Official: No link in Minneapolis, Milwaukee bridge failures
Published: August 13, 2007
Madison – A Milwaukee bridge that failed in 2000 did not have flawed gusset plates such as the ones investigators are focusing on in last week’s collapse in Minneapolis, the state bridge engineer said Thursday.
The National Transportation Safety Board said Wednesday that it found a design issue with the collapsed Interstate 35W bridge’s steel gusset plates, which tie steel beams together.
Media reports tried to link similar problems to the failure of Milwaukee’s Hoan Bridge in December 2000, but Wisconsin Department of Transportation’s chief bridge engineer Finn Hubbard said there was no connection.
“It’s an interesting corollary, but they are actually completely and totally different,” he said.
The Hoan Bridge, a freeway span over Milwaukee’s Lake Michigan harbor, failed after cracks developed in two steel beams holding up the structure, causing the road to drop four feet.
An investigation determined the bridge’s design contributed to the sudden cracks and said heavy traffic and freezing cold temperatures played a role.
The cracks developed in part because of the way steel connection plates were attached to the girders, Hubbard said. That design caused stress to build up, leading to the cracks.
But the plates did not bear weight and did not fail themselves, Hubbard said.
In contrast, the gusset plates on the deck truss bridge in Minneapolis helped hold the bridge up by connecting key parts.
“If a gusset plate was to fail, it could bring down a bridge of that type,” Hubbard said. “It could be an issue with strength, it could be an issue with corrosion, it could be an issue with connections.”
The NTSB said Wednesday its investigators are reviewing the loads and stresses on the gusset plates in that bridge and the materials used to construct them. Federal officials told state transportation officials to re-examine bridges with a similar design.
Wisconsin Department of Transportation employees are reviewing the state’s 15 deck truss bridges — which are held up by parallel trusses beneath the roadway — to detect any issues with gusset plates, Hubbard said.
“We are looking at how the gusset plates were used on our bridges, the size of them, the thickness of them,” he said. “It could lead to some maintenance and modifications.”