Copula-based seismic fragility assessment of base-isolated structures under near-fault forward-directivity ground motions

The seismic fragility of base-isolated structures subjected to near-fault forward-directivity ground motions is investigated. A general framework for deriving the system-level fragility curve is proposed, where the vulnerability contributions of multiple interrelated components to the overall system are incorporated. In this framework, the joint probabilistic seismic demand model (JPSDM) is established, where the joint probability distribution of multiple engineering demand parameters (EDPs), conditioned on the intensity measure (IM) level, is characterized via copula approach, and the sampling-based JPSDM, along with the capacity models of these components, is employed to generate the overall system fragility curves. This proposed framework is applied in the case study of a typical seismically-isolated RC frame structure, where the peak lateral displacement in the base-isolation layer and the maximum inter-story drift in the superstructure are considered as the two major component EDPs. The analysis results indicate that the combination of t copula, which is quantitatively identified as the best-fit copula function, and the conditional lognormal marginal distribution adequately captures the joint probability distribution of these two EDPs conditioned on the IM level. Moreover, the impact of different copulas selection on the system-level fragility varies depending on the relative fragility contributions of different components in the overall system.