Seismic vulnerability of box girder continuous bridges under spatially variable ground motions

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In seismic design of extended multi-span bridges the question always arises whether using either natural or artificially generated earthquake records that are identical at all bridge supports is valid or not. It is likely that earthquake ground motion remarkably differs at the various support/pier locations in terms of amplitude, frequency content and arrival time, inducing under certain circumstances significant forces and deformations that would not develop if the assumption of synchronous excitation was adopted. This paper hence illustrates the impact of ground motion spatial variability on the seismic performance and vulnerability of extended continuous box girder bridges in both bridge orthogonal directions (longitudinal and transverse). For illustration purposes, a nine-span bridge with a total length of 430 m is adopted. Non-linear time history analyses are carried out using opensees software. The effects of the spatial variability in the ground motions at the different bridge supports are investigated using a set of 20 artificially simulated seismic ground motions generated using sim software developed in the mid-nineties, considering different degrees of loss in coherency and various soil types (i.e., 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.