846 experienced significant damages of 3,200 and 823 respectively. The survey indi- building failures. It is observed from Fig. cates that Nantou City, Tsautuen Village, 6 that these damages in the Taichung Jushan Village, and Mingjian Village County were concentrated at Dungshr received substantial damages, because Village, Shrgang Village, Fengyuan City, of the direct impact of the Chelungpu and Shinshe Village. The regional damage ratios of these four areas are 28%, Fault. 24%, 19% and 14% respectively. 0% 53% 0% Wufeng Dadu (13) Dali (108) (151) 4% 0% 0% Taiping 4% Fengyuan 5% (159) 0% 0% (548) 6% 2% 2% 19% 1% 2% Shrgang (653) Tantz (8) 24% 4% 0% 2% 32% Shinshe Taichung City(110) Taichung County(2815) (397) Taipei City(157) Taipei County(337) 14% Nantou County(4637) Miaoli County(348) Taoyuanj County(13) Yunlin County(156) Hsinchu City(24) Hsinchu county(15) Shengang Wur (12) Dungshr Chiai City(2) Chiai County(7) (11) 0% (766) Changhua County(152) 0% 28% Fig. 4 Distribution of damaged buildings Fig. 6 Distribution of damaged buildings in Taiwan (number and percent- in Taichung County (number and age) percentage) Taipei County Taipei County, a special seismic zone Tsautuen Village Puli Village 18% in the north region, is applicable to a 10% Guoshing Village different earthquake design requirement 4% because of the Taipei basin location.