MnDOT Concerns shut down DeSoto Bridge.

With the Interstate 35W bridge collapse of August 1 fresh on their minds, Minnesota Dept. of Transportation officials shut down a 51-year old continuous steel deck truss arch bridge in Minnesota indefinitely after inspectors found bent gusset plates March 20. MnDOT officials say they are now working to expedite an estimated $30-million bridge replacement originally scheduled for bid letting in 2010.

The 889-ft-long, 70-ft-wide Minnesota Highway 23 DeSoto Bridge, located in St. Cloud over the Mississippi River, is classified as fracture-critical—meaning it has no redundancy—but is not rated structurally deficient or functionally obsolete, acting MnDOT Commissioner Bob McFarlin said during a March 21 press conference. Inspectors found quarter-inch bends in the 5 ft x 8 ft gusset plates, located on the lower chord of the truss main span, that they attribute to loading. “This type of distortion was new to us,” said Dan Dorgan, MnDOT bridge engineer. Engineers will determine if steel reinforcing can be bolted to the gusset to stiffen it, “similar to a brace on a leg,” he said.

McFarlin said MnDOT engineers were working with University of Minnesota’s civil engineering department to explore options.  “The bridge will remain closed until and unless a repair [method] can be found,” he said. If no suitable repair job can be determined, then the bridge, which carried more than 30,000 vehicles a day, will stay closed until a replacement is finished.

Dorgan noted that inspection crews were doing ultrasound testing on the gusset plates for corrosion as part of a systematic review, but “it was visual inspection that found those abnormalities.” The state is planning to complete reviews of all its 25 truss bridges—with gusset plates—by June.

The Federal Highway Administration issued an advisory on gusset plates in January after the National Transportation Safety Board chairman Mark V. Rosenker noted that the I-35W bridge’s half-inch plates were 50% too thin. “It’s somewhat a learning process for states, these in-depth gusset plate reviews,” said Dorgan. “We are confirming that what was designed was adequate throughout the state. On this bridge, they are half-inch plates. The 35W is much larger. So you can’t compare the two. But we did do design checks here and the half-inch plates appear to be the correct size intended.”

Jim Povich, MnDOT district program delivery engineer, says that permitting agencies have contacted his office and pledged support for an expedited process for a new DeSoto bridge.