Seismic Performance of Highway Bridges 5555 Volume 2, Number 1, March 2000, Pp

Earthquake Engineering Chang, Chang,and Engineering Tsai, Sung: Seismology Seismic performance of highway bridges 5555 Volume 2, Number 1, March 2000, pp. 55–77

Seismic Performance of Highway Bridges

1) 2) 3) 4) Kuo-Chun Chang Dyi-Wei Chang Meng-Hao Tsai Yu-Chi Sung

1) Professor, Department of Civil Engineering, National Taiwan University, Taipei, Taiwan 10617, R.O.C. 2) Manager, Structure Department I, China Engineering Consultants Inc., No. 185, Sec. 2, Hsin-Hai Road, Taipei, Taiwan 106, R.O.C. 3) Postdoctoral Research Fellow, Department of Civil Engineering, National Taiwan University, Taipei, Taiwan 10617, R.O.C. 4) Project Vice-Manager, Structure Department I, China Engineering Consultants Inc., No. 185, Sec. 2, Hsin-Hai Road, Taipei, Taiwan 106, R.O.C.

ABSTRACT

The 921 Taiwan Chi-Chi earthquake incurred tremendous disaster to the central region of the island, particularly to Taichung and Nantou counties. Most of the bridges on the provincial and county routes in Taichung, Nantou, Changhua, and Yunlin counties escaped from serious damage, while approximately 20% of them suffered minor-to-major damage. The construction completion dates of those bridges range from 1960 through 1999. Damage to these bridges include fault rupturing, collapsed spans, landslides, soil settlement, slope failures, flexural and/or shear failures, and liquefaction. This paper begins with a brief description on the evolution of seismic design code for bridges in Taiwan, followed by a general depiction of the performance of highway bridges located in the four counties during the earthquake. Several major damaged bridges with typical damage modes are illustrated and explored. Lessons learned from the field observations and suggestions made on research needs are also discussed.

INTRODUCTION buildings or houses collapsed and 7,500 were damaged. About 20% of the bridge A devastating earthquake with the inventory on the provincial and county magnitude of ML = 7.3 struck the central routes in the disaster area suffered region of Taiwan in the early morning on minor-to-major damage. September 21, 1999. It was known as There are approximately 1,000 the 921 or Chi-Chi earthquake. As highway bridges spread on the provincial exhibited by the statistics [1], over 2,350 and county routes in Taichung, Nantou, people were killed, 10,000 were injured, Changhua, and Yunlin counties, which and 35 were missing in the earthquake. were the regions with major catastrophe, Moreover, approximately 10,000 especially Taichung and Nantou counties. 56 Earthquake Engineering and Engineering Seismology, Vol. 2, No. 1

Most of the bridges escaped from serious out-of-date and did not conform to the damage, while approximately 20% of progress made in bridge engineering, the them suffered minor-to-major damage Ministry of Transportation of Taiwan due to fault rupturing, collapsed spans, issued the “Design Code for Highway landslides, soil settlement, slope failures, Bridges” in 1987 [3]. Seismic resistant flexural and/or shear failures, and conception, geological characteristics of liquefaction. The construction local sites, and dynamic amplification completion dates of those bridges range factors were included in that design code from 1960 through 1999. announced in 1987. Complying with This paper begins with a brief more understanding of structural description on the evolution of seismic dynamic behavior and development on design code for bridges in Taiwan, bridge engineering techniques, a need to followed by a general depiction of the revise the seismic resistant design performance of highway bridges located approach described in the “Design Code in the four counties during the for Highway Bridges” was realized. earthquake. Several major damaged Based on the research on earthquake bridges with typical damage modes are engineer- ing in Taiwan with reference to illustrated and explored. Lessons the seismic design methodologies for learned from the field observations and bridges in Japan and United State, the suggestions made on research needs are current “Seismic Resistant Design Code also discussed. for Highway Bridges” was elaborated and announced by the Ministry of Transportation in 1995 [4]. EVOLUTION OF SEISMIC DESIGN CODES HIGHWAY BRIDGE In Taiwan, consideration of seismic PERFORMANCE demand on bridge structures may be traced back to the “Pocket-Size General Observation Engineering Handbook” issued by Taipei The primary disaster area in the 921 Branch of the Chinese Engineer Chi-Chi earthquake is composed of four Association in 1954. It was suggested counties, Taichung, Nantou, Changhua that there were two different seismic and Yunlin. According to the zones in Taiwan, corresponding to two preliminary bridge-disaster horizontal seismic design coefficients of reconnaissance reports [5,6], there are 1.0 and 1.5, respectively. In 1960, the approximately 1,000 highway bridges “Engineering Design Code for Highway spread on the main provincial and Bridges” was announced by the Ministry county routes in the disaster area. The of Transportation [2]. However, there construction completion dates of those was no specific guideline established for bridges range from 1960 through 1999, seismic design in that design code. and hence are involved in the Therefore, the suggestion provided by the evolutionary history of seismic design “Pocket- Size Engineering Handbook” was codes. As shown in Table 1, most of the adopted in that era. damaged highway bridges are located in Since the simple seismic coefficient Taichung and Nantou because of being design approach adopted in 1960s was close to the epicenter. There were 52

