Seismic Soil Structure Interaction Response of Midrise Concrete Structures on Silty Sandy Soil

The seismic response of midrise frame structures rested on soft soils is susceptible to the dynamic interaction between structure, foundation and soil, called soil structure interaction (SSI). While seismic codes provide design acceleration charts based on 1D free-field response analysis, research into SSI is limited compared to other structural and geotechnical topics and does not consider all parameters covering the interaction between structure, foundation and soil. In this study, a series of 3D finite element analyses were conducted using Abaqus to investigate the effects of midrise structures' number of stories as well as raft and column dimensions while considering SSI effects divided between inertial and kinematic. In the analyses, the frame structure was assumed to be rested on a raft foundation and silty sandy soil block and the model was hit at bottom by El-Centro (1940) and Northridge (1994) earthquakes. Moreover, the response of the structure was studied for (1) fixed-based and (2) flexible-based structures. Results, presented in terms of storey lateral displacement, inter-storey drifts, shear force, foundation rocking and response spectrum, indicate that SSI effects divide midrise structures into 2 categories: (1) 5 <= N <= 10 and (2) 10 <= N <= 15. In fact, SSI was beneficial for the first category, while it was detrimental to the second. Moreover, different behaviours were obtained for different column sizes when varying raft sizes and thicknesses. Thus, engineers should carefully optimize their design between these different studied parameters, where an increase in column size with raft dimension caused a decrease in foundation rocking angle, while structural stability and failure depended highly on column size.