Nonlinear seismic responses of a long-span railway suspension bridge crossing strike-slip fault rupture zones

Abstract

This study investigates the nonlinear seismic responses of a long-span suspension bridge crossing strike-slip faults and their sensitivity to various parameters and conditions. Four sets of near-fault records containing forward directivity and fling-step effects are selected, corrected, and then incorporated into the analysis. Empirical model is proposed using 54 sets of near-fault records to evaluate the dynamic pulse amplitude in fling-step ground motions. A simplified scaling method considering self-consistency is proposed to obtain the fling-step ground motions with different fault static offset and dynamic pulse amplitude. Multiple parameter method is derived by theoretical analysis to consider the effect of the fault-crossing angle on the wave propagation distance and direction. A nonlinear numerical model of double-pylon suspension bridge is established to study the seismic responses of near-fault and fault-crossing suspension bridges. The results indicate that the girder of the fault-crossing bridge rotates around the midspan, and the most of the bridge responses are slightly smaller than those of the near-fault case, whereas the transverse residual bearing displacement is substantially higher. Moreover, the sensitivity of the bridge responses to the fault-crossing angle, fault-crossing location, permanent displacement, dynamic pulse-type motion, ground motion component, and wave passage effect is comprehensively investigated. The minimum bridge responses, except for the residual bearing displacement, are obtained at a fault-crossing angle of 90° and a fault-crossing location at the middle span. The effects of the permanent displacement, dynamic pulse-type motion, and apparent wave velocity on the bridge responses are highly correlated with the fault-crossing angle. At a fault-crossing angle of 90°, all bridge responses are insensitive to these parameters, except that the peak transverse pylon drift and the bearing displacement increase considerably with the increasing amplitude of dynamic pulse-type motion. When the fault-crossing angle is acute (obtuse), the wave passage has a considerable effect on the bridge responses, and most of the bridge responses increase significantly with an increase in the permanent displacement and dynamic pulse-type motion. However, the permanent displacement has a negligible effect on the peak pylon drift. The wave passage effect is more reasonable to be considered using the apparent wave velocity in the horizontal direction than in the bridge axis direction because the influence of the fault-crossing angle on the propagation distance and direction is taken into account. Furthermore, the vertical component exerts a minimal effect on all bridge responses, whereas the fault-normal component significantly affects most bridge responses.