Seismic demand and capacity models, and fragility estimates for underground structures considering spatially varying soil properties

This study proposes probabilistic seismic demand and capacity models for underground structures considering the spatial distribution characteristics of soil properties. The proposed models are then applied in a reliability analysis to estimate the seismic fragility of underground structures. First, the paper uses physics-based random fields to model different spatially varying soil properties. The random fields of soil properties are used to establish numerical models of the soil-underground structure system and simulate the structural response under pushover and seismic loads. Next, the paper develops probabilistic demand and capacity models based on the structural responses obtained from the numerical analyses. Utilizing the demand and capacity models, the paper estimates the seismic fragility of underground structures, and provides confidence intervals to capture the epistemic uncertainties in the estimates. As an illustration, the paper applies the proposed models to analyze the fragility of a typical rectangular underground structure and compares the fragility estimates under four selected seismic intensity levels. The results indicate that the proposed seismic demand model captures well the dependency between the maximum inter-story drift and peak ground velocity. The obtained fragility estimates can approximately assess the seismic fragility of similar underground structures located in similar sites.