Ductility of Non-Compact Steel Bridge Beams

Since the fifties, composite construction in steel bridges has been used in simple as well as in continuous structures. Composite construction incorporates the concrete deck with the top flange of the supporting rolled beams or welded plate girders. The final result consists of the two components from different materials acting as a unit. The composite action can be accomplished by mechanical bond between the flange and deck in the form of welded channels, studs, or other devices. Either shored or unshored construction can be specified. Composite construction for highway bridge design is most economical for spans in the range of 40-50 ft. and longer. The increase in the moment of inertia of the composite section helps in reducing deflection due to live load and impact. For shorter spans, the saving in steel weight does not usually pay for the additional weight of the shear connections and labor involved. Composite steel girder bridges in the United States are usually designed using the Load Factor Design procedure of AASHTOs Standard Specifications for Highway Bridges. Current AASHTO allows the bending moment capacity of compact sections in positive bending to be based on the full plastic moment. The moment capacity at first yield is used, however, for non-compact sections. In most practical cases, the full plastic moment of composite sections is significantly larger than the yield moment.