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.
In this research project, an investigation into the after-fracture redundancy of simple span and continuous, composite and noncomposite steel two-girder highway bridges was conducted. A near full depth fracture was assumed to occur at any position along the length of one of the two girders. The fracture was assumed to penetrate the tension flange and full depth of the web, but not the compression flange. An analytical model was developed consisting of the fractured and unfractured girders together with a redundant bracing system consisting of top and bottom laterals and diaphragms. Results showed that (1) the fractured structure carries dead and live loads as a "pseudo space truss," (2) 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, and (3) a properly designed and configured bracing system provides effective and efficient redundancy to a two-girder highway bridge.
Procedures, equations, and worked examples were developed 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.
Research is complete, and the final report has been published as NCHRP Report 319, "Recommended Guidelines for Redundancy Design and Rating of Two-Girder Steel Bridges."
Guidelines are provided for the design and rating of a redundant bracing system including suggested design and rating loads, allowable stresses, load factors, serviceability criteria, probable fracture locations, strength of connections and allowable fatigue stresses. Guidelines are provided for performing the retrofit of an existing bracing system, for the use of a composite deck as top lateral bracing, for special previsions for continuous two-girder bridges, and for the use of redundant tension cables or rods in lieu of a redundant bracing system for two-girder deck-type bridges and through girder bridges.
Guidelines, procedures, and equations are also presented for the redundancy design or rating of new or existing two-girder steel bridges using the results of a finite element computer analysis of the entire three-dimensional structure. They are intended for bridge engineers who wish to provide redundancy design or rating by computer rather than by using the simplified procedures and equations developed in the remainder of the report.
