Analysis of Eccentrically Loaded Adjacent Box Girders

Abstract

Research has shown that exterior girders of adjacent box (AB) beam bridges act compositely with their barrier wall and curb slab assemblies, resulting in a highly asymmetric cross section. Additionally, impact damage from vehicle collision often severs or exposes strands in the exterior soffit corner of exterior girders. These effects individually (and more so in combination) have the effect of rotating the neutral axis of the girder, resulting in significant, unaccounted for biaxial flexure. Common structural rating practices analyze the member about its horizontal axis, neglecting the effects of neutral axis rotation, and thus overestimate the actual capacity of the member. A parametric study that analyzed the effects of varying levels of damage to eight prototype AB girders was conducted. In all, 106 analyses were carried out. From this study, a relationship is proposed to determine the capacity of an AB beam subject to asymmetric loading, which includes the composite behavior of the barrier wall assembly and the effect of prestressing strands lost as a result of damage to the girder. This relationship is based on easily obtained capacity predictions and is shown to result in predictions of capacity having an average absolute error of less than 6%. This study also examines, in the context of complete AB bridges, the potential for in-service loading to exceed assumed design loads.