ST. PAUL – A highway bridge over the Mississippi River at St. Cloud was shut down on Thursday after inspectors found slight bowing in four gusset plates, the steel plates that help connect the bridge's girders.
Transportation officials said a visual inspection determined the gusset plates had "distorted" by about a quarter of an inch.
Flawed gusset plates have been called a critical factor in the collapse last August of the Interstate 35W bridge in Minneapolis that killed 13 people. Both bridges are steel-truss bridges classified as "fracture critical," which means that the failure of a single critical member can bring down the whole bridge.
Asked whether the 35W collapse affected the decision to close the bridge, state bridge engineer Dan Dorgan said it did.
"Our approach is very conservative given the tragedy we experienced last year," Dorgan said.
Federal investigators have said some gussets on the 35W bridge may have failed because they were designed too thin for the loads they had to carry. Dorgan said that wasn't the case with the gussets on the St. Cloud bridge.
Both bridges were designed by Sverdrup & Parcel.
The problematic gussets are about 5 by 8 feet, and about half an inch thick.
Acting Transportation Commissioner Bob McFarlin said it wasn't clear how the bridge would be repaired or when it would reopen.
"The bridge will be closed until it's deemed to be safe. If repairs cannot be completed or repairs are not possible, it will remain closed until it can be replaced," McFarlin said.
McFarlin said the transportation department's bridge office made the decision to close the bridge. He said he immediately agreed and passed the word to Gov. Tim Pawlenty.
McFarlin said the bridge was scheduled for replacement in 2015. The Minnesota Department of Transportation had been trying to accelerate that even before Thursday's emergency closure, and now will examine its options to speed replacement even more, he said.
The bridge at St. Cloud was built in 1957. The four-lane span, 890 feet long, carries 31,000 vehicles a day to and from downtown, but transportation officials said they don't expect its closure to significantly slow traffic because a detour will be only a few blocks long.
The Highway 23 bridge, also known as the DeSoto bridge, was inspected right after the Minneapolis collapse as part of statewide bridge inspections ordered by Pawlenty.
After the National Transportation Safety Board spotlighted flawed gussets in the Minneapolis bridge in January, MnDOT undertook another review of the design of gusset plates on similar truss bridges around the state — 59 in all.
The designs of the gussets on the Highway 23 bridge were checked and "no deficiency has been found," MnDOT reported in a Jan. 15 statement on its Web site. But MnDOT also said at the time it had hired two engineering firms to do load ratings on the state's largest truss bridges, including the Highway 23 bridge.
The agency has a June target date to complete those additional checks.