Inelastic Displacement Ratios for Design of Structures on Soft Soils Sites

Abstract

This paper summarizes the results of a comprehensive statistical study of inelastic displacement ratios that allow the estimation of maximum lateral inelastic displacement demands from maximum elastic displacement demands for structures built on soft soil sites. These ratios were computed for single-degree-of-freedom systems undergoing six levels of inelastic deformation when subjected to 116 earthquake ground motions recorded on bay-mud sites of the San Francisco Bay Area and on sites in the former lake-bed zone of Mexico City. These soft soil deposits are characterized by low shear wave velocities, high water contents, and high plasticity indices. The influence of period of vibration normalized by the predominant period of the ground motion, the level of inelastic deformation, earthquake magnitude, and epicentral distance are evaluated and discussed. Mean inelastic displacement ratios and their corresponding dispersion are presented. The effect of stiffness degradation on inelastic displacement ratios is also considered. For this purpose, mean ratios of maximum inelastic displacement demands of stiffness degrading systems to maximum inelastic displacement demands of nondegrading systems are presented. Finally, a simplified equation to estimate mean inelastic displacement ratios obtained through nonlinear regression analyses is provided to aid designers estimate inelastic displacement demands of structures built on soft soil sites.