Soil Damping Formulation in Nonlinear Site Response Analysis

Abstract

Nonlinear time domain site response analysis is used to capture soil hysteretic response and nonlinearity due to medium and large ground motions. Soil damping is captured primarily through hysteretic energy dissipating response. Viscous damping, using Rayleigh damping formulation, is often added to represent damping at very small strains where many soil models are primarily linear. The Rayleigh damping formulation results in frequency dependent damping, in contrast to experiments that show that damping of soil is mostly frequency independent. The extended Rayleigh damping formulation is introduced to reduce the overdamping at high frequencies. With appropriate choice of frequency range, both formulations provide similar. The proposed formulations used in non-linear site response analysis show that the equivalent linear frequency domain solution commonly used to approximate non-linear site response underestimates surface ground motion within a period range relevant to engineering applications.