Why is it important to consider the nonlinearity of soil behavior in SSI analysis for seismic design?

It is important to consider the nonlinearity of soil behavior in Soil-Structure Interaction (SSI) analysis for seismic design because soil exhibits significant nonlinear behavior under strong earthquake shaking, and this nonlinearity can substantially affect the dynamic response of both the soil and the structure. During an earthquake, soil can undergo large deformations, develop plastic strains, and exhibit stiffness degradation and hysteretic damping. A linear soil model, which assumes a constant stiffness and damping, cannot accurately capture these phenomena. Ignoring soil nonlinearity can lead to an underestimation of ground motion amplification, an inaccurate prediction of the structure's natural frequency and damping, and an incorrect assessment of the forces and deformations in the structure. For example, soil liquefaction, a highly nonlinear phenomenon where soil loses its strength and stiffness, can cause significant settlement and instability of the structure. Capturing such behavior requires advanced nonlinear soil models that account for factors like confining pressure, shear stress level, and pore water pressure. Therefore, incorporating nonlinear soil behavior in SSI analysis is crucial for obtaining a realistic and reliable assessment of the seismic performance of the soil-structure system.