Incorporating soil-structure interaction into simplified numerical models for fragility analysis of RC structures

Simplified building models are a valuable option for seismic assessment at the regional scale. These models often use calibrated springs to model column behaviour, and recent advances have made them suitable for capturing torsional response in Reinforced-Concrete Moment-Resisting-Frames. Nevertheless, their validation is typically achieved using fixed-base models, which do not include the influence of soil-structure interaction (SSI). This study introduces a novel approach to quantify the accuracy of a recently developed simplified model while accounting for dynamic SSI, using a newly implemented, refined 3D Finite Element non-linear soil model in OpenSees. The accuracy of the simplified structural model is assessed by comparing the results of non-linear dynamic analyses with those of a refined model in terms of (i) a peak structural demand parameter such as the interstorey-drift ratio and (ii) fragility curves computed from cloud analysis and accounting for collapse cases. The study presents details of the proposed refined approach for 3D soil modelling in OpenSees, focusing on implementing free-field boundary conditions and structure-to-soil connections. Results show that the accuracy of the simplified model is maintained, even in the presence of SSI, and it successfully captures the overall structural response measured at peak demand. For the proposed case study, the difference between the simplified and refined models’ fragility curves’ medians is 4% and 2% for fixed and SSI models, respectively. The simplified structural model, combined with the refined soil model for SSI effects, presents an innovative and conservative, yet computationally efficient, alternative for seismic risk analysis, even in the presence of structural irregularity.