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Causes of Showa Bridge Collapse in the 1964 Niigata Earthquake Based on Eyewitness Testimony

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Causes of Showa Bridge Collapse in the 1964 Niigata Earthquake Based on Eyewitness Testimony SOILS AND FOUNDATIONS Vol. 47, No. 6, 1075–1087, Dec. 2007 Japanese Geotechnical Society CAUSES OF SHOWA BRIDGE COLLAPSE IN THE 1964 NIIGATA EARTHQUAKE BASED ON EYEWITNESS TESTIMONY NOZOMU YOSHIDAi),TAKASHI TAZOHii),KAZUE WAKAMATSUiii),SUSUMU YASUDAiv), IKUO TOWHATAv),HIROSHI NAKAZAWAvi) and HIROYOSHI KIKUvii) ABSTRACT Testimony from eyewitnesses of the Showa Bridge collapse was collected with the objective of pinning down the cause of the collapse. On the basis of this testimony, a chronology of events was established. The bridge collapsed about 70 seconds from the beginning of the earthquake, i.e., long after the strong shock. The liquefaction-induced ‰ow occurred after the collapse of the bridge. By studying these times of occurrence, as well as observed earthquake records, the cause of the Showa Bridge collapse was deduced. The possibility of inertia force in the superstructure and liquefaction-induced ‰ow is low as the main cause of the collapse of the Showa Bridge. There is a high possibility that it was due to increased displacement of the ground in circumstances where pile deformation occurred more easily due to liquefaction. Key words: bridge, earthquake, foundation, liquefaction, (liquefaction-induced ‰ow) (IGC:E8) Showa Bridge was designed based on the most advanced INTRODUCTION earthquake-resistant engineering at that time and was The 16 June 1964 Niigata earthquake is well known as completed just before the earthquake. A brief review of the earthquake during which liquefaction caused serious past researches is as follows: damage to structures and is thus important in geotechni- The Japan Society of Civil Engineers (JSCE) study cal engineering. The collapse of the Showa Bridge was showed the following as possible causes of the bridge col- one of the damages caused by the earthquake that attract- lapse, from the damage observed to the superstructure ed considerable attention (Fig. 1, Photo 1), but even and foundation structure of the bridge and to the ground now, more than 40 years later, no common under- (JSCE, 1966): 1) Horizontal movement of the left bank standing has been obtained in identifying the cause of the side ground of about 3 m towards the center of the river, collapse. 2) liquefaction of the ground, 3) horizontal external force Many studies took place immediately after the earth- worked on the bridge girder resulting from horizontal quake to investigate the cause of collapse, because the movement of the abutment on left bank side, 4) large Fig. 1. Elevation view of the collapsed Showa Bridge (modiˆed from JSCE, 1966) (unit: m) (M: moveable bearing, F: ˆxed bearing, numbers at the bottom of the ˆgure represent horizontal displacement of the ground at each pier) i) Tohoku Gakuin University, Japan (yoshidan＠tjcc.tohoku-gakuin.ac.jp). ii) Shimizu Corporation, Japan. iii) National Research Institute for Earth Science and Disaster Prevention, Japan. iv) Tokyo Denki University, Japan. v) The University of Tokyo, Japan. vi) Port and Airport Research Institute, Japan. vii) Kanto Gakuin University, Japan. The manuscript for this paper was received for review on August 24, 2006; approved on July 18, 2007. Written discussions on this paper should be submitted before July 1, 2008 to the Japanese Geotechnical Society, 4-38-2, Sengoku, Bunkyo-ku, Tokyo 112-0011, Japan. Upon request the closing date may be extended one month. 1075 1076 YOSHIDA ET AL. sliding of the left bank into the river, which caused large deformation of the P6 pier resulting in the G6 girder col- lapse. Kubo (1981) has pointed out several competing factors: 1) a reduction in bearing capacity of sand layer due to liquefaction, 2) insu‹cient width to carry the gird- er at the top of the piers, 3) roller support of G6 and G7 girders on P6 pier. Several researches put the cause down to liquefaction- induced ‰ow. Hamada et al. (1986) showed that the earthquake caused considerable horizontal ‰ow displace- ment of the revetments along the Shinano River from measurements using aerial photographs taken before and after the earthquake, the horizontal displacement of the revetments in the vicinity of the Showa Bridge towards the center of the river was 8 m, which is extremely large. Hamada (1992) focused on the fact that, from the tes- timony of witnesses, the Showa Bridge collapsed not at the time of the main shock but after the main shock, which makes it easier to explain the damage as occurring due to liquefaction-induced ‰ow. But he does not men- tion exactly how liquefaction-induced ‰ow in‰uenced the collapse of the bridge. Sento et al. (2001) have suggested that liquefaction-induced ‰ow occurred because of the failure at the top of the liqueˆed layer by the ‰ow of ex- Photo 1. Collapsed Showa Bridge (Modiˆed from National Informa- cess pore water pressure from downward in the