Influence of the nonlinear behaviour of shallow foundations on the seismic response of concentrically braced steel frames

The dynamic response of structural systems subjected to seismic excitation can be estimated considering flexible supports. The total response is characterized by base displacements that are induced by the soil movement, the inertial response of the structure and kinematics effects. The non-linear response of flexible foundations can be modeled considering the Beam on Nonlinear Winkler Foundation (BNWF) approach which allows using backbone curves that are generated through a combination of an elastic zone response having a slope that represents the dynamic stiffness of rigid foundations, and a non-linear zone that has been calibrated via field tests. In the backbone curves for vertical response on footings, the tension zone has been considered by other authors that may range from 0- 10% of the ultimate bearing capacity (qu), considering a non-linear response mechanism equivalent to that one developed in the compression zone. Nonetheless, it has been demonstrated that considering the soil stress state on site and the compaction level of the soil located over the footing, it is feasible to model the non-linear response of the tension zone taking into consideration the construction process used in several countries characterized by deeper embedment depths ranging from 1m-3m. It has been proposed a procedure validated through nonlinear time-history analyses, which is able to consider the influence of the soil-foundation system on the superstructure response, using non-linear springs calibrated according to field and laboratory experimental data. The proposed model could be used to calibrate the stress-deformation behavior in the tension zone of the foundation, considering the properties of the soil located on the footing and the construction method