CEE Students Inspect Area Bridges

sligo bridgeIn late November, Tennessee Tech University Civil and Environmental Engineering assistant professor Matthew Yarnold took 12 students out for a closer look at a couple of area bridges in need of repair. The students, all seniors with the exception of one master’s student, took the field trip as part of bridge design class.

Their first visit was to Hurricane Bridge, which spans the Caney Fork River in De Kalb County on state Route 56; built in 1949, it has a four-span Warren deck truss construction. The bridge was rehabilitated in 2011, at a cost of $26 million.

“When Hurricane Bridge was rehabbed, all of the members were painted, the decking was replaced with a lightweight concrete material, and the connection plates and bearings were all replaced,” said Tim Huff of the Tennessee Department of Transportation. “It took about four years to complete the rehab, with traffic over the bridge cut down to one lane with a signal at each end.”

Several miles away, Sligo Bridge faces a different set of circumstances. Sligo Bridge is of particular interest to Yarnold’s students, as they are doing a project on modeling the truss structures of the bridge.

Built in 1948, Sligo Bridge is a five-span Parker through-truss bridge over the Caney Fork on U.S. Route 70. The bridge is posted for a limited load condition, with a reinforcing bar exposed at various locations on the curbs and on the underside of the bridge’s decking. Nets have been installed to catch pieces of concrete that intermittently fall from the bridge’s deck, so as to not injure boaters or construction workers underneath. Sligo Bridge is slated for demolition, and construction has begun on its replacement.

“Part of the problem with older truss bridges is that it’s near impossible to widen them or do any really significant updates on them,” Yarnold said. “It’s a problem that’s just inherent in the design and you can see that this bridge is pretty narrow for modern traffic demands.”

Replacing Sligo Bridge entails some special engineering challenges, as the Caney Fork River is impounded and is actually 140 feet deep in spots. The contractor will have to drill another 40 feet into bedrock to set the bridge’s drilled shaft foundations, with strict quality control on the concrete. Sligo Bridge replacement project will cost roughly $40 million, barring any unforeseen challenges during construction.

“It’s the first project of this kind attempted in the state of Tennessee,” said Huff. “It’s really an incredibly complex undertaking, with all sorts of environmental concerns that enter into it.”

Along with the costs of the new bridge, demolition of the existing bridge alone will cost $2.1 million by itself.

Yarnold and students were pleased with the experience. “The feedback thus far has been positive,” said Yarnold. “I asked many of the students if this was worth the time and if it should be included in next year’s course. All said yes. I think most were happy to get out of the classroom and get some real-world information from practicing engineers and contractors.”

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