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Sensors will monitor strain on Highway 51 bridge

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Stacy Vogel
October 23, 2007
— Thud. Thud. THUD!

Workers under the bridge on Highway 51 heard a dull thump every time a car passed overhead. They heard a heavier sound whenever a truck or other large vehicle crossed the bridge over the Rock River in Fulton Township.


It’s the heavy thuds that interest officials at the Wisconsin Department of Transportation, said Matt Murphy, a bridge maintenance and inspection engineer with the department.


Workers installed four “strain gauges” under the deck truss bridge Monday to collect data on traffic loads. Researchers will analyze the data to see if heavy vehicles are causing too much stress on the structure.


The move is part of a statewide reaction to the Aug. 1 bridge collapse in Minneapolis. Thirteen people died and 100 were injured after the deck truss bridge collapsed over the Mississippi River.


A week later, Gov. Jim Doyle ordered inspections of Wisconsin’s 15 deck truss bridges, including the 69-year-old Highway 51 bridge. (In October, one of the 15 bridges was torn down for a scheduled replacement, lowering the number to 14.)


After the governor’s order, Murphy and his crew examined the Highway 51 bridge using “The Snooper,” a machine that maneuvers workers in a basket around and under the bridge. Murphy gave the bridge satisfactory rankings for its steel and substructure and a good ranking for its deck.


“The Snooper” made another appearance Monday as workers installed the strain gauges. The gauges will send information to a data collection box at the top of the hill next to the bridge.


Once a week, contractors will take the data collected from the Highway 51 bridge and the other deck truss bridges around the state to researches at UW-Madison, UW-Milwaukee and Marquette University in Milwaukee. Researchers will be looking at the “elasticity” of the bridges, Murphy said.


For example, it’s OK if a bridge lowers an inch when a heavy truck drives over it, as long as the bridge returns to its original state afterward, he said.


The state isn’t expecting to find any problems with the Highway 51 bridge or any of the others, Murphy said.


“This is a result of what happened in the Twin Cities, primarily,” he said. “The state decided to take the precautionary measure.”


If the state does discover problems, it could lower the weight limit on a bridge or try to strengthen its structure, Murphy said. The Highway 51 bridge currently has a weight limit of 45 tons.


The state is paying contractors $240,000 to collect the data from the 14 bridges for three months, said Finn Hubbard, state bridge engineer. Depending on the results, it might or might not continue the data collection afterward.


“We’re going to let the bridges tell us what we do next,” he said.



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