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.