The Route 52 bridge over the Wallkill River in the village of Walden, NY, is not your typical bridge. Instead of being constructed with steel girders lying flat across the river, it boasts a concrete arch design dating back to Roman times.
Arch bridges — one of the oldest types of bridges — possess great natural strength. Instead of pushing straight down, the bridge’s weight is carried outward along the curve of the arch to the supports at each end. These supports, or abutments, carry the load and keep the ends of the bridge from spreading out.
Constructing an arch bridge can be tricky, because the structure is completely unstable until the two spans meet in the middle. One technique is to build elaborate scaffolding or “centering” before the spans to support them until they meet. A newer method supports the spans using cables anchored to the ground on either side of the bridge. In situations where there is no active roadway or water below, this method allows contractors to work without disrupting traffic.
Why an Arch?
“Aesthetically it will be a nicer looking bridge. The arch design is a perfect fit for this location and it’s appealing to the eye,” explained Aaron Pethic, project engineer of Cianbro Corp., of Pittsfield, ME, the project’s contractor.
“Steel girders could have been used, but it would have been a long span for steel. By no means is this arch long. It is only 200 feet but any time you try to place a couple of pieces of steel together … you bring in a whole different element. Typically you would want a center pier in the river, which wasn’t called for with this project. We aren’t putting anything in the river, which may be another advantage to using this design,” Pethic continued.
Out With the Old
To avoid disrupting the approximately 11,000 vehicles that travel Route 52 daily, traffic will be detoured to the nearby Oak Street bridge. Before doing so, however, “the profile of the road coming back out to Route 52 had to be improved,” reported James Corwin, engineer-in-charge of New York State’s Department of Transportation (NYSDOT). “We built the road up about 18 inches to make the grade less steep. The cross slopes also were changed because it was steep there, too. Houses were on that road so a retaining wall and new sidewalks had to be built.”
Demolishing the existing truss bridge will take several weeks. First the deck will be removed by saw cutting it over the top of the existing steel. Next the deck slabs will be taken away using an excavator and Pac-man bucket. “The bucket has a space notched in its sides so when a piece of deck is lifted up you can grab it. The bucket raises it up, allowing you to swing it rather than picking it up with the teeth,” explained Pethic.
Following the demolition, Cianbro will create new foundations. “That will involve performing rock excavation where the new thrust blocks will go. They will be placed where the arch of the bridge ties into the existing rock,” Pethic reported. “We will carve out the rock and make a pocket, then pour the concrete with a rebar and establish the new foundation.”
An HP concrete containing a corrosive inhibitor will be used. “Although the corrosive inhibitor is costly, if it isn’t used the life of the concrete will decrease,” said Corwin. “Eventually salt gets into the concrete, which is usually what causes the bars to rust.”
Challenges Abound
One challenge this $6.5-million project presents is moving the existing water and sewer lines. Cianbro is meeting that challenge in a rather unique way.
“We are using a No. 7 Manitowoc boom crane,” explained Pethic. “After foundations are established on either side of the river several steel frame columns will be erected. Then, resting the lattice boom on the columns, we will span the river with it. The water and sewer lines will be transferred by strapping the lines to the boom, which then will be used as a bridge. We had the necessary equipment on our lot so we decided to put it to good use. Doing it this way will save time, money and labor. It also will keep us out of the river and the more we stay out of the river the better.”
Abutments also are a challenge. “The abutments that sit at the end of the bridge are tall and go down into rock, making them difficult to access. If we encounter bad rock — rock that is soft and peels away easily — we won’t be able to pour a foundation,” said Pethic. “Ideally, we want bedrock. It is tough and hard. If the rock isn’t hard we will have to go deeper or drill rock anchors for stabilization.”
Another challenge involves working on two sides of the river. “Both sides must be inline with each other. The same concrete block has to be built for sides A and B. They must be perfectly opposite one another, not skewed either way. When you stand on one of the two blocks and stare across the river you should see a mirror image. To assure proper alignment a surveyor will place grade-set stakes or pins into the ground indicating the limits of the excavation,” Pethic added.
Anticipated completion date is December 2004.
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