Later this week, federal safety officials will be discussing last year’s Minneapolis bridge collapse that killed 13 people and injured 145 others. The National Transportation Safety Board (NTSB) has scheduled a meeting for Thursday, where it is expected to release a draft report identifying the cause of the I-35 West bridge collapse.
The Minneapolis Bridge collapsed on August 1, at 6:05 p.m. It was the height of Minneapolis’ evening rush hour, and cars were lined up bumper-to- bumper across the span. At least 88 vehicles and hundreds of people fell 60 feet into the Mississippi River below. Thirteen people died and at least 100 others were injured. It would be nearly three weeks before the final victim of the Minneapolis Bridge Collapse was pulled from the river.
Since the catastrophe, a design flaw in the structure’s gusset plates has been cited as a possible cause of the collapse. Gusset plates act like braces to hold bridge joints together, but these structures can be troublesome. Water, dirt and salt can collect in them, and they can corrode and rust. Over time, this can cause the plates to weaken, and suffer fatigue cracks from excessive weight.
Deck truss bridges, like the I-35 W Bridge that collapsed, are especially vulnerable if gusset plates fail. That type of bridge design has no back up features to prevent a collapse if one component of the structure fails. For that reason, bridges like the 40-year-old Minneapolis Bridge are rarely built today.
In January, the Associated Press reported that NTSB Chairman Mark V. Rosenker said the gusset plates on the Minneapolis bridge were roughly half the thickness they should have been. Investigators found 16 fractured gusset plates from the bridge’s center span, he said. Rosenker said the agency’s investigation found no evidence that cracking, corrosion or other wear “played any role in the collapse of the bridge.” They also found no flaws in the steel and concrete material used in the bridge.
At that time, Rosenker said it wasn’t clear how the design flaw made it into the bridge because investigators couldn’t find the design calculations. He also said that the faulty gusset plates could not have been discovered during regular inspection s of the Minneapolis Bridge. Rather, the poor design of the gusset plates could only have been discovered when the I-35 W Bridge was in the early stages of construction.
At its Thursday meeting, which is expected to last two days, the NTSB will discuss the findings of its investigation and make recommendations to prevent similar disasters.
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