Cross-Frame Connection Details for Skewed Steel Bridges

This report documents a research investigation on connection details and bracing layouts for stability bracing of steel bridges with skewed supports. Cross-frames and diaphragms play an important role in stabilizing steel girders, particularly during construction. The commonly used bent plate connection between skewed braces and steel girders can introduce flexibility that can have detrimental effects on the bracing behavior. An alternative detail investigated in this study is a split pipe stiffener used to connect cross-frames to girders at a skew. The split pipe stiffener allows perpendicular connections to the cross-frame connection tab, regardless of the skew angle. The split pipe provides a stiffer connection between the cross-frame and the girder. More importantly, the split pipe stiffener increases the torsional stiffness of the girder by introducing substantial warping restraint. This increases the lateral torsional buckling capacity of the girder and allows the first line of intermediate cross-frames to be moved farther from the support. Overall, the increase in girder torsional stiffness and buckling capacity that results from the use of the split pipe stiffener will enhance the safety of the girder at all stages of construction: during transportation, lifting, erection, and placement of the concrete deck. This study also examined layout patterns for intermediate cross-frames in skewed bridges. Results showed that staggering the intermediate cross-frames reduces live load induced forces in the cross-frame members and mitigates the potential for associated fatigue cracking. This report also provides design recommendations for the split pipe stiffener and provides a procedure for computing the buckling capacity of girders with split pipe stiffeners.