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Building Performance in the Taiwan Earthquake A Review of Nantou County by Peter Yin March 2000 City of Los Angeles Department of Public Works Bureau of Engineering City of Los Angeles Mayor Richard Riordan Board of Public Works Ellen Stein, President Valerie Shaw, Vice-President Maribel Marin Tod Burnett Woody Fleming Bureau of Engineering Vitaly B. Troyan, City Engineer Acknowledgments The author wishes to express his appreciation to the City of Los Angeles Board of Public Works and the Bureau of Engineering for their support of the relief mission to the Taiwan Earthquake. He is also grateful to the Taipei Economical and Cultural Offices in Los Angeles for their coordination of all arrangements for the trip. Particular acknowledgment is due to fellow relief team members: Mr. Henry Huang of the Los Angeles County Public Works Department; Mr. Albert Chen of Black & Veatch; and Mr. Albert Tsai of W. E. Moscicki and Associates, and to Dr. Jing-Wen Jaw of the City of Los Angeles Bureau of Engineering for their review and critique of this paper. Thanks also go to Winifred Harano and the Administrative Services Division of the Bureau of Engineering for assistance in the editing, layout and production of this report. Building Performance in the Taiwan Earthquake A review of Nantou County by Peter Yin, PE, SE Preface The devastating earthquake that struck central Taiwan on September 21, 1999, was the largest in recent Taiwan history. Measured at 7.6 on Richter Scale, it killed more than 2,300 people, destroyed 10,000 buildings, seriously damaged many streets, bridges, public facilities, power distributions, and industrial operations. The initial estimate of reconstruction cost was well beyond one hundred billion U.S. dollars. In responding to the disaster, structural engineers in Southern California formed a relief team to assist the Taiwanese people. With the emphasis on the experience in reinforced concrete structures and high-rise building design background, a group of eight structural professionals from public agencies and the pri- vate sector across the Southland, were chosen. I was one of the members. The team was given one of the most challenging tasks: Assigned to Nantou County, the epicenter to evaluate mid-rise/high-rise buildings, examine governmental buildings, schools, hospitals, libraries, and review low-rise residential buildings. From October 6 to October 10, 1999, we visited six hard-hit cities or townships, reviewed and inspected more than one hundred buildings. We eyewitnessed the ground rupture, the wiping out of entire blocks, and the destruction of all kinds of structures, large and small, public and privately-owned. We also saw many buildings with minor or no damage, in the same neighborhoods as those that failed. The difference in these buildings may have been just due to engineering, but the results between those that succeeded and those that failed was life or death. This report, reflecting my personal observation and opinion, is focused on buildings in Nantou County. No detailed geotechnical data or seismology theory will be discussed here, except for a few instances where it is closely associated with the building case studies included in this report. By reviewing these cases and discussing their successes and failures, it is hoped that we would have a better idea on what worked and what did not in the Taiwan earthquake. And we all know that what happened in Taiwan yesterday, might happen to California tomorrow. Buildings in Taiwan Taiwan is an island country situated in a humid subtropical region approximately 8,000 miles west of Los Angeles. It is one fourteenth the size of California but has a population of 22 million, or about two thirds of California. Almost two thirds of the 1 island are mountainous area or hillside, thus land is extremely valuable. Because of the economic benefits, an “Open Space” policy was initiated by the government which allowed building owners to build more square footage vertically as long as the space on the street level was open to the public. This, however, created a weak-story for a vast majority of buildings. Further, because of the humid environment and cost considerations, excepting high-rise buildings in major cities, almost all buildings are made of rigid concrete frames with non-reinforced brick in-fills. These in-fills played a crucial role in the Taiwan earthquake. They might have either saved lives or destroyed the building, depending on how they were used. Building design and construction in Taiwan Taiwan’s building code closely resembles the UBC and ACI practiced in the United States. (See Figure A). In fact, many of the university professors, engineers, architects, and governmental officials studied and even worked in the U.S. before they went back to Taiwan. During our visit to Taiwan, our team had opportunities to talk to professionals in Nantou County and the City of Taipei. With the qualification of our team having two members who were in fact chairs of subcommittees of the Structural Engineers Association of Southern California, we were impressed with the knowledge and ability of the professionals we met. It is obvious that there are many qualified professional engineers and geologists in Taiwan, practicing a code similar to the building code in the United States. However, in Nantou County the architect is in charge of the structural design for any building whose height is less than 35 meters. He/she employs the engineers and geologists to perform structural calculations or soil reports, sometimes under cutthroat type bidding wars. The structural engineer and the soil engineer have no authority in building design and construction in these