It is well known that the effect of soil – structure interaction (SSI) on the elastic and inelastic response of buildings is prominent. Most investigations considered SSI are mainly focused on SDOF and regular MDOF systems. In this research, the influence of foundation uplift on the ductility demand distribution along the height of the structure is investigated. The seismic demands of regular and irregular shear-buildings with different number of stories, fundamental period of vibration, irregularity ratio, soil flexibility and level of ductility are evaluated. Besides, the soil medium beneath the structure is considered as a homogeneous half-space and is simulated by beam on Winkler foundation approach. Specifically, combined vertical stiffness-strength irregularities are introduced by reducing the lateral properties separately at the first story and roof level of the regular structure. Inelastic response-history analyses are conducted to compare the ductility demands of the regular and irregular structures in fixed-base and flexible base conditions. It is concluded that soil flexibility and foundation uplift can reduce the ductility demand of vertically irregular structures, especially for systems with short period. This beneficial role is more pronounced for the irregular story. However, distribution of ductility demand ratio at stories further away from the modified story is significantly dependent on the position of vertical irregularity.

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