Soil–structure interaction (SSI) is key to the elastic and inelastic seismic responses of buildings. In this research, the ductility and strength demands of vertically irregular MDOF buildings, considering nonlinear SSI effects, are parametrically investigated. The superstructure is modeled as a nonlinear multi-story shear building, and the beam on nonlinear Winkler foundation (BNWF) concept is employed to simulate the response of shallow foundations. Specifically, combined stiffness–strength irregularities are introduced by reducing lateral properties at a specific story of the regular (reference) models. Soil–structure systems with 5, 10, and 15 stories are analyzed under three sets of earthquake records corresponding to different soil classes. A wide range of key parameters including number of stories, fundamental period, level of inelasticity, aspect ratio, and site class are scrutinized through nonlinear time history analyses. The results reveal that SSI can reduce the strength and ductility demands of the vertically irregular structures, especially those with short periods. This beneficial effect becomes even more significant for systems with low ductility ratios. It is also concluded that the median ductility demand increases in the modified story owing to the softness/weakness of the first story. Furthermore, this increase due to the code strength regularity limit reached up to 78% and 36% in fixed-base and flexible-base conditions, respectively. Finally, simplified equations are proposed to estimate the maximum ductility demands of regular and irregular structures with flexible-base conditions.

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