Influence of soil-structure interaction and ground motion parameters on the seismic vulnerability of RC buildings

Seismic performance of reinforced concrete (RC) structures is strongly influenced by soil–structure interaction (SSI), the effects of which depend on soil properties, foundation flexibility, and seismic input characteristics. This study investigates the seismic response and vulnerability of a five-story RC frame structure by explicitly considering SSI under seismic loading conditions. A three-dimensional numerical model is developed in PLAXIS3D to simulate the interaction between the structure and underlying dense sand and soft clay soils. The influence of SSI on lateral displacements, interstory drifts, and seismic fragility is evaluated. The Mohr–Coulomb (MC) model is adopted for clayey soils, while the Hardening Soil model with small-strain stiffness (HSS) is employed for sandy soils, with results indicating that the HSS model more accurately captures the seismic response of sandy deposits. For the soil profiles and structural configuration considered, the results indicate that SSI may lead to increased lateral displacements and interstory drifts, particularly in the presence of soft clay soils and flexible foundation conditions. Fragility curves are developed to quantify structural vulnerability under both near-fault and far-field ground motions. The findings show that the combined effects of SSI and near-fault seismic characteristics increase the probability of exceeding damage states at lower peak ground acceleration (PGA) levels for the studied system. These results highlight that the influence of SSI on seismic response is scenario-dependent and emphasize the importance of incorporating site-specific soil conditions and realistic soil–structure interaction modeling in performance-based seismic design and vulnerability assessment.