How does soil-structure interaction affect structural analysis?

Soil-structure interaction (SSI) modifies structural response by: (1) Changing foundation stiffness - fixed base assumption is unconservative for soft soils, (2) Period lengthening due to foundation flexibility, (3) Radiation and material damping in soil, (4) Kinematic interaction affecting ground motion input. Modeling approaches include spring models (Winkler), finite element with soil elements, impedance functions, and hybrid methods. SSI is critical for nuclear plants, tall buildings on soft soil, and structures with significant mass at foundation level.