Damage to Building Structures Caused by the 1999 Chi-chi Earthquake in Taiwan

Transcription

1 Damage to Building Structures Caused by the 1999 Chi-chi Earthquake in Taiwan Koji YOSHIMURA* and Masayuki KUROKI** Based on the field investigation conducted in severely damaged areas in the central districts of the Republic of China, Taiwan, damage to building structures caused by the 1999 Chi-chi earthquake in Taiwan is briefly introduced. Key words: Chi-chi earthquake, Taiwan, earthquake damage, building structures 1. INTRODUCTION On September 21, 1999, an earthquake of Richter Magnitude of 7.7 occurred in Taiwan, the Republic of China. As of the 9th of December 1999, the death toll was 2,405 and 11,306 people were injured during this earthquake [Source: and 9,909 dwelling and non-dwelling building structures were totally collapsed [ In order to investigate the structural damage to the building structures caused by this earthquake, authors of the present study arrived at Taipei, the Capital of the Republic of China, on October 6, 1999, and spent five days at around the damaged areas in the central districts of Taiwan to conduct the field investigation. Non-damaged building structures were also investigated in Taichung City as well as the downtown of Taipei City on the last two days before leaving Taiwan on October 11, Main objective of the field investigation conducted by authors was to collect the general information about the damage to the building structures which were caused by this shallow-focus earthquake. Herein, based on this field investigation results, typical structural damage to some of the representative building structures are briefly introduced. In Reference 1, the effects of the structural systems adopted in typical reinforced concrete (R/C) moment resisting frames in Taiwan are discussed mainly focusing on the extensive earthquake damage to the low- and medium-rise residential buildings with weak (or soft) first-stories. 2. BRIEF DESCRIPTION OF EARTHQUAKE The main shock of this earthquake occurred at 1:47 a.m. on Tuesday of the 21st of September 1999, and it s Richter Magnitude was Ms=7.7. Location of the epicenter was the Town of Chi-chi (or Jiji), about 40 km to the South-South East of the City of Taichung, the Capital of Taichung County in Taiwan (see Photo 1 and Figure 1). The depth of the main shock was about 10 km according to the information given by the Central Weather Bureau in Taiwan [ Received on December 28, 1999 * Professor of the Department of Architectural Engineering, Oita University, Oita, , Japan Photo 1 Earthquake Damage to Chi-chi Station ** Research Associate of the Department of Architectural Engineering, Located within Epicentral Area (Chi-chi) Oita University, Oita, , Japan -32-

2 Figure 1 Location of Epicenter and Visited Areas for Field Investigation During the main shock of this earthquake, a large number of earthquake accelerograms were recorded because National Network System in Taiwan has more than 700 strongmotion observation stations distributed over the countries of Taiwan. Among these, peak accelerations in E-W, U-D (Vertical) and N-S directions recorded at the TCU129 Station, which is one of the Nantou City Stations located at about 10km North-West of the epicenter, were 981cm/sec 2, 335cm/sec 2 and 610cm/sec 2, respectively [ WAVACC.jpg]. These values are the maximum recorded accelerations obtained during this earthquake which have been already published officially until the end of the year of After the occurrence of this earthquake, a large number of ground movements as shown in Photos 2 and 3 have been observed or found on the numerous ground surfaces which appeared along or close to the active fault lines. 921.ncree.gov.tw/chi-main.php3]. According to the web-site information on October 13, 1999, which was given by the Central Weather Bureau in Taiwan, the total number of dead people was 2,333 persons and 10,003 people were injured, while 9,909 dwelling and non-dwelling building structures were totally collapsed and 7,575 building structures were partially collapsed [ 921.ncree.gov.tw/chi-main.php3]. Table 1 Damage Statistics [as of the 25 th of September by ncree.gov.tw/chi-main.php3] City Injured Dead Building total Partial TAIPEI CITY HSINCHU CITY TAICHUNG CITY CHIAI CITY 10 1 TAINAN CITY TAIPEI CITY TAOYUAN COUNTY HSINCHU COUNTY MIAOLI COUNTY NANTOU COUNTY TAICHUNG COUNTY CHANGHUS COUNTY YUNLIN COUNTY CHAI COUNTY TAINAN COUNTY 1 1 LLAN COUNTY 7 5 Total Photo 2 Vertical Ground Movement Appeared on a School Playground in Wufen Photo 3 Vertical Ground Movement Crossing Route 3 between Chi-chi and Chushan -33-