Chang, Chang, Tsai, Sung: Seismic performance of highway bridges 57 and 13 bridges that experienced survey. Most of those damaged highway minor-to-moderate and major damage, bridges have simply-supported, respectively, in Taichung, as well as 82 reinforced or prestressed concrete and 13 bridges, respectively, in Nantou. slab-and- girder superstructures, In fact, these two counties are the although continuous steel plate-girders disaster zones with the most casualties and long-span cable-stayed girders are during the Chi-Chi earthquake. In also included. addition, there were 17 highway bridges Table 2 lists all the major damaged in Changhua and 18 in Yunlin that highway bridges in the inventory, the suffered moderate damage, while no associated routes, and their primary severely damaged bridge was found. It damage modes. Approximate locations should be noted that the damage degree of these bridges and their proximity to the was evaluated by somewhat subjective Chelungpu fault are shown in Fig. 1, observation in the preliminary damage

Table 1 An aggregation of highway bridges by located counties and damaged extent

Number Total inspected Minor-to-moderate Major County bridges damaged damaged Taichung 196 52 (26.5%)* 13 (6.6%) Changhua 199 17 (8.5%) 0 Nantou 410 82 (20.0%) 13 (3.2%) Yunlin 176 18 (10.2%) 0

‘ * ’: the percentage is based on the total inspected bridges in each county.

Table 2 Major damaged highway bridge

Name Route Year Span (m) Length (m) Damaged mode Shi-wei Provincial 3 1994 25 75 collapse Chang-geng local 1987 25 300 collapse Dong-feng Provincial 3 1962/1988 26 572 girder/column Pi-feng local 1991 25 300 collapse E-jian County 129 1972 11 264 collapse Wu-shi Provincial 3 1981/1983 34.7 624 collapse/column Mao-loh-shi Provincial 3 1999 40 ~ 70 500 column Ming-tsu Provincial 3 1990 25 700 collapse Ji-lu local 1999 150 300 pylon/bearing Tong-tou County 149 1980 40 160 collapse Guang-long local 1986 28 56 deck/abutment Guan-de local 1977 20 60 collapse Bei-keng County 129 1959 5.7 5.7 deck/abutment Long-an County 129 1986 35 280 column Cheng-feng County 136 1986 25.6 184 column/abutment Yan-feng Provincial 14 1984 35 455 column Pu-ji Provincial 16 1979 35 105 pier cap Hsing-shi-nan County 127 1994 50 500 column/bearing Yan-ping Provincial 3 1986 13 78 abutment Hsin-yi Provincial 21 1981 29 180 column Long-men Tou 53 1982 40 480 collapse Li-yu Tou 53 1988 39 546 bearing Ping-lin Tou 6 1969 25 500 collapse/column 58 Earthquake Engineering and Engineering Seismology, Vol. 2, No. 1

Mo-keng No.1 Provincial 16 1996 14.6 14.6 abutment Mo-keng No.2 Provincial 16 1996 40 40 abutment Da-feng Chung 105 1992 deck

Fig. 1 Location of the major damaged bridges and the Chelungpu fault [5]

Chang, Chang, Tsai, Sung: Seismic performance of highway bridges 55 where some peak ground accelerations (PGA) measured from ground motion sensing stations by the Central Weather fault rupture and land dislocation. The Bureau, the main national freeway distorted rail and a vertical drop of about (Chung-shan Freeway), and some provin- 3.0 meters are shown in Fig. 2(c) and Fig. cial highways are shown as well. As 2(d). This branch of the railway still observed from Table 2 and Fig. 1, most of remains closed today. the major damaged highway bridges are located on or very close to the Chelungpu fault, or are in the vicinity of the epicenter at Chichi. It was noticed that for those bridge sites where the Chelungpu fault passes through, the highway bridges experienced collapsed spans in most cases, such as the Chang-geng bridge, Pi-feng bridge, E-jian bridge, Wu-shi bridge, Ming-tsu bridge, and Tong-tou bridge. Most of these collapsed bridges were installed with unseating prevention devices. Moreover, it is seen that the PGAs measured in the east of or close to Fig. 2(a) Damage to the column of the the Chelungpu fault are in the range of Ta-chia-shi bridge 400 gal to 980 gal, and are generally larger than those in the west of it. Thus, most of the damaged highway bridges are close to or in the east of the Chelungpu fault. It should be noted that the main north-south lengthwise freeway running from Taipei in the north to Kaohsiung in the south is far away from the Chelungpu fault, so that it experienced only little damage. Bridges of the north-south lengthwise railway running from Taipei to Pingtung also sustained only minor injury compared to those highway bridges. Only the Ta-Chia-shi bridge suffered Fig. 2(b) The repaired column of the some damage to its bridge columns (Fig. Ta-chia-shi bridge 2(a)) [7]. Those damaged bridge columns have been repaired by steel jacket (Fig. 2(b)). However, the north-south lengthwise railway was closed for two weeks because of the damage to the San-i tunnel and some buckled rails. Moreover, the east-west Chi-Chi Branch of the railway, which only runs for tourism nowadays, suffered devastating Chang, Chang, Tsai, Sung: Seismic performance of highway bridges 59

