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The move follows the Jan. 15 revelation by federal officials that a bridge element typically assumed to be the strongest link in such structures was actually a source of weakness that contributed to the Minnesota bridge failure.
The decision by Wisconsin officials increases by nearly
tenfold the number of state bridges under added scrutiny because of that deadly tragedy and follows the guidance of federal officials looking into the collapse.
The problem in the I-35W bridge appeared to be an isolated case, but the state isn’t taking any chances, said Beth Cannestra, director of the Department of Transportation’s Bureau of Structures, which oversees the design and inspection of Wisconsin’s bridges. The change matters because until now, no inspector would have looked for the design flaw that caused the Minneapolis collapse.
“Nobody would have known that this was different than what the design should have been,” Cannestra said. “Something that should have been the strongest link was a weaker link than folks thought.”
The problem was hidden in a series of steel “gusset” plates that can be riveted or bolted to the beams and members of a bridge to hold them together to form a larger part called a truss. The failure at the I-35W bridge came in part because decades ago a consultant designed some of its plates to be only a half-inch thick when they should been a full inch thick, said Cannestra and Finn Hubbard, a Wisconsin state bridge engineer.
“The concerning part of it is that what happened there could happen somewhere else,” Hubbard said. “This was a bomb sitting in the weeds waiting for somebody to kick it.”
After the collapse, Wisconsin put extra scrutiny on 14 bridges in the state with designs most similar to the I-35W bridge — so-called “deck truss” bridges, which have trusses running beneath the roadway they support. Now the state is expanding the extra effort to other types of truss bridges — 127 bridges in all — because all truss bridges have gusset plates.
If one truss fails, as in the case of the I-35W bridge, the whole structure can collapse.
The federal National Transportation Safety Board is still investigating what triggered the collapse.
The state’s 13,677 bridges include 127 truss bridges, Cannestra said. All of the truss bridges have been inspected within the last two years.
About 35 of the bridges are in the south-central or southwest Wisconsin, including the Highway 51 bridge over the Rock River in Rock County. Only one is in Dane County, a more than 100-year-old bridge over the Yahara River on East Dyreson Road in the Town of Dunn.
To make sure the truss bridges are sound, state officials will:
Hubbard said that the NTSB and the Federal Highway Administration had recommended these precautions.
“Their point is if you’re increasing the bridge load in any significant way you should really check all the elements,” said Hubbard, adding that no further steps were needed since the Minneapolis failure appeared to isolated.
Milwaukee structural engineer John Goetter, who helped put out a recent report on the state’s bridges for the American Society of Civil Engineers, said the precautions appeared “very reasonable” given that the I-35W bridge failure was apparently isolated. “If anything, it’s conservative to me.”
Michael Oliva, a structural engineer at UW-Madison, said such checks should have already been happening around the country. “It’s unfortunate it takes a disaster like this to wake us up.”
In addition to the 127 truss bridges, the state is also looking at the structural parts of an additional up to 88 so-called “fracture critical” bridges. Those are structures that, like the I-35W bridge, could also collapse entirely if one of their supporting members failed, Hubbard and Cannestra said.
In August, the state established special monitoring sensors to track the stresses on 14 deck-truss bridges in the state. For the four bridges analyzed so far, the sensors found that the stresses are less than what they are designed to take, Hubbard said. Data on the final 10 bridges should be ready by Feb. 21, he said.
Wisconsin received good marks in a recent national report by msnbc.com for doing much better than most other states in completing bridge inspections within two years and not seeking to defer those inspections for longer than that.
Jason Stein is a reporter for the Wisconsin State Journal in Madison.
