Application of Confined Concrete for Double-Composite Steel Box Girders

Double-composite steel box girder bridges present many advantages, especially in the negative-moment region, over single-composite steel box girders by increasing the load-carrying capacity and stiffness. However, additional steel used in traditional double-composite designs has been conservatively applied to prevent buckling of the steel plates. This study investigates a novel double-composite box girder bridge design using confined concrete in a continuous concrete bottom slab. An analytical finite-element analysis (FEA) model of a three-span straight box girder bridge is used to obtain the loads applied to the proposed design and analyze the bottom slab confinement in the region of the maximum negative moment. A cost, strength, and constructability analysis was also conducted for the proposed design, which was more favorable than traditional double- and single-composite bridge designs. The proposed design was found to have material cost savings of 14%, increased plastic moment capacity, simplified fabrication details, and reduced cross-frame requirements compared with a single-composite design. A strength increase of 23% was also found for the novel continuous bottom slab design when high-performance steel was used for the box girder. The study also provides recommendations on the pattern and spacing of confinement to achieve the required concrete strength and ductility in the negative-moment region.