failures due to slope instabilities, joint failures in column-to-girder connections, cable fracture, fault rupture, and liquefaction. However, only Lyu-mei and Wan-lun bridges located in Yuen-lin town were observed to have liquefied appearance since it was not easy to distinguish when it occurred in a flowing river. Figure 3(a) shows a summary of com- parison among 11 different damage modes. It should be noted that an Fig. 2(c) Distorted railway of the injured bridge often has more than one Chi-Chi Branch damaged mode, so the total amount of all

terms exceeds 195, which is the number of all injured bridges in the inventory. It

Fig. 2(d) A vertical drop at the Chi-Chi Branch Fig. 3(a) A summary of different Major Damage Modes damaged modes

Most of the highway bridges in is seen that in these 195 bridges, the Taichung, Nantou, Changhua, and ratios of damage to auxiliary facility, Yunlin counties escaped from severe abutment, deck, and approach slab are damage and experienced only minor 35.6%, 32.5%, 26.3%, and 21.1%, distress such as the settlement of respectively, significantly larger than approach fills behind abutment those of other damage modes. From the back-walls. Approximately 20% of the field observation, it was found that bridge invento- ry suffered pounding on abutments due to excessive minor-to-major damage. Damage to longitudi- nal vibration of the these bridges include collapse of superstructure usually occurred with superstructures, displaced bearings, damage to the approach slabs and/or unseated girders from bearing supports, bridge decks. The deck damage defined shear failures in columns, pier walls, and in this paper includes the caissons, abutment back-wall failure, expansion-joint failure. In addition, the settlement of approach slab, foundation 60 Earthquake Engineering and Engineering Seismology, Vol. 2, No. 1 ratio of column or pier damage (10.8%) is Table 4 is the result of an aggregation greater than those of bearing (9.3%), of some damaged modes by the completed girder (8.2%), and foundation (7.2%). year of bridges. It is observed that the From the reconnaissance results [5], it amount of damaged abutment and deck was observed that half of those bridges are usually in the first two ranks for any with injured bearings were also with construction period. Figure 3(b) exhibits damage to the corresponding columns or the relative percentage of damaged modes piers. Some damaged bearings were for a specific construction period. It is due to crack of their pedestals, and seen that for any construction period some were due to dislodg- ment by the considered, the abutment and deck are supported girder. more liable to damage than other components under the earthquake. The Damaged Bridges Related to susceptibility of column or pier damage Construction Era decreases in the latest three construction periods, while it is not the case for the Table 3 shows the classification of bearing systems. A decreasing trend of damaged bridges based on their complet- ed years, extent of damage, and structural types. The percentage in the parenthesis is obtained by dividing the value by the total number of the damaged bridges with same structural type. It is seen that over half of those damaged bridges were constructed before 1989, generally before the issue of “Design Code for Highway Bridges” in 1987. Moreover, over 95% of those injured bridges before 1989 were simply supported. The decreasing percentage of injured, simply- supported bridges after 1989 may be attributed to the decreasing of simply- supported bridges and better seismic design methods as well as construction technologies.

Table 3 An aggregation of damaged bridges by their completed year, damaged extent, and structural types

Classification of damaged bridges by completed year and damaged extent

1975 earlier 1975 ~ 1982 1983 ~ 1989 1990 ~ 1996 1996 later total minor-to-moderate 33 44 48 19 25 169 major 3 6 7 7 2 25 total 36 50 55 26 27 194 Classification of damaged bridges by completed year and structural type

single span 19 (17%) 31 (28%) 33 (29%) 14 (12%) 16 (14%) 113

Chang, Chang, Tsai, Sung: Seismic performance of highway bridges 61 simply supported multi-span 15 (23%) 17 (27%) 20 (31%) 8 (13%) 4 (6%) 64 continuous multi-span 2 (22%) 1 (11%) 0 (0%) 3 (33%) 4 (44%) 9 other 0 1 2 1 3 6

Table 4 An aggregation of damaged bridges by their completed year and damaged modes

