Effects of Site Classes on Damping Reduction Factors

Abstract

A statistical study of the damping reduction factors considered the effects of site classes from 1,037 acceleration time histories are presented. The factors are computed from the displacement and acceleration response spectra of single-degree-of-freedom systems undergoing different levels of damping ratios. Considered here is three types of soil condition characterized by site Classes A–D in accordance with the NEHRP 2000 Provisions. Effects of period of vibration, level of damping ratio and site class on the damping reduction factors are evaluated and discussed. The study shows that if an analytic error of 5% for structural design is acceptable, the effect of site conditions on the damping reduction factor derived from the displacement response spectrum can be neglected only for systems with damping ratios smaller than 20%, whereas this effect can be disregarded even for the 50% damped systems if an analytic error of 10% can be endured. Furthermore, the factors derived from the acceleration response spectrum are more sensitive to site classes than those derived from the displacement response spectrum. For the damping reduction factors obtained from the acceleration response spectrum, the effect of soil conditions can be neglected only for systems with damping ratios smaller than 10% if an error of 5% for structural design is allowable while this effect can be omitted just for systems with damping ratios smaller than 20% if an error of 10% is permissible. Expressions obtained from nonlinear regression analyses are proposed in the end of this paper to estimate the damping reduction factors derived from the displacement and acceleration responses.