During earthquake, taking different phenomena into account could reduce fatalities and financial loss. In order to examine the phenomenon of earthquake more thoroughly, effects of different soil layers as structural support should be addressed. The mutual effects of soil and structure during a seismic phenomenon are reviewed in an extensive field of study called “soil-structure interaction”. Taking soil-structure interaction into account adds to flexibility of the system and increases energy dampening factors in a system. Consideration of soil-structure interactions requires a thorough knowledge of the soil. In order to offer a more precise estimate of behavior of soil, different behavioral models have been developed which can describe elastic behavior. More advanced behavioral models are able to describe plastic behavior of the soil as well. In the present study, effects of flexibility on structural response are studied by presuming elastoplastic behavior of the soil as well as non-linear behavior of the structure for a limited number of near-fault and far-fault excitations. Then, this case is compared with another case in which elastic behavior of the soil is presumed. In order to study the effect of periodic time of the structure on dynamic response of studied system, four steel frames with 1, 3, 7 and 14 floors were selected to represent short, medium and high structures. Nonlinear behavior of frames was taken into account by modelling beams through plastic joint. In addition, modelling of columns was done through fiber element. Finally, nonlinear analysis contributed to determination of structural responses. One could observe that performance of short structure with lateral load resistant system of flexural frame which is located on soft soil will remain in secure range. In such a case, effects of soil-structure interactions could be ignored in elastic and non-elastic analyses. Presuming rigid foundation during near-fault and far-fault excitations, medium-height structure with 7 floors and high structure with 14 floors were in the safe range. Taking the effects of elastoplastic soil-structure interaction into account takes the structural performance of the last two structures out of safe range. In addition, consideration of behavior of foundation as elastoplastic increases lateral deformation of medium-height and high structures up to 20 percent but reduces shear of different floors. In response to most records, basic shear reduces. In the case of high structures, lower shear wave speed and higher softening of the system are observably followed by higher displacement of lower floors than top ones. Another analysis revealed that increase in height of a structure is correlated with lower rocking motion as one moves from lower floors into top ones. Keywords: Nonlinear Soil-Structure interaction, inelastic behaviour, bounding surface.