Seismic Performance Assessment of Base-Isolated Steel Moment-Resisting Frames Considering Soil–Structure Interaction via the Endurance Time Method

Concurrent consideration of Soil–Structure Interaction (SSI) and seismic isolation is crucial for an accurate seismic performance assessment of structures, yet it remains a computationally challenging task, particularly for irregular systems on soft soils. This study aims to quantify the seismic performance of base-isolated steel moment-resisting frames, explicitly incorporating SSI effects, by employing the Endurance Time Method (ETM) as an innovative and computationally efficient assessment tool. A series of nonlinear dynamic analyses were conducted on three-story and 12-story steel frames with regular and irregular configurations. Four soil types, ranging from dense to very soft, were modeled using a direct approach to capture nonlinear SSI effects. The isolation system was modeled using Lead Rubber Bearings (LRBs) with bilinear hysteretic behavior. Results indicate that SSI significantly modulates the effectiveness of base isolation. For the three-story regular building, the reduction in Peak Relative Floor Displacement (PRFD) decreased from 80.79% on a fixed-base to 66.92% on very soft soil. This performance degradation was most pronounced in the 12-story irregular structure, where PRFD and Peak Floor Acceleration (PFA) reductions fell to as low as 31.07% and 23.02%, respectively, on very soft soil. The findings reveal that neglecting SSI can lead to a significant underestimation of isolator displacement demands, thereby compromising the stability of the isolation layer and potentially inducing non-conservative failure modes. Furthermore, this study validates the ETM as a computationally efficient and robust framework for quantifying complex interactions that govern the dynamic stability of isolated structures on flexible soils.