Soil–Structure Interaction in Dual Wall–Frame Systems: Seismic Response and Code-Based Classification

Soil–Structure Interaction (SSI) is known to influence the seismic response of structures; however, its implications for the classification of dual wall–frame systems within the framework of Eurocode 8 remain insufficiently understood. This study investigates how SSI affects not only the global response but also the code-based classification of a reinforced concrete dual wall–frame system. A 9-storey prototype building is analyzed using fixed-base and flexible-base models, considering linear-elastic, nonlinear static (pushover), and nonlinear dynamic (time-history) analyses. As expected, the results show that SSI induces a significant redistribution of seismic forces, reducing the contribution of shear walls and increasing the role of frames. As a consequence, the system classification shifts from wall-equivalent dual to frame-equivalent dual, or even toward frame-dominated behavior under Eurocode 8. A comparison with ASCE/SEI 7-16 reveals that such classification changes are less pronounced due to broader system definition limits. The findings highlight that SSI influences not only structural demand but also key design parameters, including behavior factors and force distribution assumptions. This underscores the need for consistent consideration of SSI effects in both analysis and system classification within seismic design codes.