Behavior of steel tub girders with optimized bracing layouts

Armijos Moya, Stalin Vinicio
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Composite steel tub girders are a popular alternative for straight and horizontally curved bridges due to their high torsional stiffness and aesthetic appearance. While the closed girder in the finished bridge has a high torsional stiffness, prior to curing of the concrete deck the steel girder alone is relatively flexible. Consequently, steel tub girders require extensive amounts of bracing to avoid instability and cross section distortion due to the relatively low torsional stiffness during construction. The primary bracing systems include a top flange lateral truss as well as internal and external cross frames. The quasi-closed shape that results with the added bracing, results in a stiff girder system capable of resisting significant torsional moments and deformations. However, current design and detailing practices for steel tub girders commonly include several aspects that may lead to unnecessary fabrication costs and structural inefficiencies. The top flange lateral truss is commonly installed along the entire length of steel tub girders to increase the girder torsional stiffness. However, the horizontal truss is mainly effective near the ends of the girders where the shear deformations are the largest. Thus, partial top lateral bracing systems can be an alternative for straight and mildly horizontally curved bridge applications. Additionally, the internal K-frame braces are usually installed at every panel point to control cross-sectional distortion of the girders. However, the amount of internal K-frames can be significantly reduced with little impact on the structural performance. By optimizing the amount of bracing depending on the application, steel tub girders can be more cost-effective without undermining their structural integrity. In addition to modifications in the bracing, current AASHTO Specifications have several geometric restrictions for the section geometry of steel tub girders that may not be warranted. AASHTO restricts the web slope not to exceed the limit of 1 Horizontal to 4 Vertical. Tub girders with flatter web slopes can result in girders with higher tributary widths that could potentially reduce the number of girder lines on a bridge. In addition, top flanges are required to be centered on the top of the webs. If the top flanges are permitted to offset towards inside of the tub, more space would be available to connect directly the horizontal truss and avoid undesired eccentricities due to large gusset plates. Since these cross-sectional and bracing details are not currently allowed by AASHTO Specifications, a TxDOT-sponsored research project was performed to evaluate the impact of the aforementioned details using large-scale experimental tests and parametric finite element analyses. This dissertation evaluates the feasibility of optimized bracing layouts for straight and horizontally curved steel tub girders, in addition to analyzing the impact of modified cross-sectional geometries in the ultimate flexural capacity of steel tub girders.