3 In case of this earthquake, most of those ground movements were dominant especially in vertical directions rather than the horizontal directions. Photos 4, 5 and 6 are typical examples of the earthquake damage to the structures observed in a residential building, electric transmission tower and road bridge, which were mainly caused by the predominant vertical ground movements around the foundation sites of those structures located on, close or along the active fault lines. Those kinds of earthquake damage to the structures which were caused by the extensive ground movements along the active fault lines must be discussed separately from the ordinary structural damage caused by the dynamic earthquake ground shaking. Photo 7 shows one of a few examples of the actual structural damage to the bridges which were not caused by the ground movement along the active fault line but by the ground shaking during this earthquake. Photo 6 Structural Damage to a Bridge Caused by Considerable Ground Movement on Active Fault Line in Hsinsha Photo 7 Shear Failure of an R/C Bridge Pier Caused by Ground Shaking (Wufeng) 3. FIELD INVESTIGATION Field investigation in severely damaged areas by this earthquake was conducted during about five days beginning from the 6th of October, Figure 1 shows the Photo 4 Rigid-body Displacement of a House Caused by Extensive Ground Movement visited areas for the present field investigation by authors, on and close to Active Fault in Hsinsha which were widely scattered over the central districts of Taiwan. Numeral in parenthesis attached to each area represents the total number of buildings investigated in the corresponding area. During this field investigation, a total of 285 damaged buildings were investigated in these areas, all of which are located within the Taichung and Nantou Counties. Among these, more than 80 percent of the buildings were low- and medium-rise buildings with less than five stories. Table 2 shows the structural systems adopted in the investigated buildings and total number of stories, Photo 5 Rigid-body Rotation of an Electric Transmission and usage of those buildings is classified in Table 3. In addition to those damaged buildings, some of the non-dam- Tower Caused by Ground Movement near Active Fault between Chi-chi and Chushan aged medium-rise residential buildings as well as R/C mo- -34-

4 ment resisting frames with soft lower stories located in Taichung and Taipei Cities were also investigated in last two days of the present investigation. Table 2 Investigated Building Structures N.S. qb tql bl v r SRC N.C. Total N.C Total TYPICAL DAMAGE TO BUILDING STRUCTURES 4.1 R/C Residential Buildings with Soft or Weak Lower-stories There were a large number of low-, medium- and highrise residential buildings damaged severely in their first stories (Photos 8, 9 and 10). Most of the structural systems adopted in those buildings were R/C moment resisting frames with a soft or weak first-story. Effects of those structural systems on severe earthquake damage to the typical residential buildings in Taiwan is discussed in detail in Reference (1). N.S. = Number of stories RC = Reinforced concrete buildings URM = Unreinforced masonry buildings CM = Confined masonry buildings W = Wood or timber buildings S = Steel buildings SRC = Steel reinforced concrete buildings N.C. = Not cleared Table 3 Classification of Investigated Buildings due to Usage Type of Use Total Residential 72 Store + Residential 52 House 46 Factory 11 School 11 Store 11 Warehouse 8 Hotel 6 Office 5 Store + House 5 Gas Station 3 Department Store 2 Restaurant 2 Station or Terminal 2 Temple 2 Bank 1 Factory + House 1 Guardsman Station 1 Hospital 1 Masonry Garden Wall 1 Museum 1 Parking 1 Store + Office 1 Warehouse + Factory 1 Not Cleared 38 Total 285 Photo 8 Damage to a Low-rise Residential Building with Soft First-story (Puli) Photo 9 Damage to a Medium-rise Residential Building with Soft First-story (Dali) -35-

5 Photo 12 Total Collapse of First-story of a School Building in Wufeng Photo 10 Damage to a High-rise Residential Building with Soft First-story (Dapin City) 4.2 R/C School Buildings There were a large number of R/C school buildings located in the severely damaged areas during this earthquake, and some of them were considerably damaged. Photo 11 is a structural damage to a three-story washroom tower attached to the adjacent three-story school building in Wufeng, and Photo 12 shows a total collapse occurred in the firststory of a three-story building located in the same school yard in Photo 11. Photo 13 is an another example of a firststory columns damaged severely in a three-story school building in Wufeng, which were located very near to the active fault lines shown in Photo 2. Photo 13 First-story R/C Columns in a Severely Damaged Three-story School Building in Wufeng 4.3 R/C Hotel Buildings Sun Moon Lake is one of the famous resort areas in Taiwan which is located about 10 to 15 km West from the epicenter of this earthquake, and there were many hotels around the Lake. Among these, several R/C hotel- and hostel-buildings were severely damaged during the earthquake (Photos 14 through 17). Photo 14 shows a total collapse of a threestory R/C building where many R/C long columns were severely damaged. Photo 11 Damage to a Three-story Washroom Tower in Wufeng Photo 14 Damage to Sun Moon Lake Teacher s Hostel in Sun Moon Lake -36-

