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Designers blamed for I-35 collapse

Official report on Minneapolis bridge collapse puts spotlight on load path connections.

Bridge engineers this week urged greater vigilance in the design of vital bridge components after an official investigation revealed that under designed gusset plates contributed to the catastrophic collapse of the I35W bridge in Minneapolis in 2007.

The collapse of the bridge’s 300m main span of the I35W in August 2007 killed 13 people (NCE 9/16 August 2007). The final report into the cause of the collapse from the US National Transportation Safety Board (NTSB) says that gusset plates used to connect load bearing columns and trusses had inadequate load bearing capacity.

Bridge designer Sverdrup & Parcel & Associates, now owned by Jacobs, was found to be at fault. Independent steel bridge consultant Jolyon Gill told NCE that UK bridge designers would need to be more vigilant as a result of the report. "People will look more carefully at each component of the bridge and especially at gusset plates," said Gill. "It’s particularly important to put the nuts and bolts in the right place.

"The report will put the spotlight back on load path connections." Head of consultant Benaim Simon Bourne added that the report showed that small details in bridge design needed more attention. "There is a tendency with a lot of schemes for people to concentrate on the main elements, such as flanges, but there is very, very rarely a problem with the main sections," said Bourne. "It’s invariably the details of the design or maintenance that can lead to problems and these are the things that need more engineering."

The NTSB found 24 underdesigned gusset plates on the I- 35 structure. They were about half the thickness of properly sized gusset plates, and escaped discovery during the original design review. Sverdrup & Parcel & Associates were found to have failed to ensure that the appropriate main truss gusset plate calculations were performed and inadequate design review was found to have been carried out by federal and state transportation officials.

NTSB acting chairman Mark Rosenker said: "Bridge designers, builders, owners, and inspectors will never look at gusset plates quite the same again, and as a result, these critical connections in a bridge will receive the attention they deserve in the design process, in future inspections, and when bridge load rating analyses are performed."

The NTSB report also found that "substantial increases in the weight of the bridge, which resulted from previous bridge modifications" put further strain on the inadequate gusset plates. Concentrated construction loads on the bridge on the day of the collapse as a result of works being carried out added to the problem it said. Inspections carried out had not identified the problem because gusset plates had been largely ignored.

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