Cross frames and diaphragms are essential to the stability of straight steel girder bridge systems as they help to resist lateral torsional buckling during construction and horizontal loading conditions. In skewed bridge systems, cross frames are often oriented parallel to the supports and hence, at an angle to the girder. To facilitate construction fit-up, plates, bent to match the skew angle, form the cross frame to stiffener connection. While the bent plate connection is a simple solution, it could introduce undesirable flexibility into the system, potentially compromising the effective brace stiffness. A proposed detail utilizing half pipe stiffeners may provide enhanced structural performance, while possibly reducing overall fabrication costs. Field and laboratory tests to determine the stiffness of both connection types are presented in the thesis. / text

Lateral bracing systems consisting of cross frames and their connections play a significant role in the elastic buckling strength of steel girder bridges. By providing lateral and torsional stability, they prevent lateral torsional buckling of the girder during bridge construction prior to the concrete bridge deck curing. To perform this function, the bracing system must possess adequate strength and stiffness. And since each component of the bracing system acts in series, the overall stiffness of the system is less than the least stiff component. In skewed bridges, cross frames at the ends of the girders are installed parallel to the bridge skew angle, and their connection to the girder requires that the cross frames be at an angle that prohibits welding a stiffener from the cross frame directly to the girder web. To make this connection, many states use a bent plate to span the angle between the web stiffener and cross frame. While this bent plate connection is now being widely used, it has never been rationally designed to account for its strength or stiffness in the bracing system. Results from field studies show that the bent plate connection may be limiting the cross frame stiffness thereby hampering its ability to provide stability to the girder during construction. The result is significant girder end rotations. The purpose of this research is to classify the impact of the bent plate connection on the end cross frame stiffness in skewed straight steel girder bridges and propose methods to improve the end cross frame’s structural efficiency. This research uses laboratory testing, finite element modeling, and parametric studies to recommend design guidance and construction practices related to the end cross frames of skewed steel girder bridges. In addition to recommending methods to stiffen the existing bent plate connection, an alternative pipe stiffener connection is evaluated. The pipe stiffener not only offers the possibility of a stiffer connection, but can also provide warping restraint to the end of the girder which may significantly increase the girder elastic buckling capacity. / text