Time-Dependent Risk Assessment of a Containment Building Subjected to Mainshock-Aftershock Seismic Sequences

The additional hazard from aftershocks and the progressive degradation of structural capacity induced by aftershock sequences can increase the seismic risk of structures. This paper aims to assess the time-dependent risk of a containment building subjected to mainshock-aftershock sequences. To quantify the seismic risk at the system level, a new Markov-based framework for the system analysis is proposed in this study. In this proposed framework, the seismic performance of internal equipment is characterized by the fragility of nonstructural components, and the contributions of various failure events and accident sequences are identified using the event tree and fault tree. To consider the seismic performance of damaged structures, state-dependent fragility curves are generated through a proposed fragility method, which can incorporate the effect of initial damage states on the residual capacity. Further, in order to measure the seismic performance of a system, the system-level fragility is derived using fragilities of all basic components and accident sequences. Finally, the Markov chain is introduced to account for the stochastic process describing the damage progression and uncertainty in the aftershock occurrence, and then the time-dependent risk of a containment building at a system level is obtained in both post- and pre-mainshock scenarios. To gain insight and familiarity with the risk of a system under seismic sequences, a series of parametric studies are also conducted in this study. The results provide quantitative information about the effect of aftershocks on the seismic risk of a system, and it is suggested that the contribution of aftershocks should be considered in the seismic risk management to ensure the seismic safety of containment buildings during the service life.