cases. (In most metropolitan areas in Taiwan any building taller than 35 meters, or seven-stories, are mandated to be designed by structural engineers and reviewed by a structural review committee formed of local professionals.) The architect would incorporate the sketches and details from the engineers to his/ her plan, and apply for building permits from the government. Figure A (Data source: UCSD Ji-Ji, Taiwan Earthquake of September 21, 1999.) Recommended design base shear by building code. 2 Figure B (Data Source: Central Weather Bureau, R.O.C.) After the permit was obtained, the owner/developer took over and hired a contractor. The developer and the contractor both should have licensed engineers to interpret the plan and execute the project by regulation. The owner normally hired his architect as representative. The local enforcement agency would inspect the construction at few key stages, though in most cases, it was never as thorough, nor with the follow-through as practiced in the United States. Construction quality control has been largely the responsibility of the architect, who might or might not receive cooperation from the contractor. Another common practice was the use of subcontracts. Taiwan is no exception in this regard. After the contract was awarded, the general contractor would sub the project out to several low bid subcontractors, whose performance would really determine the quality of the construction. In such cases, if a stringent inspection system was not implemented or enforced by the government, the quality of the construction most likely would be in serious jeopardy. The Earthquake Taiwan is situated in a subduction zone between the Philippine Sea Plate and the Eurasian Plates. (See Figure B) The East Coast experienced most of the serious ground motion Figure C 3 in the past, while the Nantou County at the central part of Taiwan, was considered a moderate seismic zone. The strong ground motion that ruptured at Hsuangtung Fault and the Chelungpu Fault, (See Figure C) had the initial rumble measure 7.6 on the Richter Scale, followed by three aftershocks bigger than the Northridge Earthquake (6.8) in the first 48 hours, and 9,000 aftershocks in the next two weeks. In the cities we visited, many of the areas experienced Peak Ground Acceleration of 0.5g in the horizontal direction, and a 0.3g in the vertical direction with the duration exceeding 30 seconds, according to data from the Central Weather Bureau of Taiwan. The relief team visited three cities: Nantou, Puli, Tsao-Tun, and three townships: Ming-Jen, Chung-Liao and Ji-Ji. The last one was the quake’s epicenter. How the buildings were evaluated This report discusses the performances of buildings in Nantou County in 8 categories: 1. 3 to 4 story non-ductile, concrete frame buildings. 2. Low-rise commercial buildings. 3. High rise buildings 4. School buildings 5. Municipal Buildings 6. Hospitals 7. Libraries 8. Convalescent Homes Since there are so many buildings, only a few examples will be given of each. Each instance will refer to the five principles in the buildings seismic design: building configuration; structural redundancy; structural continuity, ductile detailing and construction quality. 4 Building Case Studies Section 1 Three to four story, non-ductile, concrete framed buildings. These types of buildings are commonly built for residential/commercial use and are most popular in Taiwan. Usually a family occupies the upper floors of the unit with a high percentage using the ground level for commercial purposes. The following were observed: Features: Outcomes: 1. Wide open store front at street level. 1. Creates weak story/soft story on the first floor. 2. Brick in fill between units. 2. Very strong resistance to earthquake forces in the longitudinal direction. 3. Reinforced concrete frame and pour-inplace slab. 3. Load paths from floors to frames have no problem. 4. Insufficient column size, low brick wall 4. Brick wall above beam creates strongadded to the beam at building front. beam/weak-column system; column will fail by shear or flexural forces. 5. Beam reinforcement extends to the face of 5. Insufficient lap length and concrete cold building for future add-on neighboring joint on concrete beam bad for load paths. building. Re-bar splice at joint against ductile detailing. Buildings banged to each other due to these weak links; impact was greater when adjacent buildings were of different heights. 6. Illegal addition to the top. 6. Further burdens the structural system which was not built seismically sufficient to start with. 7. Concrete stairway in the middle of floor 7. Acting as K-truss to resist transverse (seisplan. mic) forces. 8. Brick in-fill wall at the rear of building. 8. Acting as shear wall may have saved many buildings from collapse. 5 1-1 (Puli) Beam reinforcements extended for future neighboring building. Noted column width (parallel to the street) reduced to the minimum to increase the store open space. Typical 3-4 Story Concrete Buildings. 1-2. and 1-3 (Puli) Weak story/soft story and lack of ductile detailing made these columns vulnerable, which in turn could cause building collapse. 6 1-4 (Puli) Stairway at middle floor plan provides lateral support to the building. See typical floor plan 1-5 (Puli) Though not by design, the brick wall at rear was the actual lateral support to the building. See typical floor plan. 3-4 Story Building Typical Floor Plan 7 1-6 and 1-7 (Puli) Ductile detailing was absent. 8 1-8 above (Chung-Liao, 10km from epicenter) All buildings on the block lost their first floor. On average, every family lost a member. 1-9 (ChungLiao) Despite the loss of the first floor, this was the only building on the block still standing. 