Across-fault ground motions and their effects on some bridges in the 1999 Chi-Chi earthquake

Simply-supported bridges are vulnerable to surface fault rupture as evidenced by several fault-crossing bridges in the 1999 Chi-Chi earthquake. To investigate the seismic collapse mechanism of simply-supported bridges crossing the fault, across-fault ground motions are firstly simulated in the present study. In particular, based on a previously developed fault model of the 1999 Chi-Chi earthquake, broadband across-fault ground motions at six fault-crossing bridges are simulated using the hybrid deterministic-stochastic method, in which the low- and high-frequency components are computed using the deterministic Green’s function method and the stochastic finite-fault modeling method, respectively. The simulation results indicate that the hybrid deterministic-stochastic method can give reasonable predictions to the across-fault ground motions. Furthermore, utilizing the explicit dynamic finite element (FE) code LS-DYNA, behaviors of a three-span simply-supported bridge under a selected pair of across-fault ground motions are numerically simulated. Numerical results indicate that the structural responses and collapse mechanisms are dominated by the low-frequency ground motions. The large differential static offset across the fault is the main reason for the collapse of the simply-supported bridges. This study contributes understandings for the across-fault ground motions and the collapse mechanism of some bridges in the 1999 Chi-Chi earthquake.