A parametric study on the evaluation of ductility demand distribution in multi-degree-of-freedom systems considering soil–structure interaction effects

The current code-compliant design lateral load patterns are based on the elastic behavior of fixed-base structures without considering soil–structure interaction (SSI) effects. As a result, the implementation of such a load pattern in seismic design of soil–structure systems may not be appropriate. Moreover, recently several new optimum loading patterns have been proposed by researchers for fixed-base systems while their adequacy for soil–structure systems have not been evaluated yet. This paper performs intensive parametric analyses of 7200 nonlinear multi-degree-of freedom (MDOF) systems with SSI subjected to a group of 30 earthquakes recorded on alluvium and soft soils to investigate the effect of SSI on height-wise distribution of ductility demands. Effect of many parameters including fundamental period, level of inelastic behavior, number of stories, damping model, damping ratio, structural strain hardening, earthquake excitation, level of soil flexibility, aspect ratio on height-wise distribution of damage (ductility demand) are intensively investigated. In addition, the adequacy of three different code-complaint lateral loading patterns including UBC-97, IBC-2009 and EuroCode-8 as well as three recently proposed optimum loading patterns for fixed-base structures are parametrically investigated for soil–structure systems by two methods associated to the economy of the seismic-resistant system. Results of this study indicate that among the aforementioned code-specified design lateral load patterns, UBC-97, generally, has the best performance in soil–structure systems. However, all of them loose their efficiency when the SSI effect is severe and inelastic response is pronounced. It is also demonstrated that although the structures designed according to some recently proposed optimum load patterns may have generally better seismic performance when compared to those designed by code-specified load patterns, their seismic performances are far from the optimum if the SSI effects are considered, and their efficiency significantly reduces with increasing the soil flexibility.