Fragility analysis of nonlinear soil-structure systems including foundation uplifting and soil yielding

The present study is focused on fragility analysis of steel moment resisting frames (MRFs) incorporating nonlinear soil-structure interaction (SSI) effects. To this end, incremental dynamic analyses are performed using a suit of real ground motion records. A MRF structure is considered which is supported by single footings. To evaluate the SSI effects, four cases are compared including (i) fixed base, (ii) linear SSI and uncoupled footings (i.e. without tie beams), (iii) nonlinear SSI and uncoupled footings and (iv) nonlinear SSI and coupled footings (i.e. with tie beams). The SSI effect is represented by modified Beam-on-nonlinear Winkler foundation (BNWF) model. An appropriate structural damage index based on summation of cumulative plastic hinges’ rotations is employed. The seismic fragility curves of the structures are derived and compared for the above-mentioned cases. The results show that nonlinear SSI has significant effects on seismic fragility curves. Evidently, these effects are mitigating especially in case of footings with tie beams. To assess the effect of ground motion type, fragility curves are also derived for each type of ground motion comparatively. It is observed that near field pulselike records are more destructive than far field or near field no-pulse records in terms of fragility curves. Overall, based on findings of this study, the obtained modified fragility curves are supposed to be helpful for the earthquake engineers to conduct more realistic loss estimations considering SSI effects. These modification factors need to be generalized with respect to a variety of structural systems, site types and foundation configuration.