Recommended guidelines for redundancy design and rating of two-girder steel bridges

Article 10.3.1 of the 13th Edition of the AASHTO Standard Specifications for Highway Bridges classifies all two-girder steel highway bridges as nonredundant load path structures. This classification is based on unrealistic concepts widely held by bridge engineers, resulting from the oversimplified assumptions normally used in design, and not on the realistic behavior of the as-built three-dimensional structure. This report presents the results of an investigation into the after-fracture redundancy of simple span and continuous, composite and noncomposite steel two-girder highway bridges. A near full depth fracture is assumed to occur at any position along the length of one of the two girders. The fracture is assumed to penetrate the tension flange and full depth of the web, but not the compression flange. An analytical model is developed consisting of the fractured and unfractured girders together with a redundant bracing system consisting of top and bottom laterals and diaphragms. It is shown that the fractured structure carries dead and live loads as a "pseudo space truss". It is shown that the after-fracture behavior of the structure is primarily dependent on the strength and stiffness of the redundant bracing system and its connections to the girder flanges. It is shown that a properly designed and configured bracing system provides effective and efficient redundancy to a two-girder highway bridge. Procedures, equations and worked examples are provided for application to the design of a new or retrofitted redundant bracing system for new or existing bridges and for application to the redundancy rating of a properly configured existing bracing system in terms of AASHTO truck loading. Guidelines are provided for the design and rating of a redundant bracing system, for performing the retrofit of an existing bracing system, for the use of a composite deck as top lateral bracing, for special provisions for continuous two-girder bridges, and for the use of redundant tension cables or rods in lieu of a redundant bracing system. Recommendations for further research are made.

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