Influence of Soil–Structure Interaction on the Nonlinear Seismic Response of Mixed RC–Steel Buildings under Near-Fault Ground Motions

Conventional seismic design approaches are generally based on idealized assumptions, including uniform structural material distribution along the building height and fixed-base foundation conditions for usual buildings. However, contemporary construction practices frequently adopt mixed structural systems, typically consisting of a reinforced concrete (RC) lower structure and a steel upper superstructure, founded on deformable soil deposits. Such configurations introduce significant vertical irregularities and soil–structure interaction (SSI) effects that are not adequately addressed by current seismic design provisions.

This study investigates the nonlinear seismic behavior of three-dimensional mixed-material buildings with vertical structural irregularities, explicitly accounting for SSI under near-fault earthquake excitation. Two representative soft-soil profiles are considered and compared with the typical fixed-base assumption. Particular attention is given to the RC–steel interface, which is modeled as a pinned connection to represent a critical, conservative design condition. The influence of earthquake incidence angle on the structural response is also examined.

A series of nonlinear time-history analyses is performed using selected near-fault ground motion records. The results reveal that SSI substantially modifies seismic demand and deformation patterns in mixed structural systems. In particular, soil flexibility amplifies response variability and may significantly alter the distribution of internal forces and displacements. Consequently, neglecting SSI effects can lead to inaccurate performance predictions and potentially unconservative design decisions.

The findings highlight the importance of realistic numerical modeling for achieving improved structural performance and more efficient design solutions. Enhanced prediction of seismic response contributes to the development of safer, more durable buildings by reducing damage potential, minimizing repair requirements, and extending service life. By addressing the combined influence of SSI and mixed RC–steel structural configurations, this research provides valuable insights for advancing the performance-based seismic design of modern building systems.