I-35W Bridge Collapse

The I-35W bridge over the Mississippi River in Minneapolis, Minnesota, collapsed suddenly on August 1, 2007. This note briefly summarizes an analysis based on original design drawings, an investigation of material evidence provided by the National Transportation Safety Board (NTSB), and a full-scale load rating of the bridge superstructure. The results of the investigation and conclusions of the analysis include. (i) The thickness of gusset and the thickness of the side wall of the upper chords were designed proportional to the bending moment solution of a one-dimensional influence line analysis. This fact reveals that the NTSB-disclosed undersized gusset plates are the consequence of a bias toward a “one-dimensional model” in the original design that did not give sufficient consideration to the effects of the forces from diagonal truss members. (ii) Although the bridge’s truss-cell structure was appropriately designed, the design of the node that connected the floor members to the main truss-frame was inadequate to effectively distribute live and dead loads. Consequently, the local redundancy provided by the truss-cells was significantly reduced. (iii) A three-dimensional, nonlinear, finite-element, computation-based load rating indicates that some of the gusset plates had almost reached their yield limit when the bridge experienced the design load condition. The bridge was sustained by the additional safety margin provided by the ultimate strength of the ductile steel that comprised the gusset plates.