Classification of damaged bridges by completed year and damaged mode

1975 earlier 1975 ~ 1982 1983 ~ 1989 1990 ~ 1996 1996 later total column or pier 2 5 9 3 2 21 girder 2 7 3 2 2 16 abutment 8 10 26 9 10 63 bearing 2 4 4 5 3 18 deck 16 12 9 7 7 51 foundation 2 7 2 3 0 14 the damage ratio of column or pier in the Fig. 3(b) Relative percentage of damaged latest three construction periods may be modes for a specific attributed to the improvement on seismic construction period design approaches. Figure 3(c) shows the variation of the six damaged modes with the construction periods. It is observed that most of the six damaged modes occur in terms of higher damage percentage in a certain construction period than in others, except for the bearing-damaged mode. Generally speaking, over 70% of the damaged bridges for each mode other than the bearing-damaged mode were completed before 1989. Those bridges with damaged bearing systems disperse Fig. 3(c) Variation of the six damaged in each construction period with a nearly modes with the constructed pe- uniform way. riods

DETAILED DESCRIPTIONS OF SEVERELY DAMAGED HIGHWAY BRIDGES

Since the Chi-Chi earthquake was mainly induced by rupture of the Chelungpu fault passing through Taichung and Nantou counties, those bridges located on the highways near the 62 Earthquake Engineering and Engineering Seismology, Vol. 2, No. 1 fault experienced more severe damage. Closing to the southern abutments, two Provincial Route 3 running from Taipei in consecutive spans of the southbound the north to Pingtung in the south is one bridge and one of the other collapsed, as of the main provincial routes. Since the shown in Fig. 4(a). The retaining wall section of Provincial Route 3 in Taichung near the southern abutment collapsed, as and Nantou counties is close to the shown in Fig. 4(b). The first pier includ- Chelungpu fault, some highway bridges ing caisson in both bridges tilted away on that route were damaged seriously. from the southern abutment As for those bridges located across the northeastward, and the abutment tilted Chelungpu fault, it is probable to result in north- wards (Fig. 5). Figure 6(a) shows collapse under the action of both fault that some shear cracks occurred on the rupture and strong earthquake ground first column bent extending from the motion. Insufficient unseating northern abutment of the northbound prevention device, on the other hand, bridge. Moreover, almost all the may be responsible for collapsed spans concrete stoppers on the pier cap of the when the bridge is not located on or very second pier were crushed (Fig. 6(b)) and close to the ruptured fault. transverse movement of the middle span, Some typical major damaged bridges which escaped from collapse, was as mentioned above, as well as an almost observed. completed, cable-stayed bridge as well as Since there is no significant flexural or a viaduct newly completed in 1999, are shear crack observed on the tilted column described as follows. bents, it seems that fault rupture near the southern abutment may be responsible to Shi-wei Bridge those tilted piers. It thus appears The Shi-wei bridge is located at the that those two collapsed spans close to milepost of 163km + 278m on Provincial the abutment were induced by the Route 3 is the main line for connecting tiltingcolumn bents. In addition, the Dong-shih and Drao-lan. It was measured horizontal PGA is as high as reconstructed in 1994 as two individual, 500 gal in the transverse direction and 3- span curved bridges on single column 350 gal in the other. Supported by one bents supported by caissons. It has an tilted column bent, collapse of the middle identical span length of 25m and each span of the southbound bridge may be span comprises five simply-supported due to excessive in-plane displacement prestressed concrete girders and deck under the strong ground motion. slab. Those beam girders were support- However, the middle span of the ed on elastomeric pads with shear keys northbound bridge may have been for restricting transverse movement. experienced less displacement and thus The Chelungpu fault crossed the bridge in escaped from collapse. the vicinity of the southern abutments.

Chang, Chang, Tsai, Sung: Seismic performance of highway bridges 63

Fig. 4(a) Three spans collapsed (Shi-wei Fig. 4(b) The southern retaining wall bridge) collapsed (Shi-wei bridge)

Fig. 5 The southern abutment tilted northwards 64 Earthquake Engineering and Engineering Seismology, Vol. 2, No. 1

Fig. 6(a) Shear cracks at the first Fig. 6(b) Crushed concrete stoppers on column bent of the the pier caps northbound bridge

Dong-feng Bridge wall-type piers and single column bents, The 22-span Tong-feng bridge is respectively, on spread footings. Under located at the milepost of 168km + 337m the strike of the Chi-Chi earthquake, on Provincial Route 3, and is actually strong transverse shaking moved the three parallel bridges with a length of superstructures southward and some 572m for each. The central bridge is the bearings were dislodged (Fig. 7). Even original one constructed in 1962, while more, as shown in Fig. 7, the prestressed the other two were widened in 1982. concrete girder was fractured near the Each span of the original bridge and of beam end. Figure 8 shows settlement of the other two is consisted of 5 and 4 the bridge deck due to loss of support by prestressed concrete girders, respectively. bearings. Significant flexural/shear All spans have an identical length of 26m cracking and spalling occurred in both and are supported by bearings at each the sixth column bents extending from pier cap. The substructures of the the eastern abutments of those widened original and widened bridges comprise bridges (Fig. 9(a) and 9(b)).