9 1-10 above (Nantou) Five-story building showed column flexural failure. Note masonry wall added to top of girder. 1-11 right (Nantou) Stirrup space too large, electrical conduit reduced the column effective area. 1-12 below (Nantou) Shear failure at column and wall in this five-story building. 10 1-13 (Nantou) No seismic detailing. 1-14 right (Chung-Liao) Column flexural failure in this three-story building. 11 1-15 Three-story apartment at Ji-Ji, the epicenter. 1-16 Detail of flexural failure at column. No ductile detailing, weak-story. 12 Section 2 Low-Rise Commercial Buildings Due to Taiwan’s “open space” policy and for commercial reasons, most buildings have wideopen space on the street level. This, along with in-fill-brick walls added to the floor beam and lack of ductile detailing, caused numerous commercial buildings to collapse when their first floor failed. Unlike residential buildings, which have stairways and brick walls (in the rear) as redundancies, commercial buildings were mostly supported by columns. 2-1 and 2-2 (Puli) Eightstory twin buildings lost first floor. Note that the columns had no seismic detailings. 13 2-3 and 2-4 (Puli) Detail of the twin buildings showed lack of seismic ties. Many required detailings were missing in the columns. Concrete quality was questionable. 14 2-5 Commercial buildings at Puli. Like many others, these two buildings lost their first floor due to weak story/soft story. 2-6 Market at Puli. The buried first floor was normally jammed with people during business hours. Luckily, the quake occured at 1:47 am. 15 2-7 (Puli) Another example of the devastating effect of the “strong beamweak column” frame system. 2-8 This street in Puli is a perfect example of structural continuity problems. All buildings in this block lost the first floor due to weak story. Meanwhile, the shorter building (built at a different time) hit the adjacent taller building when movement occurred in a different direction. The deformation of the taller building demonstrates this. (Also see Section 1, Feature 5 for comments.) 16 2-9 Seven-story commercial/residential building. See details below. 2-10 and 2-11 Column shear failure in this building. The building did not collapse due to the large amount of structural redundancies. 17 Section 3 High-Rise Buildings Most of these buildings were built over the past 5 to 10 years. Reinforced special moment resisting frame system was supposed to be used. However, no ductile detailing was found in many columns abutting the street. The unfavorable locations on the floor plan and poor detailing made these columns vulnerable. Lack of quality control also contributed to many column failures 3-1 and 3-2 Twelve-story commercial building in Nantou. Due to wide use of brick in-fill walls throughout every floor, this building only suffered damage to columns on the first floor, street front. 18 3-3 and 3-4 (Nantou) Columns showed crowded reinforcements. Utility and drainage conduit took away further space. There were no seismic ties, and improper lap splices. Also, concrete quality was questionable. 19 3-5, 3-6 and 3-7 (Nantou) Similar problems occurred in this building as those in 3-1 and 3-2. Columns showed poor construction as well. 20 3-8 at right (Puli) Sixteen-story Kuo-Pao Plaza 3-9 Largest drift occurred at the east corner. Southeast column damages should not be surprising. Note that stirrups were missing. (See Typical Floor Plan) 3-12 Column at middle of south wing was damaged due to bad detailing. Shear failure occurred above the short brick wall. 21 3-10 Wall elements suffered most damage in the north wing where the floor plan bent. Note that the shear wall on the first floor was severely damaged and removed. In the photo a worker is starting to rebuild the wall. (See Typical Floor Plan.) 3-11 Though the concrete peeled off, this link beam in the north wing was considered to be successful. 22 Typical Floor Plan Indicates beam column rigid frame system with brick-in-fill at all places. “C” shape configuration and bend at plan layout caused plan irregularity. The areas that suffered damage were predictable. 23 Section 4 School Buildings Tsau-Tun Trade School The school construction has several features: a. b. c. d. Non-ductile concrete frame throughout. Brick in-fill wall at transverse direction full weight. Short brick wall above girders at longitudinal direction. Column weak axis bends along longitudinal direction. Building has no shear wall in this direction. 4-1 Three-story moment frame system. All columns became “short column” due to the built-up of short brick walls on girder. Structural system had no chance to deform and to absorb energy before the sudden shear failure at columns. These short brick wall was never considered in the design analysis. 24 4-2 A close up of the first column of building on previous page. 4-3 Show failure at “d” from joint face. 4-4 A combination of shear and compression failure on columns. 25 4-5 The result of a “weak story”. 4-6 Note the short wall above girder, second-floor made column flexural failure inevitable. 26 4-7 Shear failure at “short column.” 4-8 Deformed columns. Ductile detailing would have mitigated the damage. 27 Section 5 Municipal Buildings Almost all governmental buildings were non-ductile concrete moment frames with brick infill walls. Poor construction quality and “short column” were common problems at these buildings. 5-1 Puli City Hall. This three-story reinforced concrete building pancaked. The high brick in-fill wall made the column very short. Columns were sheared off before any frame deformation could start. 5-2 Tsau-Tun City Hall had a better fate. Structural redundancy and flexibility made the difference. 