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Metro's Silver Line bridge project casts a big shadow in Tysons Corner

Trusses are building the aboveground tracks for Metro's Silver Line.
Trusses are building the aboveground tracks for Metro's Silver Line. (Jahi Chikwendiu/the Washington Post)
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Tuesday, August 3, 2010

If your parents drive you to the malls in Tysons Corner this summer, you'll see a lot of tall cranes, backhoes and orange cones and barrels. The chaos is from construction of a new Metro line to Dulles International Airport.

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Workers are gearing up to tackle the biggest high-wire act of the six-year project: A new bridge that will carry Silver Line trains 30 feet above the Capital Beltway on t heir way to the airport. At its highest point, the bridge will rise 65 feet above the sidewalks of Tysons.

The Beltway bridge is also known as an aerial guideway, or flyover, since it will swoop over the concrete of highways. Making things even more complex and dangerous, construction is already going on under the Beltway right at this spot -- on Route 123. Ironworkers and pile drivers (machines that pound support beams called piles into the ground) are busy building giant supports called piers along Route 123 to support a separate project that is widening the Beltway for carpool lanes.

Engineers had to figure out a way to build the flyover for the trains over the Beltway and Route 123 while 12 lanes of traffic are moving underneath. That's because closing down the Beltway at Tysons wasn't an option. The technique the engineers chose is called sequential assembly. It means the flyover will be built in small pieces rather than by laying big slabs of concrete between the piers. The pieces will hold up the subway tracks.

First, workers are building a massive overhead crane called a truss at the edge of the Beltway. Then they will hoist hundreds of 25-ton blocks of concrete from a truck and suspend them from the crane, one by one.

The blocks are being made by pouring concrete into special molds at Dulles Airport. The segments come out looking the same, but each one is different, designed for a particular spot on the bridge.

The segments will be held together by steel cables called tendons. They're locked in place and glued with a super-strong epoxy. After that, the remote-controlled truss moves on to the next part.

The truss looks like a giant yellow Lego block on stilts. Three of these custom-built cranes will be needed to build the first 11.5 miles of the rail line, which will go out to Reston by 2013. In all, six miles of track (three inbound and three outbound) will sit aboveground. There will also be elevated sections along the second half of the rail line, from Reston to the airport. When the entire Silver Line is completed in 2016, you will be able to take a train from Washington or Maryland all the way out to Dulles Airport. How old will you be then?

The truss should be finished this fall. But Metro has been showing it off this summer at community fairs in Fairfax County, using blown-up photographs on a big board.

Marcia McAllister, project spokeswoman, says kids are interested in the work. "They can't wait until the truss starts to move down the road," McAllister said. "They will have a lot more to see very soon."

-- Lisa Rein


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