Interactive Base‐Isolation Foundation System: I. Finite Element Formulation

A finite element model to study the effects of seismic base isolation on the response of structures supported on pile foundations is presented. The model takes into account both pile flexibility and energy dissipation and radiation, realized through the mechanism of soil‐pile‐structure interaction. The resulting dynamic characteristics are considerably different from those of the structure with a fixed base. The model is applied to study the influence of soil‐pile‐structure interaction on the response of structures, including nonlinear pile‐soil effects. To incorporate nonlinearities in the soil‐pile system that might arise from large deformations under severe earthquake loading, the piles are modeled as beam‐column finite elements with material and geometric nonlinearities, while the soil is idealized by nonlinear springs and dashpots uniformly attached to the piles along their length. For the soil springs, a modified Ramberg‐Osgood model is employed. For the material damping of the soil, a simple relationship between the shear modulus and the damping is utilized. To simulate radiation damping effects in the soil, viscous dampers are attached to the pile. The model is tested against documented solutions for verification.