This bridge spans smart and practical
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THis 'smart' bridge is on Route 12 in Middlesex. KYLE MARTEL/TIMES ARGUS |
Toolbox
By Peter Hirschfeld Vermont Press Bureau - Published: November 26, 2009
MIDDLESEX – The 143-foot span under construction on Route 12 in Middlesex is smarter than the average bridge.
It can tell the temperature, detect its own movement, and even sense the amount of pressure pushing against its concrete abutments. Most impressively, though, the $2.5 million structure, under construction since early spring, will actually tell state engineers how to design better bridges in the future.
"What we're trying to do here, and the reason for all the science, is we're trying to understand how to build bridges as simple as possible," says John Zicconi, director of planning and communications for the Vermont Agency of Transportation. "Because simplicity means fewer costs in maintenance and potentially a longer life for the structure."
The new, two-lane bridge replaces a deteriorating structure that spans a small ravine with a brook emptying into the Wrightsville Reservoir. Situated near the intersection of Route 12 and Shady Rill Road, the bridge, according to Wayne Simmons, a structures engineer for the state of Vermont, is part of an innovation in bridge design.
"Bridges typically over the last 100 years have been designed with provisions in them to accommodate for expansion and contraction due to temperature," Simmons says.
Conventional bridges, he says, have accommodated for that expansion and contraction by separating the superstructure – the part you drive over – from the abutments that hold it in place.
"So what we've done traditionally is to use what we call bearings, which allow for movement of the superstructure to be separate from abutments that hold the superstructure in place," Simmons explains.
It's a been a workable arrangement, Simmons says, but not ideal. The bearings that separate the abutment from the superstructure are typically among the first components to fail.
"Things like bearings in expansion joints typically are first thing that becomes a problem in the life of the bridge," Simmons says. "Because expansion joints, no matter how hard we try, eventually will leak and when they leak, the road salt gets down on the ends of the beams and inside the bridge and can cause rusting and other damage that shortens the life of bridge."
So Vermont is trying out the "integral abutment" bridge, an approximately decade-old technology now being used in a few spots in Vermont – including the Route 12 replacement bridge in Middlesex. The plan marries the superstructure and abutment into a single, unified structure. The temperature, motion and pressure sensors built into the Middlesex bridge, Simmons says, will help Vermont's bridge engineers figure out just how far they can take the technology.
"We're starting out with modest use of integral abutment technology," Simmons says. "But we're really interested in seeing how far we can stretch in terms of the size of the bridge and the length of the bridge. And that's what we're trying to do with these instruments – to see how it performs and how far we can push the technology."
Integral abutment bridges offer significant advantages over the conventional structures they'll one day replace. Not only are the bridges a little cheaper to construct, transportation officials say they could save serious money in long-term maintenance costs.
The high-tech instruments that detect motion and pressure changes have been installed in three integral abutment bridges – the others are in East Montpelier and Stockbridge. Engineers will download and analyze the data from the bridges beginning next year.
"We all know the cost of repairing major infrastructure now is enormous, and we also know the history of people doing the proper maintenance over the years is not as stellar as it should be," Zicconi says. "Anytime we can get rid of things that are going to deteriorate or lock up or erode and cause you problems, we want to take advantage of that. And that's exactly what we're after with this project."
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