Time-dependent seismic risk assessment of highway bridges in western Canada under mainshock-aftershock subduction earthquakes

This study conducts the time-dependent seismic risk assessment of highway bridges in western Canada when subjected to Cascadia subduction earthquakes (CSE) and aftershocks (AS). The performance-based earthquake engineering framework is extended to assess the expected annual repair cost ratio and annual restoration time of a benchmark bridge class under CSE mainshock (MS) and AS events within a 5-day period. The epidemic-type aftershock sequence (ETAS) model is utilized to simulate the temporal evolution of AS events after the MS of CSE. Seismic hazard models for MS and AS are used to select CSE-consistent MS ground motions and build an AS ground motion database for selecting and pairing ETAS-matching MS-AS sequences. High-fidelity numerical bridge models are then developed and paired with these MS-AS motion series for non-linear dynamic response analyses. The Park and Ang damage index is modified into a demand-capacity ratio model to account for the long-duration-induced cumulative damage to bridge columns. Day-to-day time-dependent fragility models are then developed for the bridge class at both component and system levels under MS and time-changing AS events. Furthermore, these fragility models are integrated with the CSE hazard model and loss functions to estimate the risk metrics for the bridge class. The study highlights the significantly higher bridge risk when the AS hazard is considered, with the elevated risk being stabilized by the third day after the MS. However, this study also indicates that the commonly adopted one-MS-one-AS approach underestimates the bridge AS risk when facing CSE.