process of tion Service for Earthquake Engineering) excess porewater pressure dissipation, and that this mechanism also caused the collapse of the Showa Bridge. But their research assumes a gradient in the liqueˆed layer horizontal displacement of the foundation resulting from in the direction perpendicular to ‰ow, while ignoring the ground displacement, 5) reduction in lateral ground gradient in the direction of ‰ow. resistance due to liquefaction, and 6) loss of bearing The causes set out above for the earthquake damage to capacity due to poorly welded pile joints. the Showa Bridge can be classiˆed as follows from the Several possibilities have been proposed that inertia point of view of external load: force played an important role. One of them is that piles 1) Inertia force: signiˆcant inertia force worked at yielded at the river bed position (Kuribayashi, 1987). the time of the earthquake. Another cause is that, because the predominant period of 2) Liquefaction: liquefaction reduced lateral sub- the P6 pier, by which girders are roller supported, was grade reaction, or pile displacement increased diŠerent from the other piers such as P5 and P7, by which when ground displacement occurred as apparent one girder is roller supported and the other simply sup- length above the ground surface became large. ported; phase diŠerence was generated in the piers, and 3) Liquefaction-induced ‰ow: liquefaction-induced this caused the collapse at the roller supported G6 and G7 ‰ow aŠected the pile foundations causing con- girders loaded on the P6 pier (Public Works Research In- siderable deformation to the piles, or the horizon- stitute, 1965; Arakawa et al., 1982). Tazoh et al. (1985) tal movement of the left bank pushed bridge gird- have pointed out the possibility that the damage resulted ers towards the center of the river. from ground deformation, i.e., kinematic interaction, Considering the relation between these causes and the because the location of residual bending deformation on time of the bridge collapse, it can be predicted that if iner- the P4 pier piles, matches the boundary between the soft tia force had caused the collapse, the collapse would have and stiŠer soil layers. started from the time of largest acceleration; if liquefac- As researches attributing the cause to liquefaction, tion were the cause, the collapse would have started from Okubo et al. (1966) suggested from seismic response anal- the time ground displacement increased, and if ysis, as a possible cause of the collapse, the increase in liquefaction-induced ‰ow were the cause it would have displacement due to decrease of soil rigidity through li- started after ‰ow had occurred. However, it is unclear at quefaction as well as increase of the predominant period what time the bridge collapsed and liquefaction-induced of the pier and piles structural system, combined with the ‰ow occurred. horizontal displacement of the bridge abutment. Okamo- The existing research mentioned above only suggests to (1971) has argued that liquefaction occurred in the possible causes of the damage without substantiation. coarse sand with 10 meters thickness in the river, reducing Furthermore, where analytical studies have been under- the length of the embedded section of the piers, and mak- taken, they state only how the collapse might be ex- ing the pier supports easier to bend. Added to this was the plained through hypothetical mechanisms, and have not CAUSES OF BRIDGE COLLAPSE IN NIIGATA EARTHQUAKE 1077 considered how this relates to the actual time of the col- identiˆed from G6 to the ˆnal girder on the left bank side. lapse. Thus, although a number of causes have been But the girders on the right bank side suŠered no major oŠered on the collapse of the Showa Bridge, none has damage except deformation of handrail above pier P9 reached the stage of gaining universal acceptance. (JSCE, 1966). The three phenomena that have so far been suggested The damaged P4 pier was pulled out for examination; as causes: the inertia force, liquefaction, and liquefac- there was no residual deformation in the pile for about 4 tion-induced ‰ow, are important factors for earthquake- m up from the tips, that local buckling occurred about 4 resistant design of foundations. If they are all taken into m from the pile head. The pile deformation suggested account together, the result may be over-design, but if that horizontal displacement of about 1 m in the direction they are taken into account individually the design may of the right bank (center of river) occurred around the risk danger. In order to utilize the Showa Bridge collapse pile heads (JSCE, 1966). as a lesson for future earthquake-resistant design, it is vital to clarify whether the cause was a single factor or the combination of a number of factors. However, more EYEWITNESS TESTIMONY than 40 years have passed since the Niigata Earthquake Eyewitnesses and it is now impossible to obtain any physical evidence For this study, 27 people who witnessed the collapse of relating to the cause of the bridge collapse.
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