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|Publication Number: Date: January/February 2002|
Issue No: Vol. 65 No. 4
Date: January/February 2002
On Sept. 21, 1999, at 1:47 a.m. (local time), central Taiwan experienced a destructive earthquake. As a result of this earthquake, more than 2,400 lives were lost, and more than 10,000 people were injured, according to the Taiwanese official report. Approximately 10,000 buildings and homes collapsed, and about 7,000 were severely damaged.
Highway bridges, including those constructed under modern seismic design codes, were severe damaged. According to the Taiwanese Highway Bureau's preliminary report, at least nine bridges were severely damaged, including three bridges that were under construction. Five bridges collapsed due to fault rupture, and seven bridges were moderately damaged.
This earthquake, called the Chi-chi Earthquake, measured 7.6 on the moment magnitude (Mw) scale. Moment magnitude is based on the concept of seismic movement, and although it is more difficult to compute than other magnitude scales, it is more uniformly applicable to all sizes of earthquakes. In particular, for very large earthquakes, moment magnitude gives the most reliable estimate of earthquake size. However, all magnitude scales should yield approximately the same value for any given earthquake.
The U.S. Federal Highway Administration (FHWA) and the Ministry of Transportation and Communication (MOTC) of Taiwan formed an investigative team to evaluate Taiwanese highway bridge performance under the Chi-chi earthquake. The team members came from FHWA, the Taiwanese Highway Bureau (THB), and the National Expressway Engineering Bureau (NEEB) of MOTC. The team visited 10 bridge sites, including two NEEB bridge sites and eight THB bridge sites.
This paper presents the preliminary findings and lessons learned from the team's investigation, which was conducted Nov. 15-17, 1999.
Fault Rupture Type
Taiwan is located at the junction of the Manila and Ryukyu trenches in the Western Philippine Sea, where the Philippine plate is being forced under the Eurasia plate. The Philippine plate is moving northwest, which causes a significant strike-slip component along the northern portion of the Manila Trench and essentially creates a "transpressional" effect that has popped up the island of Taiwan microplate relative to its larger tectonic neighbors. This "thrust fault" - also called a reverse-slip fault - pushed up the ground at several locations, and some bridge sites were lifted as much as nine meters (30 feet).
Bridge Design Codes
|A pot bearing on this pier sheared and resulted in the displacement of the structure.|
Bridge design specifications used in Taiwan have been revised three times since 1960. Prior to 1960, several design guide specifications were used for practical design. Some of them were based on Japanese design codes. In 1960, MOTC issued standard specifications titled "Highway Bridge Engineering Design Specifications" for the design and construction of highway bridges. These design specifications were based on the American Association of State Highway and Transportation Officials (AASHTO) bridge design specifications issued in 1953. In 1987, MOTC revised this design code based on the 1977 AASHTO bridge design specifications. Although this code was revised again in 1995 based the 1992 AASHTO specifications, the seismic design part of the code was not changed much.
The bridges in the epicenter area were designed to accommodate seismic design forces of about 0.15 g to 0.2 g. (G or g is the force of gravity - an acceleration of 9.78 meters/second2. In an earthquake, the forces caused by the shaking can be measured as a percentage of the force of gravity.) With the largest peak ground acceleration of the Chi-chi Earthquake measuring greater than 1.0 g, damage to the bridge structures was inevitable.
The team first visited two NEEB bridge sites, Neotsou-si and Neotsopu Kenshi bridges. These two bridges are continuous spans with a pre-stressed box girder superstructure. They are about 60 to 70 kilometers (37 to 43 miles) away from the epicenter. They were still under construction and suffered similar damage. In general, both bridges performed well. However, the pot bearings of both bridges were severely damaged, and superstructures were offset 2 to 30 centimeters (0.8 to 12 inches). These bridges are parallel to the direction of the fault, and this may have mitigated the effect of this large earthquake. Some concrete bridge foundations and piers were poured one or two days before the earthquake, and extensive nondestructive evaluation (NDE) examinations are needed to investigate bonding issues between reinforced steel bars and concrete.
The team visited eight THB bridge sites, including the site of a new bridge. Six bridges had collapsed due to a fault rupture underneath or adjacent to the bridges. The average ground movement was more than 2 meters (6.5 feet).
|The Shi-wei Bridge collapsed due to piers tilting and large ground offsets. The fault rupture was directly underneath the south abutment area.|
It is located on Route 3, which was constructed in September 1994. It consists of northbound and southbound twin bridges. The total length of the bridge is 75 meters (246 feet) and was divided into three simply supported spans of 25 meters (82 feet) each. It is a curved bridge with a width of 24 meters (78 feet) and is supported by five Precast/Prestressed Concrete Institute-certified (PCI) girders. Each girder is supported on elastomeric bearing pads with shear keys to provide transverse constraints. The second pier of both bridges was tilting, and the first pier of the northbound bridge revealed shear cracks. The second and third spans of the southbound bridge and the third span of the northbound bridge collapsed due to piers tilting and large ground offsets. The fault rupture was directly underneath the south abutment area. Because the bridge is skewed and curved, large ground motion might have caused the bridge deck to rotate and damage the substructures. The bridges, designed with a simple support beam, almost certainly would not be able to accommodate ground movements as large as the Chi-chi Earthquake.
