Collapse of Showa Bridge during1964 Niigata earthquake: a quantitative reappraisal on the failure mechanisms

Collapse of Showa Bridge during the 1964 Niigata earthquake has been, over many years, an iconic case study for demonstrating the devastating effects of liquefaction. Inertial forces during the initial shock (within the first 7 s of the ground shaking) or lateral spreading of the surrounding ground (which started at 83 s after the start of the shaking) cannot explain the failure of Showa Bridge as the bridge failed at about 70 s following the main shock and before the lateral spreading of the ground started. In this study, quantitative analysis is carried out for various failure mechanisms that may have contributed to the failure. The study shows that at about 70 s after the onset of the earthquake shaking, the increased natural period of the bridge (due to the elongation of unsupported length of the pile caused by soil liquefaction) tuned with the period of the liquefied ground causing resonance between the bridge and the ground motion. This tuning effect (resonance) caused excessive deflection at the pile head, resulting in unseating of the bridge deck from the supporting pier and thereby initiating the collapse of the bridge.