Collapse performance and subsystem response of dual SMF–SCBF systems under nonlinear soil–structure interaction and near-fault motions

Existing collapse assessments do not clarify whether nonlinear soil–structure interaction (SSI) and near-fault ground motions with forward-directivity characteristics only alter global demand or also modify subsystem interaction in dual Special Moment Frame–Special Concentrically Braced Frame (SMF–SCBF) systems. Accordingly, this study aims to quantify how nonlinear SSI and near-fault excitation affect global seismic demand, collapse fragility, and SMF–SCBF subsystem interaction across different structural heights, while distinguishing increased relative SMF participation from an actual synchronized increase in SMF demand. Six 2-, 4-, 8-, 12-, 16-, and 20-story archetypes were designed according to ASCE 7–22 and AISC provisions. Fixed-base and flexible-base conditions were assessed through nonlinear static, time-history, and incremental dynamic analyses, with SSI represented by a Beam-on-Nonlinear-Winkler-Foundation model. The central novelty combines story-wise shear participation with event-based global base-demand changes after probable brace-force degradation, distinguishing relative participation from an actual increase in SMF demand. The results demonstrate that SSI effects are strongly height- and record-dependent, while near-fault motions primarily increase response dispersion and the upper range of transient deformation rather than uniformly amplifying all demands. The SCBF remains dominant, although SMF participation reaches approximately 35–40% in the upper stories of the 20-story SSI archetype under some near-fault conditions. The event-based assessment shows a negative median SCBF base-demand change in all evaluated combinations, whereas the median SMF change generally remains near zero or negative. Thus, the increased SMF share observed in the story-shear profiles represents height- and record-dependent subsystem engagement rather than a universal one-to-one transfer of demand from the SCBF to the SMF.