Experimental Study on the Interaction of Partial Top Lateral and K-Frame Bracing on Tub Girders

Steel box girder systems, which consist of steel tub girders with a cast in-place concrete deck on top, are a popular alternative for straight and horizontally curved bridges due to their high torsional stiffness and aesthetic appearance. However, steel tub girders possess a relatively low torsional stiffness during transport, erection and construction because of the thin-walled open section. Additionally, during the casting of concrete, the upper portion the tub girder is in compression in the positive moment region and the girder is susceptible to lateral torsional buckling (LTB). Usually, top flange lateral bracing (TLB), in the form of a horizontal truss, is installed along the entire length of the steel tub girder to increase the torsional stiffness of the girder and to prevent LTB. However, for straight or nearly straight girders, the horizontal truss is mainly effective near the ends of the girders where the shear deformations are the largest. The contribution of the top lateral bracing to control lateral torsional buckling is notably reduced at the mid-span region. Also, internal K-frames are placed to control cross-sectional distortion. This paper provides an overview of on an ongoing research study focused on improving the efficiency of steel tub girders by investigating the impact of the girder geometry and bracing details on the behavior of the girders. The study includes large-scale experimental tests and parametric finite element analytical (FEA) studies. This paper highlights both the experimental tests and part of the analytical study. The interaction between partial top lateral and K-frame bracing systems is assessed by conducting multiple elastic-buckling tests on three steel tub girders with different amounts of top lateral bracing along the girder. Interaction between these two types of bracing systems was observed with variations in the forces of the top lateral truss diagonals and struts when the configuration of internal K-frames was altered. The three tub girder specimens were also subjected to vertical bending and combined bending and torsion using concentric and eccentric loads, respectively, applied by gravity load simulators. The goal of the study is to improve the efficiency of steel tub girders by optimizing the bracing while maintaining adequate safety.

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