Fig. 7 Southward movement of the superstructure and the fractured Fig. 8 Settlement of the bridge deck girder (Dong-feng bridge)

Chang, Chang, Tsai, Sung: Seismic performance of highway bridges 65

Fig. 9(a) Flexural/shear cracks at the Fig. 9(b) Flexural/shear cracks at the 6th column bent of the 6th column bent of the westbound bridge eastbound bridge

The Chelungpu fault did not pass abutment collapsed, and the second through the Tong-feng bridge, but evoked column bent along with its caisson was strong ground motion in the vicinity of the uprooted and lay down on the riverbed, bridge with a horizontal PGA of 500 gal in as shown in Fig. 10 and Fig. 11. The the longitudinal direction and of 350 gal collapsed second and third spans rotated in the other. Such large shaking made clockwise as they fell down on the the bridge girders move southwards and riverbed. Except those collapsed spans, even fall down from their bearing there is no evident damage to the rest supports. As a consequence, sinking of components of the bridge. the bridge deck resulted from the Since there is no significant flexural or southward movement of girders. shear crack observed in the lain down Furthermore, since the column bents of pier, it is believable that ground rupture the widened bridges are not as wide as through the pier toe led to its failure. A the wall-type piers of the original one, permanent drop of the riverbed, as shown they reserve less shear and flexural in Fig. 12, verifies the pass of the capacities than the old one does. It is Chelungpu fault through the bridge. thus plausible that the column bents experienced much more severe damage than the wall pier.

Pi-feng Bridge The 13-span Pi-feng bridge with a span length of 30m is located downstream of the Shih-kang dam on Ta-chia river and completed in 1991. It is consisted of 4 simply-supported, pre- stressed concrete girders for each span as the superstructure and of single column bents on caissons as the substructure. During the earthquake, 3 spans ex- tending from the southern Fig. 10 Three spans of the Pi-feng 66 Earthquake Engineering and Engineering Seismology, Vol. 2, No. 1

bridge collapsed

Fig. 12 A permanent drop of the Fig. 11 The third collapsed span of the riverbed induced by fault Pi-feng bridge rupturing

E-jian Bridge the other abutment as rigid bodies or Located at the milepost of rotated with their spread footings due to ground movement (Fig. 14). Relative 25km + 195m on County Route 129 and constructed in 1972, the 24-span E-jian movements between adjacent spans were bridge with an identical span length of observed for some of those spans 11m is an important passageway for the remaining supported by their piers, as Tai-ping city in Taichung county. Its shown in Fig. 15. superstructure is composed of a pair of A field examination indicated little simply-supported, reinforced concrete, damage to those collapsed girders and to double-T girders and deck slab. The the southern half of the bridge. It substructure comprises unreinforced appears that those collapses were caused concrete walls on spread footings. No by large ground movements of their unseating prevention device was provided foundations towards the southern between adjacent spans. The bridge was abutment rather than caused by being widened as the earthquake excessive structural vibration. occurred and new piers with a different span length from the original ones had been completed. Distinct damage to this bridge is the “seesaw-like” collapse of nine sequential spans extending from the northern abutment (Fig. 13). The Chelungpu fault crossed the northern abutment with uplift and pressed the soil layers to move along the longitudinal direction towards the other abutment. From the northern abutment to the central pier of the bridge, some piers near the abutment were crushed or snapped, while the remaining piers moved towards

Chang, Chang, Tsai, Sung: Seismic performance of highway bridges 67

Fig. 13 The “seesaw-like” collapse of the E-jian bridge

Fig. 15 Relative movement of adjacent spans (E-jian bridge) Fig. 14 Movement or rotation of the pier walls (E-jian bridge)

Wu-shi Bridge the northbound bridge did not The Wu-shi bridge, located at the experience such massive shear failure, milepost of 210km + 371m on Provincial but flexural cracks with fractured Route 3, is the main bridge connecting reinforcements were observed in the Wu-feng town in Taichung county and third pier wall from the northern Tsou-tun town in Nantou county. It is abutment (Fig. 18). Moreo- ver, the first actually two parallel 18-span bridges with two spans from the northern abutment of a uniform span length of 34.7m. The the northbound bridge were unseated northbound bridge was completed in (Fig. 19) due to the ground movement. 1981 and the southbound one in 1983. The bearings at these pier caps were Both bridges have similar heavily damaged due to strong superstructures as 5 simply-supported, transverse shaking, as shown in Fig. 20. prestressed reinforced concrete girders The third span especially exhibited a and similar substructures as wall-type westward permanent offset. In addition, piers on caissons. However, the wall the super-structure of the southbound piers of the southbound bridge were not bridge showed westward settlement (Fig. as wide as those of the northbound one. 21) due to sliding down of those injured Hence, different damaged characteristics piers and caissons along their diagonal were exhibited although fault rupturing shear cracks. occurred under the third span of both bridges, as shown in Fig. 16, and similar ground motions was experienced. Extending from the northern abutment of the southbound bridge, severe shear failure occurred at the first five pier walls and/or caissons under the strong ground motion and fault rupture, as seen in Fig. 17. Perhaps because of the higher shear capacity of its pier walls, 68 Earthquake Engineering and Engineering Seismology, Vol. 2, No. 1

