I-35W Mississippi River Bridge — Half-Inch Gusset Plates That Buckled Under Added Weight

On 1 August 2007, at 6:05 p.m., the eight-lane steel deck-truss bridge carrying Interstate 35W over the Mississippi River in Minneapolis, Minnesota collapsed in seconds during the evening rush, dropping a 456-foot main span and its approaches into the river and onto the banks. Thirteen people died and 145 were injured; 111 vehicles were on the failed deck. The National Transportation Safety Board (NTSB), the federal investigating body, traced the collapse to a single class of component: the gusset plates that connected the steel members at the bridge’s main-truss joints. At the joints designated U10, those plates were roughly half the thickness they should have been — a design error baked into the structure when it was built in the 1960s.

The mechanism was instability, not rupture from corrosion or fatigue. A gusset plate is the steel sheet that ties a truss’s diagonal, vertical, and chord members together at a joint, and it must be thick enough to carry the combined forces without buckling. The U10 plates were 0.5 inches thick where the design demanded roughly twice that, and for four decades they carried traffic because the loads stayed within the margin even an undersized plate retained. The NTSB found two slow, additive overloads erased that margin: concrete resurfacing raised the permanent dead load about 20 percent, and on the afternoon of the collapse a resurfacing project parked an estimated 578,000 pounds of equipment, sand, and aggregate over the weakest joints.

When the demand on the U10 plates finally exceeded their buckling capacity, the plates failed by lateral instability — they folded. The deck truss was non-redundant and fracture-critical: with a main-truss connection gone, there was no alternate load path, and the loss of one set of joints unzipped the center span. The collapse propagated across the full 1,907-foot bridge in roughly four seconds.

What makes I-35W a permanent case is that the fatal flaw was not wear, weather, or neglect, but an original calculation that was never done. The plates were sized below the loads they would carry, the error survived design review, and decades of added dead load plus one day of stacked construction material brought the demand to the point the deficient plates could not hold. The bridge did not fail because it grew old; it failed because it was never strong enough at one joint, and no one ever checked.