Nonlinear soil-structure interaction effects on eccentric seismic pounding and torsional response of adjacent buildings: Experimental validation and reduced-order modelling

Seismic pounding between adjacent buildings, particularly those with torsional irregularities, critically threatens structural safety during strong ground motions. This study investigates the nonlinear dynamic response of torsionally coupled moment-resisting frames subjected to eccentric pounding, explicitly considering nonlinear soil-structure interaction (SSI). A 3D time-domain finite element framework is developed, incorporating nonlinear material behavior, Hertzian contact laws, rotational inertia, and strain-dependent soil stiffness via an enhanced nonlinear Winkler foundation. The model is validated against scaled shaking table experiments of asymmetric structures on laminar soil boxes, yielding close agreement between numerical and experimental responses (errors <10% for translational and <15% for torsional metrics). Results show that, compared to fixed-base assumptions, nonlinear SSI increases peak torsional rotations by 25–40%, delays initial pounding onset by 15–20% of the dominant period, amplifies impact forces by up to 30%, and increases inter-story drifts by 20–35% in torsionally irregular configurations. Parametric analyses reveal critical separation gaps of 1.0–1.5% of building height; below this threshold, pounding-induced response amplification becomes pronounced. Furthermore, reducing equivalent soil stiffness beyond 40–50% sharply escalates torsional demand and impact intensity. By quantifying the coupled influence of torsional irregularity and SSI on seismic pounding, this study offers design-oriented thresholds to mitigate pounding risk in closely spaced buildings on soft soils, contributing to refined performance-based seismic design guidelines.