Fig. 17 Shear failure of the pier walls and/or caissons

Fig. 16 Evidence of fault crossing the Wu-shi bridge

Fig. 18 Flexural crack at the third pier wall of the northbound bridge

Fig. 19 The first two spans of the Fig. 20 Crushed bearings and northbound bridge collapsed diaphragms at the pier cap

Chang, Chang, Tsai, Sung: Seismic performance of highway bridges 69

Fig. 21 Westward settlement of the superstructure 70 Earthquake Engineering and Engineering Seismology, Vol. 2, No. 1

Mao-loh-shi Bridge In an 8-span, curved segment, 4 Just finished in 1999, the Mao-loh- single column bents are eccentrically shi bridge is a multi-span viaduct connected to their steel cap girders, as connecting the Provincial Route 3 to the shown in Fig. 22, in order to keep traffic Provincial Route 63. It is consisted of a flow fluent for the highway below it. series of continuous segments, each of During the earthquake, diagonal shear which was constructed from 4 I-type steel cracks occurred at the upper portion of girders and steel cap girders as its super- those columns with eccentric connections structure supported on reinforced (Fig. 23). Even more, the confinement concrete, single column bents. The steel hoops are exposed for some cases (Fig. cap girders were rigidly anchored into the 24). The steel cap girders were corresponding single column bents. temporarily shored to prevent sinking of Although the viaduct is not close to either the superstructures. Since there is no the Chelungpu fault or the epicenter as damage to those single column bents with compared to other major damaged symmetrical connections, it appears that bridges, some joint injuries were observed the eccentric connection may be one of in the column-to-girder connections. the reasons for the damage.

Fig. 23 Diagonal shear cracks at the Fig. 22 The eccentric connections of the column with eccentric Mao-loh-shi bridge connection

Fig. 24 Exposed confinements of a column with eccentric connection

Chang, Chang, Tsai, Sung: Seismic performance of highway bridges 71

Ming-tsu Bridge southern abutment and nine spans The 28-span, simply-supported Ming- collapsed as shown in Fig. 25. The first tsu bridge is located at the milepost of six piers extending from the southern

233km + 564m on Provincial Route 3 and abutment were tilted or fractured (Fig. was constructed in 1990. It is an 26). Moreo- ver, the back-wall of the important highway bridge connecting southern abutment was cracked by Ming-jian and Zhu-shan towns in Nantou the superstructure and thus was driven county. Each span has 4 prestressed, back into the back-fill (Fig. 27) under the reinforced concrete girders for both the strong longitudinal ground shaking. southbound and northbound lines, and From the field observation, ground those girders were made continuous by rupture may be responsible for those cast-in-place reinforced concrete deck tilted piers and collapse of the northern slab over intermediate bents. Each line segments. However, the collapse of the has its own single column bents support- spans in the southern segment appears to ed by caissons. During the earthquake, be induced by the enormous impact of the fault rupturing occurred near the superstructure on the back-wall.

Fig. 25 Collapsed spans of the Ming-tsu Fig. 26 Tilted or fractured column bridge bents of the Ming-tsu bridge

Fig. 27 Cracked back-wall of the southern abutment 72 Earthquake Engineering and Engineering Seismology, Vol. 2, No. 1

Ji-lu Bridge pier of the approach span, as shown in The Ji-lu bridge is a cable-stayed Fig. 32. The approach span was bridge with two 120m spans and a single unseated from its elastomeric pads and tower (Fig. 28). The tower, constructed the associated pedestals under the with reinforced concrete, is 58m high and strong transverse ground motion (Fig. sustains the prestressed concrete 33), and relative movement between superstructure by 17 pairs of parallel certain adjacent spans was observed (Fig. steel cables from each side of it. Each 34). In addition, one of those 68 cables end of the cable-stayed bridge is was torn off from its restrainer, as shown connected to a simply-supported in Fig. 35. approach span. At the time of the This bridge is very close to the earthquake, this bridge was almost epicenter, and the horizontal PGA completed except for the guard rails and measured nearby is about 600 gal and a cantilevered section of the 400 gal in the transverse and longitudinal superstructure at the base of the tower. directions, respectively. Such enormous The tower, superstructure, and some ground shaking may have been cables experienced significant damage. responsible for the relative movement of certain adjacent spans and for unseating As shown in Fig. 29, concrete at the base of the approach span. Moreover, since of the tower was spalled off and some there is an uninstalled cantilevered horizontal cracks occurred. At the same section at the base of the tower, the face of the tower and extending from transverse vibration of the cable-stayed where concrete was spalled off, vertical bridge may have been unsymmetrical splits along the tower height were about the longitudinal centerline of it. observed (Fig. 30). Nevertheless, Thus, damage to both the parallel faces damage to the opposite face of the tower of the tower to the longitudinal direction was not as severe. Furthermore, the is unsymmetrical and the connection at connection between the tower and the the base of the tower to the superstructure at its base was smashed superstructure was crushed due to (Fig. 31). Similar damage was observed stress discontinuity. at the end of the superstructure on the

