Equivalent Damping in Support of Direct Displacement-Based Design with Applications to Multi-Span Bridges

Abstract

This dissertation aimed at contributing to the advancement of Direct Displacement-Based Seismic Design (DDBD) method in order to ensure its wider acceptance and to enable its implementation in future codes. The concept of equivalent linearization of nonlinear system response as applied to DDBD for single-degree-of freedom (SDOF) structures was evaluated. The evaluation process revealed significant errors in approximating maximum inelastic displacements due to overestimation of the equivalent damping values in the intermediate to long period range. Conversely, underestimation of the equivalent damping led to overestimation of displacements in the short period range. Earthquake characteristics had a significant effect on the equivalent damping, resulting in a scatter in estimating peak inelastic displacements between 20% and 40% as a function of displacement ductility. New equivalent damping relationships for 4 structural systems, based upon nonlinear system ductility and maximum inelastic displacement were proposed. The accuracy of the new equivalent damping relations was assessed, yielding a significant reduction of the error in predicting peak inelastic displacements. Furthermore, a simplified approach was proposed to select target displaced shapes for continuous bridges based on the relative stiffness between the superstructure and the substructure. The approach, in some cases, minimizes the effort and time needed to design multi-span bridge structures because it eliminates the need for the iterative approach in selecting target profiles.