By Stacey DaBaldo
Four giant structures called straddle bents have been constructed in recent months on Dulles Airport property to support the aerial guideway that will carry future Silver Line trains into Dulles International Airport from Innovation Station.
As additional work and final inspections take place on the first four straddle bents, Capital Rail Constructors (CRC) crews have begun work on the project’s final four straddle bents; but this time, over the Dulles Greenway.
CRC is the contractor hired by the Metropolitan Washington Airports Authority to build the majority of Phase II of the Metro Silver Line, which will extend from the Wiehle Avenue-Reston East station through Dulles Airport and on to Ashburn. Once complete, anticipated in 2019, MWAA will turn Phase II extension over to the Washington Metropolitan Area Transit Authority, also known as Metro. Metro will conduct final testing and set an opening date, currently expected in 2020.
In the meantime, Loudouners can watch construction of the Greenway straddle bents, which started the last weekend of July.
What exactly is a straddle bent? It’s a massive concrete span that straddles a wide roadway giving support to the load on top. Straddle bents have become an unsung hero of sorts because the rail line will cross some roads do not have space for a support pier in the middle. The engineering in a straddle bent permits it to support unusually heavy loads over a significant span.
“The eight large straddle bents are a critical component of the Silver Line’s aerial guideway,” said Brad Williamson, CRC’s Structures Project Manager. “With an average of 410 cubic yards of concrete and 27 tons of rebar in each one, and a very technical post-tensioning process, a single straddle bent takes approximately eight weeks to build from start to finish.”
Inside the “belly” of the straddle bent, multiple tension cords allow for ever so subtle pressure and weight fluctuations due to weather and travel, such as that of a rail car.
John Kearney, construction manager of the Dulles Corridor Metrorail Project, said the aerial guideway not only helps carry the track over eastbound Greenway lanes and into the right of way, but also to permit the guideway to ‘spiral.’ At high speeds, the track must ‘spiral’, or avoid sharp turns, in order to ensure rider safety and provide an optimal ride quality for passengers onboard, he explained.
Since each straddle bent is unique, it must be poured in place rather than being precast. Lane and road closures will be necessary to ensure drivers safety and traffic will be temporarily diverted when construction over the Greenway begins.
Specific dates have not been set yet.
“Our crew is working in close coordination with the airport and Dulles Greenway staff to build these labor-intensive elements with as little disruption as possible,” Williamson said.
Project officials report that any major closures will be done during the night and should not have any effect on rush hour commutes. To stay informed on upcoming closures, sign up for project-specific traffic alerts at www.dullesmetro.com.
For concerns and emergencies regarding construction, call the project hotline toll free at 1-844-385-RAIL (7245).
Stacey DaBaldo is an intern with the Dulles Corridor Metrorail Project and the Metropolitan Washington Airports Authority.