Site-Dependent Strength-Reduction Factors

Strength-reduction factors that are used to reduce linear elastic design spectra to account for the hysteretic energy dissipation of the structure are evaluated. The paper presents a summary of results of a statistical analysis of strength-reduction factors computed for single-degree-of-freedom systems undergoing different levels of inelastic deformation when subjected to a relatively large number of recorded earthquake ground motions. Special emphasis is given to the influence of soil conditions. Results indicate that for a given displacement ductility demand, the use of period-independent reduction factors is inadequate. Soil conditions can have an important effect on strength reduction factors, particularly in the case of soft-soil sites. It is recommended that strength-reduction factors to be used in design be specified as a function of the period and inelastic capacity of the structure, and of at least two types of soil conditions—one for rock and relatively firm sites and another for soft-soil sites. Following these recommendations, simplified expressions to compute strength-reduction factors are proposed.