Fragility-based Seismic Assessment of Reinforced Concrete Frame–tube Tall Buildings Incorporating Soil-structure Interaction

Rapid urbanization and limited available land have increased the construction of high-rise buildings, highlighting the importance of realistic seismic vulnerability assessment in performance-based design. This study analyzes the probabilistic seismic behavior of a 33-story reinforced concrete frame–tube building with core shear walls, modeled according to Iranian seismic codes. Nonlinear Incremental Dynamic Analysis was conducted in MIDAS software under both Fixed Base and Soil–Structure Interaction conditions, using ground motions scaled between 0.1 g and 1.5 g. Fragility curves were generated based on peak inter-story drifts corresponding to four damage states: Slight, Moderate, Extensive, and Complete. Results show that including SSI leads to significantly higher seismic vulnerability, with the structure reaching equivalent damage states at lower PGA levels than the fixed-base model. SSI also causes period elongation and greater deformation demands, indicating the need for enhanced foundation stiffness, soil improvement, or the application of energy-dissipation and isolation strategies to reduce adverse effects. Moreover, comparison of long- and short-period ground motions reveals that the building is more vulnerable to long-period events, especially when SSI is considered. Median PGA thresholds consistently show lower values for SSI across all damage states, confirming the importance of accounting for soil–structure interaction in seismic evaluation. While the results are specific to the soil and structural characteristics studied, the observed trends such as increased seismic demand and vulnerability due to SSI are expected to be applicable to similar high-rise buildings and highlight the need for integrating SSI into future seismic assessments.