Effect of spatially variable ground motions on the seismic fragility of box girder continuous bridges

Abstract

Seismic design of bridges is usually performed assuming that all bridge supports experience the same ground motion (i.e. uniform input). However, strong-motion recorders of special seismograph arrays already demonstrated that significant spatial variation do exist in seismic ground motions [1]. The spatial variation of seismic ground motions is due to various sources including the loss of coherency, which results from random reflections and refractions as the waves travel through the soil, and the wave-passage effect. The present paper illustrates the impact of ground motion spatial variability on the seismic performance of continuous box girder bridges in both bridge orthogonal directions (longitudinal and transverse). For illustration purposes, a nine-span bridge with a total length of 430m is adopted. Non-linear time history analyses are carried out using OPENSEES software [2]. The wave passage effects are examined individually using a set of real records originally extracted from the PEER (Pacific Earthquake Engineering Center) Strong Motion Database using different apparent wave propagation velocities. Then, the loss of coherency effect is investigated using a set of artificially simulated seismic ground motions using the SIM software developed by Ramadan and Novak [3&4] considering different degrees of loss in coherency and various ground motion frequency contents. Results of the non-linear time history analyses performed in an incremental dynamic analysis context are hence manipulated through a probabilistic analysis framework to generate fragility curves associated with various performance levels for the case study bridge. Fragility curves giving the conditional probability of exceeding various performance levels are then integrated with generated hazard curves defining the expected seismic hazard in Egypt. The outcome of this integration process results in values of mean annual frequency of exceeding pre-defined performance levels.