This paper presents a nonlinear spring multi-Winkler model for nonlinear seismic response of foundation and superstructure system using the fiber element method. The nonlinear soil reactions are modeled by using the spring coefficients per unit area of foundation surface with 3 components consisting of normal traction and two shear fractions on foundation surface. The shapes of monotonic curves and the hysteresis loops of the 3 springs are evaluated using the Mohr-Coulomb yield criterion and no rigidity of tension, and by the concrete crack model. The proposed spring model can incorporate such effects as separation and slippage at the soil foundation interface, and also the effect of coupling between the normal traction and two shear tractions on foundation surface. To demonstrate an applicability of the proposed model, a nonlinear seismic response analysis of cable stayed bridge supported by the two caisson foundations is performed and the effects of nonlinear behaviors of soil reactions are discussed.
