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

Soil–Structure Interaction (SSI) has been extensively investigated for moment-frame or shear-wall lateral load resisting systems, yet its implications for dual wall–frame systems remain less well understood, particularly within the framework of Eurocode 8. The absence of prescriptive design provisions in Eurocode 8 adds further uncertainty regarding their seismic performance. This study examines the influence of SSI on the seismic performance of a hypothetical 9-storey reinforced concrete dual wall–frame building. Three structural models were analyzed: a fixed-base reference model and two flexible-base models with different design assumptions. Linear-elastic analysis, nonlinear static (pushover) analysis, and nonlinear dynamic (time-history) analysis were carried out to evaluate structural response. The results reveal that SSI significantly redistributes seismic forces, reducing the wall contribution and shifting the system behavior towards frame-equivalent classification (as per Eurocode 8 classification). A comparison with the American standard ASCE/SEI 7–16 shows that, because of its broader system classification range, the same structures would consistently remain classified as dual systems. This highlights how SSI considerations can influence system categorization differently depending on the adopted code framework. Overall, the findings demonstrate that SSI not only modifies global stiffness and capacity but may also affect system classification under Eurocode 8, with implications for design forces and behavior factors.