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Structure and Soil interaction

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Ehsan Ahmadi
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Consequences of near-fault ground motions to strength reduction factors of flexible-base multi-story structures are addressed by employing synthetic pulses. For this purpose, three non-dimensional parameters are adopted as the key parameters of interacting systems: (1) non-dimensional frequency as the structure-to-soil stiffness ratio, (2) aspect ratio of the superstructure and (3) structural ductility. The soil and superstructure are idealized as homogeneous elastic half space (Cone model concept) and nonlinear shear building, respectively. A deep sensitivity analysis is carried out to elucidate the impacts of various parameters on strength reduction factors of multi-degree-of-freedom (MDOF) structures considering Soil–Structure Interaction (SSI) effects. It is confirmed that pulse period extremely affects the trend of strength reduction factors while interacting parameters influence this trend at various pulse periods. Moreover, findings of this study demonstrate that amplification factors of equivalent flexible-base single-degree-of-freedom (SDOF) structure are highly impressed due to pulse period effects and interacting factors impose great changes to variations of amplification factors with respect to pulse periods.
In this study, the role of soil material damping in the engineering demand parameters (EDPs) of soil-structure systems is investigated. For this purpose, a superstructure is modeled as a two-dimensional nonlinear multi-story shear building. The soil beneath the foundation is simulated based on the concept of the cone model. The effects of various parameters are evaluated using the relative reduction ratios between demands in the presence of soil material damping and in its absence (radiation damping only). The results demonstrate that as the number of stories increases, the effects of soil material damping on the responses become more pronounced. Moreover, in the case of slender structures and higher structural ductility, the effects of soil material damping are governing in comparison to those of radiation damping. Generally, the consequences of soil material damping on the displacement demands are greater than those on the force demands. Furthermore, the roof displacement demands of the superstructure are more affected by soil material damping than by the maximum story drift angle response.