Fig. 28 The cable-stayed Ji-lu bridge

Chang, Chang, Tsai, Sung: Seismic performance of highway bridges 73

Fig. 29 Concrete spalled off and horizontal cracks at the tower base 74 Earthquake Engineering and Engineering Seismology, Vol. 2, No. 1

Fig. 30 Vertical split along the tower Fig. 31 Crush at the connection height between the tower and the superstructure

Fig. 32 Crush at the connection between the cable-stayed and approach Fig. 33 Unseated approach span from its spans bearing

Fig. 34 Relative transverse movement Fig. 35 The torn-off cable from its between adjacent spans anchorage

Chang, Chang, Tsai, Sung: Seismic performance of highway bridges 75

Tong-tou Bridge generally the longitudinal direction, and The 4 span Tong-tou bridge is located 360 gal in the other direction. During at the milepost of 13km + 633m on County the earthquake, all three single circular Route 149 and connects Zhu-shan and columns were cut off by shear failure (Fig. Tsou-ling towns in Nantou county. It 37) and the spans collapsed. It was was constructed with 3 simply-supported, noticed that those large-diameter prestressed reinforced concrete girders as caissons extended well above the riverbed the superstructure and with single and thus reduced the relative height of circular columns on caissons as the each circular column to the overall substructure. Each span is 40 meters height of both the column and its long. Most of the structural components caisson. Both abutments were damaged, were destroyed under both the strong with the northern one impacted into its ground shaking and fault rupturing (Fig. back-fill and crushed severely, as shown 36). According to the measured PGAs in Fig. 38. Furthermore, significant nearby, the PGA is approximately 750 gal settlement of the northern approach in the north-south direction, which is slab occurred (Fig. 39).

Fig. 36 A view on the collapsed Fig. 37 Shear failure of the single Tong-tou bridge circular column

Fig. 38 The crushed northern abutment Fig. 39 Settlement of the approach slab of the Tong-tou bridge of the northern abutment 76 Earthquake Engineering and Engineering Seismology, Vol. 2, No. 1

Since the columns and spans Long-men Bridge collapsed in an unsymmetrical scenario, it appears that they did not fail at the The 12-span, simply-supported Long- same instant. Extending from the men bridge was completed in 1982 and is northern abutment, the first column was located on Tou-53 Route. It comprises 2 snapped to the east, while the other two prestressed, reinforced concrete girders were snapped to the west. The north with a span length of 40 meters as the end of the second span rotated clockwise superstructure and single column bents about its south end, while the whole on caissons as the substructure. Under third span was tilted to the west and the earthquake excitation, the second partially lay on the caps of both and third spans extending from the fractured columns. It seems that the western abutment collapsed (Fig. 40). northernmost column was destroyed one Since there is no significant damage to moment before the others. Moreover, other structural components, it appears because those two spans close to the that no unseated prevention device (Fig. abutments still stayed at the longitudinal 41) may have been the reason for its centerline after their collapse, it seems collapse. that they failed a moment before the columns did.

Fig. 40 Collapse of two spans of the Fig. 41 No unseat prevention device Long-men bridge between adjacent spans

DAMAGE TO BRIDGES UNDER damage to those bridges is the fracture CONSTRUCTION or tumbling of the uninstalled, cast-in-place, reinforced concrete girders, In order to facilitate the east-to-west as shown in Fig. 42(a) and Fig. 42(b). transportation of Taiwan, several east-to- The Feng-yuan viaduct, which is located west express highways or freeways are at the Taichung Branch of the Second under construction at present. Some of Freeway, suffered moderate damage to the bridges on these new express the elastomeric bearing systems. Many highways or freeways experienced bearing pedestals of steel pot bearings minor-to- moderate damage. Common were crushed during the earthquake (Fig.

