Diaphragm Design for Limiting Exterior Girder Rotation during Overhang Construction

Bridge overhang construction often introduces torsional moment acting on the exterior girder due to the unbalanced loads coming from fresh concrete, finishing equipment, and other construction live loads. Sometimes this torsional moment can create excessive rotation on the exterior girder in the transverse direction, leading to thin deck, reduced concrete cover, and potential maintenance problems in the service stage. Permanent and temporary diaphragms are widely used in wide flange steel girder bridges to resist these loads and subsequent transverse rotation of the exterior girders. However, those diaphragms are generally designed for resisting lateral wind loads, improving vertical loads distribution, and creating lateral stability during construction, which often results in insufficient bracing for reducing the exterior girder rotation. In this study, the effective diaphragm depth, which is determined based on the size of diaphragm and the type of girder to diaphragm connection, was introduced to determine the minimum diaphragm size in order to limit the distortion of the girder web during the bridge overhang construction. A parametric study was conducted to evaluate the significance of different bridge geometries using finite element analysis. The result shows that the diaphragm depth and spacing, as well as girder section, and overhang width, are the most important parameters that affect the exterior girder rotation. To evaluate the relationship among those parameters, around three thousand finite element models were built in SAP2000 using the bridge modeling computer program developed in this study. Finally, a simple method to determine the maximum diaphragm spacing to limit the exterior girder rotation was developed. This method uses the combination of primary curves and modification coefficients based on the girder section and overhang width.