This Ingenious Way to Build Bridges Will Fix Our Crumbling Infrastructure

Construction crews replacing a bridge on I-84 in New York are using a technique called accelerated bridge construction to save time and money while minimizing headaches for motorists. Photo: New York State Department of Transportation

The I-84 corridor that connects New York and Connecticut is getting a new and improved bridge, and the final piece of the intricate puzzle will slip into place this weekend. The way it’s happening saves money, limits closures, and could revolutionize the way America remakes its crumbling infrastructure.

The technique is known as accelerated bridge construction, or ABC, and though it’s been used on hundreds of small projects, this marks the first time a state transportation agency has used it on a major interstate.

The existing bridge is a 135-foot long, two-lane structure in the town of Southeast, New York. It carries as many as 100,000 drivers a day, with big rigs accounting for 16 percent of traffic. If the state opted for a traditional replacement, it would have taken more than two years, involved multiple closures, and would require a temporary bridge that would have added $2 million to the overall cost.

“The New York State DOT didn’t have a good strategy,” says Bala Sivakumar, director of special projects for infrastructure design firm HNTB. The problem, according to Sivakumar, is Connecticut has been upgrading and widening its roads for years, while N.Y. state “hasn’t done much since the 1960s.”

“Everything pointed to a great ABC solution,” Sivakumar tells WIRED.

The process of ABC is basically to build support structures around the existing bridge, construct the new roadway on top, then demolish the old bridge while simultaneously sliding the new bridge into place and onto its new supports.

The new bridges are constructed alongside their predecessors, with a new superstructure and abutments waiting to form the basis of the new bridge. Image: NY DOT

Construction began late last year, with a combination of pre-made components being shipped in while the construction of two new superstructures on temporary supports were mounted alongside the current bridge.

“We assembled these pre-cast elements into a single span,” Sivakumar explains. “With one superstructure in the median area between the two existing bridges and another north of the westbound bridge. While all this was happening, [the construction crew] built a new superstructure underneath the existing bridge.” And all this was happening without any affect on traffic.

“People see what’s going on, but there’s no impact to them,” says Sivakumar.

At 5 p.m. on Saturday, September 21, the DOT closed I-84 and the first round of demolition began, with crews working for four hours to rip apart the dilapidated structure. And then the slide began.

Four 100-ton jacks began pushing on the new roadway plates, each set atop a “slide track” with a teflon surface that slips across the underside of the polished, stainless steel plates. But each jack only has a stroke of around 30 inches, so every two and a half feet the plates slide, the crews move the jacks forward to push the new roadway further.

“It’s a slow process,” Sivakumar admits. But the bridge was in place eight hours later, and Sivakumar says it could have been done in four to six hours, “but it was pouring that night.”

The final step was to raise the approaches of the road on either side of the bridge to match the new structure’s height and length, so an army of asphalt trucks, pavers, and workers descended on the two sides to build up the additional space and make it strong enough to handle the hundreds of thousands of tractor trailers that would roll over it in the coming years.

The westbound span was opened to traffic at 12:55 p.m. the following day, widened into a single span bridge that was 80 feet long and 57 feet wide — more than twice the width of the previous bridge — with three lanes and two full-size shoulders.

This Saturday, October 19, the eastbound span will be slid into place, and if weather isn’t an issue, Sivakumar says the new span could be open even earlier.

For the hundreds of small bridges across the country in desperate need of replacement, ABC is a perfect solution. It’s low impact, less expensive, and takes less time. Sivakumar concedes it’s not the best solution to every bridge, saying that “20 spans over a river would be hard to replace,” but for small towns and major municipalities, ABC could be the quick, cost-effective solution to making safer roads without major impacts on motorists.

Source: http://feeds.wired.com/c/35185/f/661370/s/329cd610/sc/4/l/0L0Swired0N0Cautopia0C20A130C10A0Cabc0C/story01.htm
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