This bridge is also on Route 3, about five kilometers (three miles) from the Shi-wei Bridge. The Tong-feng Bridge is 573 meters (1,880 feet) long and consists of three parts. The middle part was completed in 1966, and in 1988, the bridge was widened on both sides. The original section has 22 spans with four PCI girders to support the 9.5-meter- (31-foot-) wide bridge deck. The substructure is a pier-wall-type construction. The later additions widened the bridge deck to 30 meters (98 feet) with PCI-girder support. However, the substructure used single-column bridge piers. After the earthquake, the bridge had large vertical displacements of 10 to 20 centimeters (4 to 8 inches) and offsets of 30 to 50centimeters (12 to 20 inches) in the transverse direction. The PCI girders dislodged from the bearings because of the large transverse movements. One girder was cracked and was temporarily supported by a steel truss. Although the bridge was severely damaged, it was reopened to traffic with restricted lane use after the bearings were replaced with elastomeric pads.
This bridge, completed in 1991, has multiple spans simply supported by a PCI girder. The superstructure collapsed because the fault ruptured underneath the bridge. The fault rupture lifted the upper stream by 5 to 6 meters (15 to 20 feet) and created a new waterfall. This reverse-slip fault also shortened the bridge length and might have caused the second pier to fail.
|The Bei-fung Bridge collapsed because the fault ruptured underneath the bridge.||The fault rupture under the Bei-fung Bridge lifted the upper stream by 5 to 6 meters (15 to 20 feet) and created a new waterfall.|
Also on Route 3, this bridge has 18 spans with a total length of 624.5 meters (2,049 feet) and a width of 25 meters (82 feet). This bridge is two parallel structures that were constructed during two different periods. The superstructure of the northbound bridge was constructed in 1981, but it used the original substructure (pier-wall type) that was constructed in the 1950s. The southbound bridge was completed in 1983. Both structures use PCI girders in their simply supported superstructures, and both have pier-wall-type substructures. The fault rupture occurred behind and under the northern abutments of both bridges. Although the two bridges suffered similar ground motions, they failed in different ways. The first and second spans of the older bridge (northbound) collapsed. This failure was due to the fault rupture, which caused a large ground movement, pushing the superstructure back and forth until it fell down from the "seats" atop the piers. The bearings also failed due to large compression forces. The third pier of the northbound bridge was uplifted also. Both superstructures may have collided during the earthquake, causing some damage to the substructures. The piers of the northbound bridge suffered tension cracking and fractured, and the southbound bridge piers had severe shear cracks and failures.
|Damage to the Wu-shi Bridge was caused by pier shearing and tension failure.||This pier of the Wu-shi Bridge was sheared.|
|The Mao-lui-shi Bridge did not collapse, but it had concrete spalling and shear cracks. Most eccentric connections showed distress in the concrete columns. Some severely damaged locations were temporarily supported by steel truss.|
This bridge on Route 3 is a horizontal, curved viaduct with a steel superstructure. The superstructure consists of four plate girders supported by concrete, single-column bents. Some of these bents are "C-bent," where the column is eccentrically connected to the cross-girder. The bridge did not collapse, but it had shear cracks. Most eccentric connections showed distress in the concrete columns. Some severely damaged locations were temporarily supported by steel truss.
The cable-stayed Ji-lu Bridge is on Route 152. Approach spans at both ends are simply supported, and they lead up to the two-span, single-tower structure that is approximately 240 meters (787 feet) long. The concrete superstructure is symmetrically supported by 17 pairs of parallel cables from each side of the tower. All but one section under the tower and guardrail was completed at the time of the earthquake. Damage included one snapped cable, tower structure cracking and concrete spalling, failure of the pot bearings due to the structure pounding up and down, and approach spans offset in transverse directions. Unbalanced loading of the uncompleted bridge might have caused the vertical and lateral pounding.
This structure on Route 149 is 160 meters (525 feet) long and 9 meters (30 feet) wide. The bridge's superstructure is PCI-girder supported by single-column bents. The first and fourth spans collapsed. The second span tilted and rotated in a transverse direction. The substructures had severe shear failure and column bents sheared out. Large movements due to the fault rupture underneath the bridge caused it to fail.
|The first and fourth spans of the Tong-tou Bridge collapsed. The second span tilted and rotated in a transverse direction.||The substructures of the Tong-tou Bridge had severe shear failure and column bents sheared out..|
E-Jiang and Min-Tsu Bridges
The E-Jiang and Min-Tsu bridges had been demolished by the time that the team visited.
Through the extensive visits and evaluations of damaged and undamaged bridge sites, the following is a preliminary set of lessons learned from this investigation:
The Chi-chi Earthquake severely damaged highway bridges as a result of large ground motion and fault rupture directly underneath or adjacent to bridge sites. Even with the modern design codes, a bridge cannot resist such huge displacements or offsets of either the superstructure or substructure. The challenge left for engineers is to develop a better strategy for constructing bridges across or nearby a known fault.
Ian Buckle and Jenn-Shin Hwang. " Bridge Performance in the 921 Earthquake, Chi-Chi, Taiwan," Proceedings of the 15th US-Japan Bridge Engineering Workshop, Tsukuba, Japan, November 1999.
Dr. Wen-Huei (Phillip) Yen is a research structural engineer in FHWA's Office of Infrastructure Research and Development at the Turner-Fairbank Highway Research Center in McLean, Va. He is FHWA's representative in the National Earthquake Loss Reduction Program, and he is a technical committee member of the National Seismic Conference on Highways and Bridges. He received his bachelor's degree in civil engineering from the National Taipei Institute of Technology in Taiwan and his master's degree and doctorate in applied mechanics and civil engineering from the University of Virginia. He is a registered professional engineer in Virginia.
For more information about the work and findings of the evaluation team, contact Wen-Huei (Phillip) Yen at email@example.com or by telephone at (202) 493-3056