Seismic vulnerability assessment of RC skew bridges subjected to mainshock-aftershock sequences

Due to the potential of strong aftershocks in the increase of vulnerability of bridges damaged under mainshocks, accurate evaluation of the structural performance during the seismic sequences is essential. This paper investigates the effect of different parameters such as skew angle of deck and direction of seismic excitation on the fragility curves of RC skew bridges subjected to mainshock and aftershocks. Fragility assessment is performed using a cloud analysis method subjected to a wide range of as-recorded sequences. First, a proper engineering demand parameter (EDP) which can result in the most probability of failure at bridges employed in this study is determined. The vulnerability of the bridge is then evaluated for different geometries representing various skew bridges and also for different incidence angles. Finally, comparisons are made with the fragilities in HAZUS. Results indicate that fragilities are significantly affected by aftershocks thus considering mainshock only is found to be unconservative. Comparison of median values for bridges with different skew angles across all damage states reveals that bridges with small skewness (almost straight geometry) can be less vulnerable than those with more skewness. Furthermore, fragilities are extremely sensitive to the direction of ground motion and therefore exclusive consideration of incidence angle can underestimate the seismic vulnerability in skew bridges. Results also show that the fragilities obtained based on aftershock and excitation orientation effects are more vulnerable than predicted by HAZUS and thus it seems necessary to revise damage functions for bridges in HAZUS in order to achieve a more accurate seismic risk assessment.