Beneficial and detrimental effects of soil-structure interaction according to damage spectrum

In spite of extensive studies since 1970 on Soil Structure Interaction (SSI), there is still controversy regarding the seismic performance of the structures rested on soft soil. SSI is known as a phenomenon influencing beneficial and sometimes detrimental effects on the response of structures. The response of soil–structure system depends basically on the size of the structure, its dynamic properties, and the soil profile as well as the applied excitation. Under strong shaking the response of soil–structure system is nonlinear and a combination of different types of nonlinearity as well as sliding, uplift and soil yielding will occur. Although foundation uplift in conjunction with soil yielding may dissipate energy during earthquakes, they may also lead to excessive permanent deformations in the structures.
In such case, soil- structure interaction (SSI) is not only beneficial, but also detrimental to the seismic response of the structures. In order to investigate the beneficial and detrimental effects of SSI on the nonlinear response of building, damage spectra on the basis of Park and Ang damage Index for the SDOF models are provided by considering and neglecting the SSI effects. The bilinear SDOF models are supported by Beam on Nonlinear Winkler Foundation. Two non-dimensional parameters are used to control the modeling. (I) non-dimensional frequency a0 which is a statement of the structure-to-soil stiffness ratio (II) the aspect ratio of the structure h/r. The systems are subjected to three earthquake ground motions recorded on soft soil. For each period, first the yield strength demand of the structure is calculated by iteration in order to reach the specified target ductility in the fixed-base model within 1% of accuracy when subjected to the ground motions. The dissipated hysteretic energy in the structure is also calculated accordingly. Then, the ductility and the hysteretic energy demands are calculated for the soil–structure systems under different values of a0 and h/r subjected to the same ground motion providing the same yield strength for the structure. Consequently, the damage index is calculated for the structure in the fixed-base state as well as for the structure when located on soil. The result showed that for most of structures with long period SSI decrease the damage index. How ever It is observed that in some cases of structure to soil stiffness, SSI increases the damage index before a threshold period which is closely related to the predominant period of the ground motion. It means that the conventional fixed-base model underestimates the damages sustained by buildings having periods less than this threshold period. In particular, the SSI substantially increases the damage index of short-period buildings located on soft soils. It is also observed that increasing the aspect ratio of the structure increases this effect. However, the trend is reversed after the threshold period. it is observed that the increase in the slenderness ratio and ductility ratiob of structures leads to increase in the maximum damage.