Chang, Chang, Tsai, Sung: Seismic performance of highway bridges 77

43) and the box girders supported by them exhibited a permanent transverse dislocation (Fig. 44). 78 Earthquake Engineering and Engineering Seismology, Vol. 2, No. 1

Fig. 42(a) Fracture of uninstalled, cast- Fig. 42(b) Tumbling of uninstalled, cast- in-place, concrete girders in-place, concrete girders

Fig. 43 Damage to the pot bearing and Fig. 44 Transverse movement of the its pedestal superstructure

LESSONS LEARNED AND but to construct bridges across or near SUGGESTIONS active faults because of finite serviceable area. Under such a situation, “easy to From the bridge-disaster repair or reconstruct” should be a priority reconnaissance and field observations, in the bridge design and construction, several impressive lessons were learned. and alternative routes should be outlined First of all, fault rupture under or across in advance. Shorter spans may be bridge foundations is catastrophic to the installed in bridges to facilitate urgent structure and usually leads to collapse of restoration. spans, especially when the ground Moreover, the unusual characteristics dislocations are large. The Pi-feng bridge of near-fault earthquakes may lead to is a good example of such a case. unexpected damage to structures, However, since faults with different particularly to those older bridges. Even magnitudes are widespread in Taiwan, long-span bridges are vulnerable to near- there may sometimes be no alternative fault earthquakes. Seismic input energy

Chang, Chang, Tsai, Sung: Seismic performance of highway bridges 79 of near-fault earthquakes is usually behavior of bridges under real earthquake released in a very short instant, e.g., 1~2 ground motions is more complex than seconds, and thus considerable analytical prediction. Some design incremental velocity is brought forth. It details, such as the near-fault effect, may be necessary to conduct site specific confinement, and unseating prevention hazard analyses to have thorough device, etc., which may be roughly comprehension on the near-fault effect. estimated or overlooked, should be Furthermore, unseating prevention comprehended thoroughly. Further devices and sufficient seat widths should investigations on those major damaged be provided to protect from unintended bridges and the confirmation of fault strikes. Ground failure, skewed spans, locations may be needed. and liquefaction, etc., may induce It is necessary to proceed with both addition structural movements, such as the improvement of seismic design code the Shi-wei bridge. Well-designed shear and retrofit of old bridges. When agree- keys, restrainers, and concrete stoppers ment on the opinions on major damaged are excellent equipment for unseating modes and modified bridge design prevention. Also, as noticed in the field methodologies is reached, the seismic observation, the span next to either design code of bridges can be revised for abutment collapsed in all those bridges future construction. In addition, many with collapsed spans. Thus, adequate verified seismic retrofit techniques [8,9] design and construction of abutment should be applied to repair or strengthen back-walls and fill-backs are essential those minor damaged or old bridges. even for continuous spans. In addition, column-to-girder joints should be accurately designed and ACKNOWLEDGMENT constructed, especially for eccentric The authors are grateful to the connections, such that the seismic force National Center for Research on can be transferred by the expected load Earthquake Engineering (NCREE) and path. Also, sufficient and well- bridge-disaster reconnaissance group for constructed hoop bars or confinements providing the relevant information on the should be provided to prevent from shear 921 Chi-Chi earthquake. failure in piers.

REFERENCE SUMMARY AND DISCUSSION 1. The Ji-Ji Earthquake Web Site, The 921 Chi-Chi earthquake brought http://921.ncree.gov.tw (1999). National Center for Research on an intense impact on the seismic Earthquake Engineering, Taipei, resistant design and technical Taiwan. development of bridge engineering. 2. Design Code for Highway Bridge From the bridge- disaster Engineering, Issued No. 09499 (1960). reconnaissance, some valuable lessons Taiwan Highway Bureau, Technical were learned to review and improve Standard Committee, Ministry of the current seismic design code. Also, Transportation, November. as indicated by those observed 3. Design Code for Highway Bridges appearances, the realistic structural (1987). Ministry of Transportation, Taiwan, February. 80 Earthquake Engineering and Engineering Seismology, Vol. 2, No. 1

7. Taiwan Construction Research Institute 4. Seismic Resistant Design Code for (1999). “Engineering suffering from Highway Bridges (1995). Ministry of the 921 Chi-Chi earthquake” Special Transportation, Taiwan, January. Issue, Construction News Record, 5. National Center for Research on October. Earthquake Engineering (1999). “A 8. Scientific and Technical Consultant reconnaissance report of the 921 Chi- Division, Ministry of Transportation Chi earthquake for bridges and (1998). “A study on guidelines of seis- transportation facilities” Report No. mic resistant evaluation and retrofit for NCREE- 99-035, Taipei, Taiwan, bridges,” Technical Report, Taipei, Tai- December. wan, October. 6. China Engineering Consultants Inc. 9. Priestley, M.J.N., Sieble, F. and Calvi, (1999). “A preliminary reconnaissance G.M., Seismic Design and Retrofit of report of the 921 chi-Chi earthquake for Bridges, Wiley-Interscience Publication, bridges,” Taipei, Taiwan, September 28. John Wiley & Sons, Inc.