A design flaw in the bridge’s gusset plates was to blame.
Steel gusset plates, like those on the Minneapolis Bridge that collapsed last Wednesday, where at the center of a bridge failure in Ohio 11 years ago. Even as the National Transportation Safety Board (NTSB) tried to downplay reports that a design flaw in the bridge’s gusset plates was to blame for the I-35 W Bridge collapse, bridge inspectors around the country were taking a closer look at such plates on other bridges. And concerns about the gusset plates were serious enough that Secretary of Transportation Mary Peters formally warned states to carefully consider the loads they placed on similar bridges that are under construction or repair. The Minneapolis Bridge was being resurfaced and had several heavy construction vehicles parked on its span when it collapsed.
Deck truss bridges, like the Minneapolis 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. 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. That is what happened to an Ohio interstate bridge in 1996.
According to an article in the Cleveland Plain Dealer, a similar bridge on I-90 in Ohio failed in 1996 because of problems with its gusset plates. The bridge in Perry Township, Ohio, sank 3 inches after some of its gusset plates had corroded and buckled. The Ohio bridge had striking similarities to the Minneapolis Bridge. For one thing, they were both Warren truss bridges that used gusset plates. And according to the Cleveland Plain Dealer, both bridges were redundant, meaning that if one part fractures, the whole bridge can fall. And both the Minneapolis Bridge and the Ohio bridge were undergoing construction when they failed.
Several of the gusset plates had been corroding
In the case of the Ohio bridge, several of the gusset plates had been corroding unbeknownst to bridge engineers before the construction started. According to the Plain Dealer article, the gussets were too thin. Construction on the bridge had also weakened the plates further, because crews had used steel shot to blast the bridge in preparation of repainting. That process and the added weight of the construction vehicles caused some gusset plates to buckle in May 1996. Reports said that the bridge began to “bounce”, and it sunk three inches. Unlike the I-35 W Bridge collapse, no one was injured in Ohio.
Reports from around the country indicate that other states are taking Secretary Peter’s warning to heart. In Pennsylvania, where the state owns 28 truss bridges similar to the Minneapolis Bridge, bridge inspectors have been asked to pay special attention to gussets. The state’s transportation agency started to inspect all Pennsylvania bridges that have a design similar to the collapsed I-35 W Bridge last week.
Yet even as attention turned to the gusset plates as a possible culprit in the Minneapolis Bridge collapse, the NTSB was trying to downplay such reports. The agency said that gusset plates are only one area being looked at, and no single cause for the I-35 W Bridge collapse had been determined.
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