I-35W bridge collapsed at rush hour — 0.5-inch gusset plates, a 40-year-old design error

The I-35W Mississippi River bridge was a steel deck truss bridge designed in the early 1960s and opened in 1967 as a key Minneapolis artery carrying 140,000 vehicles daily. The original design contained a fatal error: the gusset plates connecting the truss members were only 0.5 inches thick — roughly half the thickness needed for the loads the bridge carried. The design met 1961 AASHTO specifications, but the error was never caught during design review, construction, or four decades of inspections.

By August 1, 2007, the bridge carried far more than its original design loads. A 20% increase in concrete deck weight had been added over the years. That afternoon, 578,000 pounds of sand, water tanks, vehicles, and construction equipment sat in four closed lanes during a resurfacing project. At 6:05 PM CDT, during rush hour, the south-center gusset plates on two truss nodes failed simultaneously. The entire 1,907-foot span dropped into the river in seconds — 13 people died and 145 were injured.

The NTSB investigation concluded that the primary cause was a design error in the original 1960s gusset plates. The bridge had been rated structurally deficient since 1990 due to corrosion in its bearings, and was inspected annually — but inspectors checked individual members and never flagged the gusset plate thickness. MnDOT had planned a reinforcement project in December 2006 but canceled it two months later. The replacement bridge opened September 18, 2008 at a cost of $234 million, three months ahead of schedule.