Current design practice for structures subject to earthquake loading regards dynamic Soil-foundation-Structure Interaction (SFSI) to be beneficial to the behavior of structures. Including the flexibility of the foundation and soil reduces the overall stiffness of a SFS system and can therefore reduce peak loads caused by a given ground motion. This might be true in (some) many cases. However, there is the possibility of SFS system going into resonance with the exciting earthquake motions as a result of a shift of the natural frequencies of the SFS-system. This can lead to much larger inertial forces acting on a structure. In that case, the SFSI is not beneficial, but rather detrimental to the seismic response of the structures (and of the SFS system). In this paper we present methodology and an analysis of beneficial and detrimental effects SFSI can have on seismic response. It will be shown that an interaction between SFS system and the seismic motions plays crucial role in determining if SFSI will be beneficial or detrimental. In other words, SFSI can be both beneficial and detrimental for a particular SFS system, depending on the characteristics of the seismic motions exciting the system.
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