Ductility demands of MRF structures on soft soils considering soil-structure interaction

The problem of nonlinear response of moment resisting frames on nonlinear soft soils is investigated. The soil structure interaction (SSI) is considered using the direct method. The direct method consists of concurrent modeling of structure and its supporting soil to the bedrock. The soil medium is usually modeled using the equivalent linear method where the shear modulus and damping ratio of soil are replaced with values representative of the nonlinear soil behavior. A correction of this method with regard to very large strains of soil at and around foundations is proposed in this study. The nonlinear behavior of the frame elements of structure is assumed to be concentrated at plastic hinges at the ends of members. Drifts, shear forces and ductility demands of stories of the studied buildings are calculated once assuming a rigid base and then a soft supporting soil for the structure with nonlinear time-history analysis. It is shown that SSI increases the drifts and ductility demands of the lower stories. Interim modification factors being ratios of ductility demands with SSI to those without SSI (fixed-base cases) are presented as checking tools for when SSI is important for nonlinear dynamic response calculation of structures.