A design flaw in gusset plates has been implicated in the Minneapolis Bridge Collapse that killed 13 people last August. Yet, while even pointing to the gusset plate design flaw as a contributor to the I-35 W Bridge collapse, officials from the National Transportation Safety Board (NTSB) stressed that they have yet to discover the ultimate cause of the tragedy.
The I-35 W 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.
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. Early on, faulty gusset plates where eyed as a possible cause of the Minneapolis Bridge Collapse. 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.
16 Fractured Gusset Plates
According to the Associated Press (AP), NTSB Chairman Mark V. Rosenker said the gusset plates, which connected steel beams, 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.
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
The AP is also reporting that there are about 465 other steel-deck truss bridges around the country. Rosenker said the safety board had no evidence that the deficiencies in the Minneapolis bridge design “are widespread or go beyond this bridge.” However, he did say that contractors and state transportation officials should look at the original design calculations for such bridges before they undertake “future operational changes.” When the Minneapolis Bridge collapsed, it was undergoing construction, and there were 287 tons of construction vehicles and materials on the bridge at the time.
According to the AP, Transportation Secretary Mary Peters is expected to issue an advisory urging states to check the gusset plates when modifications are made to a bridge — such as changes to the weight of the bridge or adding a guardrail, said a federal official with knowledge of the plans. Currently, such calculations are done for the entire bridge, but not down to the gusset plates.
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