A Methodology for Nonlinear Soil-Structure Interaction Effects Using Time-Domain Analysis Techniques

The dynamic response of rigid foundations on an elasto-viscoplastic half-space is investigated in the context of nonlinear finite element (FE) analysis. A deviatoric viscoplastic theory with a linear combination of isotropic and kinematic hardening is used to model the soil constitutive response. Large-scale nonlinear FE computations are made feasible by the use of a composite Newton-POG iteration technique, which requires the factorization of the consistent tangent operator no more than once during the solution process. Time-domain analyses are used to investigate the nonlinear responses of vertically oscillating circular, square, and rectangular foundations to harmonic loads, using two- and three-dimensional FE modeling. It is shown that for low frequency excitations, resonance is created which amplifies the motion of the foundation at amplitudes well above those obtained at the zero-frequency leveL

In addition, horizontal, rocking, and torsional vibration modes of strip and square foundations are considered using the same methodology developed for vertically oscillating foundations. The foundation responses for horizontal, rocking, and torsional modes are characterized by increased vibrational amplitudes due to material stiffness degradation. Furthermore, one or more resonance frequencies are created which resemble those observed for vertically oscillating finite-size foundations. Nonlinear soil effects are shown to be dominant over a wide range of excitation frequencies for foundations vibrating in torsional and horizontal modes. In contrast, nonlinear soil effects are shown to be dominant over a much narrower range of excitation frequencies for the vertical and rocking modes.