There is apopular approach in the structural fieldthis approach assumes the base of the structureas a fixed base but in reality, the soil under the structure deformsbecause the soil has the ability todeform. In the conventional seismicdesign of buildings frame, the sameapproach is also assumed. Because of the soil response,the seismic response of the structure gets changed. The soil response maydecrease the stiffness of the structure and increase natural periodsof the structure. In this paper, theeffect of soil flexibility on the performanceof medium-high RC framed structure (5bayby 5bay 6 story) with various plan irregularities resting on different types ofsoil (Hard soil, Medium soil, soft soil) is taken into consideration.

Two typesof model approaches are used in order to represent the soil- structureinteraction(flexibility). The firstapproach is Winkler model which isdeveloped by using spring stiffness equations. Every spring has six degrees offreedom. Along these six degrees of freedom,the stiffness is determined by using spring stiffness equations. The second approach is elastic continuum whichis developed by finite element method using Sap2000. The idealization of elastic continuum depends on a commonexperience which says deflections in soil media do not happen directly underthe loading area but also in certainlimited zones outside the loading area.

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Accordingto on the idealization of the elastic continuum,a soil block of 75m x 75m (1.5 times the width of the structure) is assumed.Soil block is modeled by FEM. Beams and columns are modeled as frame elementwith 2 nodes having 6 degrees of freedom at each node, the foundation is modeled as 8 nodes concreteelement and soil mass beneath the foundation is modeled as 8 nodes solidelement with 2 degrees of freedom at each node. In conclusion, when the soilflexibility increases, the response of the structure, beam moment, columnmoment, and base shear also increase.

Irregularity does not have clear effectson the response of the framed structure.Soil-structure interaction has effects on moments of columns and beams. Forinstance, has been found that the models with soil-structure interaction havehigh values of moments in columns and beams when compared with models withoutsoil-structure interaction. The results from FEM method are more accurate than Winkler method. FEM method is suggested foranalysis of structure resting on soft soil.

Finally, when the structure isresting on soft soil, it’s important to consider soil-structure interactioneffects.