Hybrid seismic performance of a steel–concrete benchmark structure under nonlinear soil–structure interaction using a virtual real-time dynamic substructuring framework

Fixed-base assumptions remain prevalent in seismic design practice, despite the fact that soil compliance and strain-dependent stiffness degradation can substantially alter the dynamic response of structures under strong ground motion. This study presents a virtual real-time dynamic substructuring (vRTDS) framework, executed in a deterministic fixed-step discrete-time environment and designed to be hardware-in-the-loop ready, to assess the seismic response of a steel–concrete benchmark structure accounting for nonlinear soil–structure interaction (SSI). The superstructure is modeled as a linear elastic system, whereas the soil–foundation interface is represented by a lumped extended Bouc–Wen macro-element calibrated against resonant column tests. Three configurations are compared: a fixed-base model (AR.1), a linear-elastic SSI model (AR.2), and a nonlinear SSI model (AR.3). Five ground motions scaled to 0.10 g and 0.25 g are considered. Incorporating foundation flexibility produces a dual-resonance response, in which the second system frequency governs peak demands under high-frequency-rich records, a feature that AR.1 cannot capture and that results in underpredicted drift and base shear demands. Conversely, linear SSI overestimates spectral amplitudes by more than 220% for these records. At 0.25 g, soil yielding in AR.3 acts as a natural force-limiting mechanism, bounding peak base shear to approximately 62–64 kN under some seismic records, compared with 80–82 kN in AR.2. The results are specific to the benchmark, the soil profile, and the ground-motion suite considered, and the framework offers a validated numerical precursor for physical hybrid testing with nonlinear soil boundaries.