The time-dependent seismic risk of bridges is assessed while account accounting for the effect of aftershocks and the uncertainty in the damage state after a mainshock event. To achieve this, a Markov risk-assessment framework is adopted to account for the probabilistic transition of the bridge structure through different damage states and time-dependent aftershock hazard. The methodology is applied to three typical California bridge configurations that differ only based on their era of design and construction. Era 11, Era 22, and Era 33 designations are used for the three bridges, which are designed and detailed to reflect pre-1971, 1971–1990 and post-1990 construction. In addition to mainshock-only evaluations (used as a benchmark to quantify the additional risk posed by aftershocks), pre-mainshock (to account for the uncertainty in the occurrence of mainshock and aftershock events) and post-mainshock (which are based on a conditioning mainshock event and bridge damage state) seismic risk assessments are performed. To support these assessments, a set of 34 pairs of ground motions from as-recorded mainshock–aftershock sequences is assembled. Sequential nonlinear response history analyses (including incremental dynamic analyses) are used to obtain the response demands when the structural models of all bridges are subjected to mainshock-only records or mainshock–aftershock record-pairs. Physical damage in both the mainshock and mainshock–aftershock environments is defined using the following mutually exclusive and collectively exhaustive limit states: Intact, Slight, Moderate, Extensive, and Complete. For both the pre- and post-mainshock assessments, the additional risk caused by aftershock hazard is found to be higher for the older bridges (i.e., Era 11 and Era 22) and more severe conditioning damage states. A direct correlation between the bridge’s age and the increase in seismic risk due to aftershock hazard was also observed for the pre-mainshock assessment. It is suggested that the proposed methodology be used to make informed decisions regarding the appropriateness and timing of bridge closures (partial and complete) following a seismic event while considering aftershock hazard.
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