From its initial design to the stockpiling of materials during subsequent renovation, Sean Brady details a succession of failures and missed opportunities that resulted in the catastrophic collapse of a highway bridge in Minnesota.In the early afternoon of August 1, 2007, Progressive Contractors Inc (PCI) were preparing for a 160m-long concrete pavement pour on the southbound lanes of the I-35W Highway Bridge in Minneapolis, Minnesota.Seven previous pours had been completed since the project’s commencement in June, and for this, the eighth, 50mm of existing concrete wearing course had been removed in preparation for the pour planned for 7pm that evening.PCI’s preparations included stockpiling gravel and sand on two of the bridge’s southbound lanes, which was not an unusual procedure, as the Minneapolis Department of Transport’s (Mn/DOT) specification provided only a one-hour window between initial concrete mixing and final screeding – thus necessitating that concrete be mixed as close to the placement site as possible.By 2.30pm that afternoon, 84t of gravel, 90t of sand, and 90t of construction vehicles, equipment and personnel – totalling 264t spread over an area of approximately 300m2 – was in place for the pour.As we now know, this pour never took place. At 6.05pm, a 300m section of the main truss span collapsed, with a 140m section falling 33m into the river below (Figure 1, main image). A total of 111 vehicles were on the collapsed section, and only 17 were recovered. Tragically, 13 people lost their lives and 145 were injured.The subsequent National Transportation Safety Board’s (NTSB) investigation would identify undersized gusset plates, due to a design error, in combination with recent increases in dead load and live load as the cause of the failure1.This article explores how the load increases occurred and the role they played in the collapse, as well as exploring the design error and, just as importantly, how it went unnoticed throughout the bridge’s 40-year service life.
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