Enhancing Seismic Displacement Prediction for Reinforced Concrete Box Girder Bridges Through Nonlinear Time History Analysis

Inelastic displacement ratios are critical parameters in the seismic design of reinforced concrete (RC) box girder bridges. Existing approaches of displacement prediction, including the Displacement Coefficient Method and the Capacity Spectrum Method, typically rely on simplified single-degree-of-freedom (SDOF) models, which do not fully account for the complex and nonlinear behavior of multi-degree-of-freedom (MDOF) bridge systems. Moreover, the AASHTO Guide Specifications apply the equal displacement rule through the inelastic displacement modification factor Rd, which may underestimate displacement demands for short-period structures. This study evaluates the accuracy of the AASHTO Rd using nonlinear time history analyses of six RC box girder bridge models subjected to 28 recorded ground motions from California. Each ground motion included two orthogonal components applied in the longitudinal and transverse direction. Both elastic and inelastic displacement demands were determined in each direction, and inelastic displacement ratios (Cμ) were computed and compared with AASHTO predictions. A new predictive equation for Cμ was developed to capture response variability. While AASHTO Rd aligns with the average behavior, it fails to provide reliable estimate across the full range of seismic conditions. A comprehensive parametric study was conducted to examine the influence of column boundary condition, column height, superstructure deck width, number of spans, and damping ratio on Cμ. While the elastic and inelastic displacement decreases with an increase in damping ratio, the result shows that Cμ increases with higher damping ratios. Accordingly, a revised amplification factor was proposed to better represent the inelastic displacement demand in MDOF bridge systems.