Performance -based design of steel moment frames using target drift and yield mechanism.

Abstract

The design procedures for regular moment-resisting frames specified in the current seismic codes are mainly based on elastic analyses under seismic horizontal forces (Equivalent Lateral Force Procedure), without attempting to directly deal with the inelastic response. These procedures offer considerable simplification and do not require the designer to have an in-depth understanding of inelastic structural behavior. The structures designed by these procedures may posses sufficient strength and stiffness to satisfy the requirements of serviceability limit state. However, the design procedures do not always lead to the intended failure mode and the expected drifts under severe ground motions. The energy dissipation capacity of the structure designed by these procedures can be less than that required to prevent collapse under severe ground motions. Therefore, alternative design procedure based on inelastic structural analysis should be adopted for the ultimate limit state. The research work presented herein focuses on developing a new design procedure based on inelastic (plastic) analysis to replace the current design procedures based on elastic analysis for moment-resisting frames in severe seismic zones. The new design concept is based on performance limit state using target drift and yield mechanism. Modified energy balance equation using a new modification factor, which was derived from the concept of force reduction factor and displacement amplification factor, was presented in order to calculate the design base shear demand using the conventional principle of energy conservation. A new distribution of the lateral forces for performance-based plastic design was developed from shear proportioning factors that were derived from the relative distribution of maximum story shears obtained from nonlinear dynamic analyses. Then, using the ultimate design base shear derived from the modified energy balance equation and the new lateral force distribution, plastic design was employed to achieve the selected mechanisms in the design of beams and columns of the structure. A parametric study was carried out to verify the validity of the proposed design procedure. The results of nonlinear static and dynamic analyses show that the proposed method can produce structures that meet pre-selected performance objectives in terms of yield mechanism and target drift.