28 5-3 Longer columns between floor (deformed before shear failure) and redundancy was observed at this building. (Tsau-Tun City Hall) 5-4 Brick in-fills were the first defense of this building. (TsauTun City Hall) 5-5 Ground rupture in front of Tsau-Tun City Hall. An even larger rupture tilted the building in the back of this picture. 29 5-6 and 5-7 Puli Police Headquarters had the same column shear failures as City Hall. 30 Section 6 Hospitals Puli Veterans Hospital This building, completed within 10 years, was not a success. It was constructed as a dual system, but neither the shear wall nor the frame has a ductile detailing. The shear wall was lightly reinforced with only one curtain rebar and no boundary member. This building did not collapse due to the large amount of structural redundancy. 6-1 A vertical irregularity (Two-story high lobby at entrance) caused all shear walls at the first and second floors to crack. No diagonal reinforcement was provided at shear wall between windows. 6-2 Columns at lobby area were sheared off. No seismic details were provided. 31 6-3 Building side view. Veneers were peeled off after concrete wall cracked. 6-4 There is no boundary member noted at the inner side of shear wall. Light reinforcement has only one curtain re-bar. 32 6-5 The interior shear wall has no required seismic detailing. 6-6 Columns (no ductile detailing) damaged after shear wall failed. 33 Section 7 Libraries Nantou County Library This eight-story reinforced concrete special moment resisting frame building was still waiting for final inspection and approval when the earthquake struck. All interior and exterior walls were non-structural and were not designed for seismic resistance. Plan irregularity indicated that torsion would govern (see page 37). Despite walls cracked at south and east sides, this project was considered a success. Quality of construction was good. 7-1 Cracks are evident in south elevation of the Library wall. 34 7-2 and 7-3 Damaged walls were not designed as lateral force resisting elements. 35 7-4 A look at the same damaged wall from the inside. Note that the columns next to the wall panels have no damage. 7-5 Northeast corner of building. The only other place that exhibited significant damage (nonstructural). 36 7-6 Even though the ceiling fell apart, no cracks at the column or beam were observed. Plan This special moment resisting frame building has plan irregularity. Centroid of rigidity is at northwest portion on the plan, thus the south and east exterior walls suffered the most damage. 37 Section 8 Convalescent Homes Jeri Retirement Center - A successful example Jeri Retirement Center, in Nantou City, has three large non-ductile reinforced concrete frame buildings with brick in-fills. It suffered no damage in this earthquake. The construction quality was good (the owner was familiar with construction practice), the building had regular configuration, and brick walls provided extra redundancy. The brick in-fills shown on the plan were constructed from ground to roof. Regular configuration and excellent wall layout made the building resistent to damage. Plan Typical floor plan 38 8-2 and 8-3 Elevation views of the other two buildings. 39 Observations and Conclusions: During our team’s stay in Nantou, only a limited number of buildings could be observed. However, based upon the magnitude of the devastation and how wide spread the damaged area was, it was clear that the Taiwan earthquake was much stronger than the Northridge quake. The following was observed: 6. 7. 1. Near the fault line rupture, ground motion could vary significantly within a very short distance. If a building performed well, it does not necessarily mean that building was designed and built better than those that collapsed at nearby sites. 8. 2. Despite the magnitude of the seismic force, well-designed shear wall and special moment frame system proved to be most effective. Except for those near the fault line, no single structures designed and built per 9. building code was found to have collapsed in Nantou County. 3. Partial height wall/brick in-fill significantly changed building behavior. One of the most damaging factors in this quake was the failure of “short columns” created by these low walls. The building failed instantly due to a frame system that had no chance to deform and absorb the energy. 4. Full height non-reinforced brick in-fills, despite it’s low engineering value, clearly prevented many buildings from collapse and thus saved lives. The importance of redundancy in building design can not be overemphasized. 5. Ductile detailing is essential for rigid frame buildings in seismic design. Many building would have survived had the code requirements been followed. The low-rise residential/commercial concrete buildings 40 not built according to code suffered the most casualties. Lack of structural continuity also caused many building failures of 3 to 4-story structures, especially where the buildings were of different heights. The impact of building collisions was tremendous, as illustrated in Section 2 of this report. Liquefaction caused foundation settlement in a significant number of buildings. Many buildings were built around fault lines and the failure of these structures was inevitable. Geotechnical data and strict regulation regarding these issues should be established and strictly enforced. The authority and responsibility of structural and geotechnical engineers should be established. Recognition of the importance of these disciplines is essential to public safety, as demonstrated in this earthquake. Ignorance in design practice and poor quality in construction was observed. Implementation of a plan review, permit processing and inspection system, similar to the one in the City of Los Angeles is recommended.