Figure 1: I-35W Mississippi RiverBridge(1967 – 2007) of Minneapolis, MN.1
The I-35W Mississippi River Bridge was constructed in 1967 to serve Interstate W35 traffic in Minneapolis, Minnesota. The bridge was provided with eight lanes for traffic which in 2007 numbered about 140,000 vehicles a day. The bridge was designed by Sverdrup & Parcel and Associates of St. Louis, Missouri in accordance with the 1961 AASHO (American Association of State Highway Officials) specifications. It consisted of two, three-span continuous deck trusses, six continuous steel-girder approach spans, two steel-beam approach spans, and three concrete voided-slab approach spans. Abutments and piers were of reinforced-concrete construction. On August 1, 2007, after nine days of high temperature that averaged about 90º, the bridge suddenly collapsed.
Figure 2: Collapsed sections of the I-35W Mississippi River Bridge.
Photo courtesy of Tim Davis.
The content and especially the conclusion of the NTSB Report on the collapse of the I-35W Bridge is shocking to an engineer who has spent almost his entire fifty-year career evaluating the behavior of real bridges. Upon examining the contents of NTSB report, the author discovered that the investigation was elementalistically focused on the behavior of only one part of the complete or "composite structure," its deck truss system (see EXHIBIT A). Missing from the investigation was any awareness of the behavior of the other parts of the composite structure (six steel-girder approach spans, two steel-beam approach spans, three concrete voided-slab approach spans, two approach embankments, two approach concrete highway pavements, substructure foundations, etc.), or how the behavior of these other parts of the structure, taking into account the forces associated with them and their various application orientations, would impact the behavior of the overall structure. Consequently, the NTSB investig- ation appears to have been merely an academic exercise that focused only on the response of a deck-truss system to hypothetical vertical loads.
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