Inelastic displacement ratios for structures on firm sites

The results of a comprehensive statistical study of inelastic displacement ratios that permit the estimation of maximum lateral inelastic displacement demands on a structure from maximum lateral elastic displacement demands are presented. These ratios were 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. The study is based on 264 acceleration time histories recorded on firm sites during various earthquakes in California. Three types of soil conditions with shear-wave velocities higher than 180 m/s rue considered. The influences of period of vibration, level of ductility demand, site conditions, earthquake magnitude, and epicentral distance are carefully evaluated and discussed. Inelastic displacement ratios associated with mean values are presented. Special emphasis is given to the disperson of the results. It is concluded that for sites with average shear-wave velocities higher than 180 m/s the influence of soil conditions is relatively small and can be neglected for design purposes. Finally, results from nonlinear regression analyses are presented that provide a simplified expression to be used in the design to approximate mean inelastic displacements ratios for structures on firm sites.