Lancaster, Pa. — The Precast/Prestressed Concrete Institute (PCI) recently announced the winners of their 2018 Design Awards. The program, in its 55th year, recognized 21 projects with awards of excellence in design. More than 100 project entries from across the U.S. were submitted for award consideration.
The Aerial Guideway project for the Dulles Corridor Metrorail received the 2018 PCI Design Award for Transportation: Best Non-Highway Bridge. Coastal Precast Systems of Chesapeake, Va., a PCI Mid-Atlantic member, was the precast concrete producer for the project.
The project owner is the Metropolitan Washington Airports Authority (MWAA). When completed, the Dulles Corridor Metrorail Phase 2 will (to the east) connect Washington Dulles International Airport to Washington, D.C., and Loudoun County, Va. (to the west) with numerous stops along the way. The project includes a station at Dulles, and an aerial guideway that will carry passengers above the busy airport.
The aerial guideway required a thoughtful design strategy, says Gregory Shafer, bridge technical manager for Parsons, in Baltimore. In addition to filling a gap in the transportation network, the guideway needed to complement the landscape of a historically significant transportation hub. It also had to be delivered at a reasonable price.
The design-build project had a $1.2 billion bid value, with low bids separated by just 1.2 percent, making it very cost-competitive, Parsons said.
Several structure types, including steel box girders, were considered for the aerial guideway structure. Precast concrete girders, supplied by Coastal Precast Systems, were selected for several reasons.
Precast concrete offered a high-durability and low-maintenance solution, which was critical to meeting the 100-year service life requirements. The use of precast concrete also minimized impacts on nearby roads and wetlands and delivered a high-quality product at a lower cost relative to the alternatives.
According to Parsons, the fully prestressed, factory-produced precast concrete elements met serviceability requirements and fit well within the historic airport architecture. The ability to quickly mobilize and erect the precast concrete girders was another advantage of precast concrete, as it minimized impacts on airport traffic and operations.
Overall, the Dulles Rail Project expansion adds 11.4 route miles to the Metrorail system, including the 6 track miles of aerial guideway that passes through the airport, over wetlands to a maintenance yard, and connects to the western section of at-grade rail.
Elements of the bridge required tight 800-foot-radius curves, along with higher live loads and overall requirements for rider comfort.
The use of prestressed concrete girders in a chorded configuration worked well with the spans of up to 150 feet on the curved alignment. The girders incorporated draped prestressing strands with flared spacing at the ends. Florida wide-flange I-beams (or FIB) sections with four different depths were used to minimize the structure depth and deliver an efficient structure.
“It was the first use of these FIB shape girders in the region,” Shafer says.
The flexibility of these precast concrete elements also accommodated various challenging elements, including track cross-overs and turnouts, as well as features for power and control systems.
The ability to adjust the girder depth also allowed the designers to “tune” the structure to meet the vibration requirements for rider comfort. To further minimize costs and expedite construction, the design used a pair of precast, prestressed concrete girders to support each track. The design of the girders considered redundancy, strength, and serviceability.
Additional details and photos for this award-winning project are available at https://www.pci.org/PCI/Project_Resources/Project_Profile/Project_Profile_Details.aspx?ID=214956.
- Owner: Metropolitan Washington Airports Authority – MWAA (Washington, D.C.)
- Precast Concrete Producer: Coastal Precast Systems (Chesapeake, Va.)
- Engineer of Record: Parsons (Baltimore)
- General Contractor: Capital Rail Constructors, a Joint Venture of Clark Construction Group, LLC and Kiewitt Infrastructure South Co. (Herndon, Va.)
- Project cost: $1.18 billion
- Bridge size: 37,831 feet.