Effect of soil and foundation stiffness on the behavior of RC structures

The fixed support assumption, which ignores the displacements in the ground and the rotations of the foundation, is frequently used by civil engineering practitioners. In this study, 25 different 4-storey reinforced concrete frame buildings with infill walls were modeled on soils with different bearing coefficients for raft and continuous foundation types. Design and analyses of the buildings are performed according to TBDY-2018 and TS-500 using the SAP2000 software. Based on the analyses results, the variation of the first natural vibration periods for different models, the ratio of strength to seismic weight, capacity curves, damage to plastic hinges, change in energy dissipation amount, change in building roof displacement demand value, the ratio of demand to building height, shear forces distribution of a typical column were investigated. Considering the analysis results, a period increase of up to 44% was observed, depending on the stiffness of the foundation and the soil, when compared to the model with the infinitely stiff foundation assumption. The horizontal strength of the buildings is less affected by the foundation and ground stiffness with results of 4% to 9% decrease. Damage distributions in plastic hinges are slightly affected by the stiffness of the foundation and soil stiffness, with figures less than 3%. Compared to the reference model, a 40% change was observed in the roof displacement demand in the soil-structure interactive models. Soil and foundation stiffness also affect the force distribution between columns. The shear force generated in small-sized columns is higher than expected compared to the reference model. This increase is between 8% and 24%. The results of the analysis using non-linear elastic method and analysis results of the 7-storey reinforced concrete building performed in the study by Yaşar and Özmen (2022) in the were compared. Using 100 values in total, a new equation, has been established to estimate the displacement increase in the building per the fixed foundation case. The equation developed within the scope of the study will provide convenience to civil engineering practitioners in the dimensioning of foundation elements for a certain displacement increase limit and soil stiffness.