Comparative assessment of seismic collapse risk for non-ductile and ductile bridges: a case study in China

Abstract

Risk assessment plays an important role in quantifying earthquake loss and has been widely used in the field of seismic engineering. However, current studies of the risk assessment rarely consider the different failure modes of piers (i.e., flexure failure, flexure-shear failure and shear failure), which will lead to an excessive estimation for the seismic performance and collapse safety of the reinforced concrete girder bridges. This paper focuses on studying the seismic behavior of bridges considering different failure modes and comparing the seismic collapse risk of the bridges with and without ductile detailing. The numerical analysis is performed in an open source finite element (FE) software framework, Open Sees. Nonlinear shear springs are introduced into the FE models of reinforced concrete (RC) columns to accommodate the shear and flexural-shear effect of the bridges. The accuracy of the FE models is validated by using the existing results of static cyclic-loading experiments. Case studies are conducted for two categories of the girder bridges, which are designed based on new and old seismic design criteria in China, respectively. The collapse fragility curves are developed by using the incremental dynamic analysis method, which considers the uncertainties of the ground-motion characteristics and the structural material parameters. Based on the seismic hazard curve of a designated region, a comparative assessment of the seismic collapse risk is performed regarding mean annual collapse rate for the non-ductile and ductile bridges. The main results of the paper are: (1) The FE model with the nonlinear shear spring provides an acceptably accurate solution for simulating the initial stiffness, shear failure point, strength degradation slope and residual shear strength of the RC columns; (2) The structural collapse safety is shown to be related not only to the collapse probability of the bridges but also to the occurring probability of the ground motion intensity exceeding a given threshold; (3) The mean annual collapse rate of the ductile girder bridges is significantly smaller than that of the non-ductile girder bridges. The results can be used as a reference to implement specific policies for appraising and mitigating the seismic risk of existing RC bridges.