Effect of Inconsistent Diaphragms on Exterior Girder Rotation During Overhang Deck Construction

The construction of bridge deck overhangs results in unbalanced eccentric loads acting on exterior girders which can cause rotation and increased stresses not accounted for during design. Permanent diaphragms and temporary bracing in bridge exterior girder lines or panels are used to resist these loads and subsequent transverse rotation of the exterior girders. The addition of extra diaphragms in the exterior panels is one potential alternative to temporary bracing which is not always effective. In this paper, a unique steel plate girder bridge in the state of Illinois with extra diaphragms in one exterior bay was instrumented with tilt sensors and strain gages to monitor transverse rotations and strains due to unbalanced loads occurring during construction. Two types of rotations were recorded; maximum and residual rotations. The extra diaphragms were included in the design of this bridge on only one side of the bridge to carry utility lines. The full bridge was modeled using the commercial finite element analysis software ABAQUS and the model was validated using field data. As expected, diaphragm spacing was found to have a high impact on exterior girder rotations that occur during bridge deck construction. The maximum obtained finite element rotation was 0.47° which occurred at mid span and on the bridge deck side that does not have extra diaphragms. Field residual rotations were found higher (approximate 50% on average) than rotation determined from the finite element analysis. These extra stable rotations were seen in the exterior girders and was a result of permanent deformation occurring when the finishing screed passed by the section under consideration.