Optimal Steel Section Length of the Composite Rigid-Frame Bridge

The composite rigid-frame bridge replaces a specific concrete section of the midspan girder of a concrete rigid-frame bridge with a steel section. Although the added steel section improves the bending moment distribution of the rigid-frame bridge, it increases the stress level and structural deformation in a few specific points along the bridge girder. Therefore, the determination of the steel section length needs to consider the comprehensive mechanical effects caused by the replacement of the concrete section with the steel materials. This article presents the results of a study on the influence of steel section lengths on the structural mechanical behavior of rigid-frame bridges, including bending moment, bending stress, and structural deformation. Based on the results of the sensitivity analysis, this study optimizes the steel section length by using the bending strain energy as the objective function. Taking a composite rigid-frame bridge with a span combination of 84 + 200 + 84 m as an example, the structural optimization yields an optimal steel section length ratio of 0.55 (110 m), with the least bending strain energy and with acceptable magnitudes of bending moment, bending stress, and structural deformation. The outcome of this study provides guidance regarding the steel section length for the structural design of composite rigid-frame bridges.