Rapid Assessment of Gusset Plate Safety in Steel Truss Bridges

Research into the cause of the failure of the I-35W Bridge in Minneapolis, Minnesota, has indicated that some of the gusset plates were significantly overstressed. The Federal Highway Administration has recommended load rating all gusset plate connections in bridges around the country using their recommended load rating procedures, which are complex and require a large commitment of resources. Further, the procedures use approximate ultimate capacity and therefore do not address serviceability or bridge longevity, which require prediction of gusset plate yielding, not collapse. A triage evaluation procedure is proposed to enable rapid, yet conservative, identification of gusset plates that may be yielding under service loads and to eliminate gusset plates with adequate capacity from needing further or more sophisticated analysis. High-resolution finite element models of gusset plate connections from Washington State bridges and the connection identified as critical from the I-35W Bridge are used to develop the proposed procedure and evaluate the current recommendations. A complex interaction of stresses is generated in gusset plates by connecting members, and this interaction can initiate gusset plate yielding when the uniaxial stresses on Whitmore sections associated with those connecting members are well below yield. It is demonstrated that the onset of gusset plate yielding may be conservatively predicted by comparing the uniaxial stresses at Whitmore sections with Fy/3. Additionally, a modification to the simple column analogy method recommended by FHWA for calculating the buckling capacity of gusset plates is proposed, and it is shown that gusset plate yielding occurs prior to buckling in all cases considered. The triage evaluation procedure is grounded in mechanics and conservative assumptions, enabling more rapid identification of problem gusset plates while also being reasonably conservative. The method is then applied to three bridges, from which 3 gusset plates out of 35 are identified as requiring additional investigation.