The impact of soil-structure interaction on the hybrid force-displacement seismic design method for steel moment-resisting frames

This study investigates the influence of soil–structure interaction (SSI) on the Hybrid Force–Displacement (HFD) seismic design method for steel moment-resisting frames (MRFs). The HFD method, which combines the benefits of force-based and displacement-based design, is extended to incorporate SSI through a beam-on-nonlinear Winkler foundation (BNWF) model. This model captures the nonlinear soil response and its interaction with the structure, enabling a more realistic performance-based design framework. To assess the effectiveness of the extended HFD method, a large-scale parametric study is conducted using 100 far-field ground motions, categorized into four suites of 25 records corresponding to EC8 soil classes A, B, C, and D. Nonlinear time history analyses indicate that neglecting SSI may overestimate seismic demands in some configurations and may therefore lead to conservative designs. Incorporating SSI through the BNWF model, particularly for soft soils, allows both global responses, including inter-storey drift and roof displacement, and local demands, including member plastic rotations, to be evaluated while accounting for foundation flexibility. The results show that the proposed extension preserves the force-based simplicity of the HFD method and provides a more complete representation of displacement and damage demands when SSI effects are significant. For soft soils, especially EC8 class D, the method generally yields lower base shear demands and higher behavior factors than the fixed base formulation, particularly for taller frames, although the magnitude of these effects varies with frame height and ground motion. The seismic design of the frames is carried out using the extended HFD method, the original HFD method, and the EC8 force-based procedure. The resulting designs are assessed through nonlinear time history analyses. Two representative numerical examples illustrate that explicit consideration of SSI can provide more realistic drift estimates and a more rational use of deformation capacity in cases where foundation flexibility is substantial, while the differences remain limited for short frames on stiff soils.