Time-Varying Seismic Fragility Analysis of Offshore Bridges with Continuous Rigid-Frame Girder under Main Aftershock Sequences

Durability damage of reinforced concrete bridges in an offshore environment often occurs in varying degrees with the extension of service life. Meanwhile, statistical data on earthquake damage show that most of the main earthquakes are accompanied by strong aftershocks, and the seismic response of bridges under the main shock and main aftershock is different. To analyze the effects of the main aftershock sequence and durability damage on the seismic performance of concrete bridges, this study establishes a nonlinear dynamic analysis model of an offshore bridge with a continuous rigid-frame girder based on the OpenSEES platform. Moreover, this work constructs the main aftershock sequence considering the degradation of the mechanical properties of materials after durability damage repair under normal maintenance conditions and the time-varying seismic effect of shortening the subsequent service period, and investigates the time-varying seismic fragility of offshore bridges under the main aftershock sequence. Results show that the seismic response of bridge structures considering the main aftershock sequence is significantly larger than that considering the main shock only. Material deterioration will lead to the increase in the seismic response of bridge components under earthquake. The seismic time-varying fragility of offshore bridges considering material deterioration and seismic time-varying effects is analyzed based on time-varying seismic action in the entire life cycle. The influences of the two factors should be considered comprehensively when evaluating the seismic performance of bridges throughout their entire life.