In conventional design and analysis of structures, it is generally assumed that they are fixedly supported and the foundation is rigid. However, in reality, structures are normally supported by flexible soil, and in the case of shallow foundations and under strong earthquakes, they could also experience foundation uplift – i.e. separation of foundation from its underlying soil. The flexibility of the soil medium beneath foundations could increase the overall flexibility of the soil-structure systems and also heighten their overall damping through the so-called radiation and hysteretic damping mechanisms of the soil. These effects, which are often known as inertial soil-structure interaction (SSI) effects, could significantly change the dynamic response of structures, especially when the structure is much less flexible than the soil medium supporting it and remains elastic.
In his M.S. thesis, Dr. Salehi evaluated the effects of soil flexibility and foundation uplift on the seismic performance of two-dimensional multi-story steel frames. More specifically, in the first part of his M.S. thesis, Dr. Salehi designed and analyzed a large number of moment resisting and concentrically braced steel frames supported by various foundation designs and soil flexibility. The analyses included both nonlinear pushover and time history analyses, considering a varied earthquake intensities. The results of hundreds of analyses performed on the selected structures revealed the separate effects of soil flexibility and foundation uplift on the seismic performance of steel frames. The key parameters of the soil-structure system influencing the significance of the above effects were also identified. In general, this research showed that foundation uplift could increase the displacement demands of steel frames, but it generally reduces their inter-story drift and ductility demands. The major findings of this part of Dr. Salehi’s research have been published in a conference paper.
In the second part of this research, Dr. Salehi investigated the ability of the nonlinear pushover analysis method to predict the seismic demands of steel frames prone to foundation uplift. This study showed that the nonlinear pushover analysis method presented in ASCE 41-06 in unable to reasonably predict the seismic demands of steel frames considering foundation uplift. Further information on this part of Dr. Salehi’s research is found in his M.S. thesis.