6 Photos 15, 16 and 17 are other examples of the extensive earthquake damage to a three-story (partially four-story) R/C hotel building which were also located in Sum Moon Lake area. 4.4 R/C Housings There were two- or three-story R/C personal houses damaged severely during the earthquake. Photo 18 shows one of the severely damaged R/C three-story houses in which all the first-story R/C columns were completely collapsed. Photo 15 Total Collapse of First-story R/C Columns in a Three-story Hotel Building in Sun Moon Lake Photo 16 Complete Collapse in Lap-joints of Longitudinal Bars in the Same Building with Photo 15 Photo 18 Totally Collapsed First-story of an R/C Three-story House in Dangshi 4.5 Other R/C Buildings Photos 19 and 20 are other types of earthquake damage observed in R/C buildings. Photo 19 shows a total collapse of an R/C slender wall building, where out-of-plane buckling was observed in both of the extreme-edge walls of the building. Photo 20 represents one of the typical failure modes observed in R/C beam-to-column connections, where there were almost no shear reinforcing bars provided into this beam-to-column connection. Photo 17 Out-of-plane Buckling of R/C long Columns in the Same Building with Photo 15 Photo 19 Total Collapse of a Slender R/C Wall Building in Puli -37-

7 Photo 20 Typical Failure in Beam-to-column Connection of an R/C Moment resisting Frame in Chi-chi Photo 22 Damage to an Adobe House in Chi-chi 4.6 Unreinforced Brick Masonry Buildings There were large number masonry buildings damaged severely in the epicentral area. Most of the damaged masonry buildings were composed of unreinforced masonry walls using clay-brick masonry units. Photo 21 is one of the damage examples observed in unreinforced masonry house located in Chi-chi. 4.8 Timber Structures There were quite few timber or wooden buildings in which severe structural damage was observed during the field investigation conducted by authors. Photo 23 is one of the severe structural damage to the timber buildings in the epicentral area. Photo 21 Damage to an Unreinforced Masonry House in Chi-chi 4.7 Adobe Houses There were also a number of adobe houses in and around the epicentral area of this earthquake, and some of them were damaged severely during the earthquake. Photo 22 is a typical damage to the non-engineering structures observed in adobe houses. Photo 23 Damage to a Timber Building (Chi-chi Station) 4.9 Steel Building Structures There were several steel buildings where total or partial collapse or damage to their structural or non-structural members were observed. All of those were low-rise smallscale buildings such as a warehouse and factory. Photos 24, 25 and 26 are structural damage to a three-story warehouse located in the severely damaged area in Dungshi. -38-

8 Photo 24 Complete Collapse of a Three-story Steel Warehouse in Dungshi. Photo 25 Details in Beam-to-Column Connection in the Same Building with Photo 24 tural walls provided along the height (or vertical direction) of the buildings, (4) Poor reinforcing details in R/C columns, beams and beam-to-column connections, as well as structural walls, (5) Low material and mechanical properties of concrete and masonry units, and (6) Low quality control in building construction site. Since most of the extensive earthquake damage to the R/ C buildings was concentrated in the lower-stories of the low- and medium-rise R/C moment resisting frames and wall structures where unreinforced brick or block masonry infill wall panels were provided in the openings enclosed by the cast-in-place R/C column- and beam-members with poor structural details, it may be concluded that, if the adequate amounts of reinforced structural walls were provided in both vertical and horizontal directions of the buildings, then those walls would be able to play an effective role to prevent or reduce such a severe structural damage to the R/ C building structures during this earthquake. 6. ACKNOWLEDGEMENTS The two papers presented herein and in Reference (1) are summarized based on the materials used for the lecture titled Damage to building structures caused by the 1999 earthquakes in Turkey, Greece and Taiwan presented by Koji YOSHIMURA, which was given on November 5, 1999 at the University of Toronto (UT), Canada. Authors wish to Photo 26 Poor Welding Details in Base-plate and Bottom thank Professors Peter C. Birkemoe and Shamim A. Sheikh of First-story Column in the Same Building with of the Department of Civil Engineering of UT who organized to open this lecture. The authors also wish to thank Photo 24 Ms. Diana. LIN, who brought authors to most of the damaged areas caused by the earthquake within the limited days. 5. CONCLUDING REMARKS Although considerable amount of structural damage to the Finally, many thanks are extended to Mr. Satoru KUDO, an building structures were caused by the 1999 Taiwan earthquake, undergraduate student of Oita University, for his assistance some of the principal causes to those severe structural damage in the preparation of this paper. to the building structures are summarized in the followings: (1) Inadequate structural system adopted in R/C moment 7. REFERENCES resisting frames with soft lower stories, 1) Koji YOSHIMURA and Masayuki KUROKI, On Structural Systems and Details Adopted in R/C Residential Build- (2) Insufficient amount of structural walls provided in both longitudinal and transverse directions in plan especially in ings Damaged Severely during the 1999 Chi-chi Earthquake lower stories of the buildings, in Taiwan, Faculty of Engineering Research Report, Oita (3) Insufficient amount and irregular arrangement of struc- University, No.41, March,