SSI effects on seismic demand of reinforced concrete moment resisting frames

We present the results of nonlinear dynamic analyses performed on reinforced concrete (RC) moment resisting frames (MRFs). The analyses were performed considering SSI by means of both a plane-strain soil-structure finite element model approach and a structure-on-springs approach. The results show that the complete FE model approach may produce significant differences in the evaluation of the seismic demand, in terms of maximum inter-storey drift, with respect to the fixed-base structure and to the structure-on-springs approach, mainly because of the different incorporation of the damping in the two modeling approaches. Differences in the soil properties, the seismic design level of the structures and the modeling technique of SSI effects are reflected as reduction in the seismic demand with respect to a fixed-base model, up to 50% in maximum inter-story drift ratio and up to 20% in maximum base shear for the complete FE model and up to 20% in both maximum base shear and maximum inter-story drift ratio for the simplified BNWF model.