Field Measurements on Steel Girder Bridge with Skewed Supports Utilizing Lean-On Bracing

There are a number of performance stages that must be considered in bridge design. The critical stage for lateral torsional buckling of steel girder bridges often occurs during deck placement. The buckling capacity is improved by installing cross-frames at intemediate points along the bridge length. Many times, standard cross-frames are used, which can result in braces that are larger than required. These stiffer braces tend to attract larger live load forces and can lead to the development of fatigue cracks around the brace locations, particularly in bridges with skewed supports. Bridges with skewed supports are often necessary because of geometric constraints of underpasses or geological issues with the terrain. Though, a major concern with skewed bridges is differential deflection, which can intensify the propensity for fatigue cracks. Using lean-on bracing concepts can alleviate these concerns by allowing designers to reduce the number of cross-frames that are required as well as minimize live load induced forces in bridges for both skewed and non-skewed supports. The Texas Department of Transportation (TxDOT) recently implemented the recommendations of a research study on lean-on bracing [Wang 2002, Helwig and Wang. 2003] for three steel bridges with support skew angles of approximately 60 degrees. One of the bridges was instrumented with strain gages on the girders and braces prior to erection. The bridge performance was monitored during placement of the concrete bridge deck as well as subsequent live loading with two trucks loaded with sand. In addition to strains in the girders and braces, girder deflections and rotations were also monitored. This paper summarizes the results from the field monitoring and demonstrates the benefits of the lean-on bracing techniques.