Soil-structure interaction effects in analysis of seismic fragility of bridges using an intensity-based ground motion selection procedure

The paper focuses on the effects of Soil-Structure Interaction (SSI) in seismic fragility analysis of reinforced concrete (RC) bridges, considering the vulnerability of multiple critical components of the bridge and different modelling approaches for soil-foundation and bridge-embankment interactions. A two-step procedure, based on the introduction of springs and dashpots at the pier foundations and the abutment to account for inertial and kinematic SSI effects, is incorporated into a component-based methodology for the derivation of bridge-specific fragility curves. The proposed methodology is applied for quantifying the fragility of a typical highway overpass at both the component and system level, while the effect of alternative procedures (of varying complexity) for modelling foundation and abutment boundary conditions is critically assessed. The rigorous SSI modelling method is compared with simpler methods and the results show that consideration of SSI may only slightly affect the probability of system failure, depending on the modelling assumptions made. However, soil-structure interaction may have a notable effect on component fragility, especially for the more critical damage states. This is an observation that is commonly overlooked when assessing the structural performance at the system level and can be particularly important when component fragility is an issue, e.g. when designing a retrofit scheme.