William B. Hurd Transit Facility (DASH)
(52,400 sq. ft. precast walls, 121,400 sq. ft. double tees)
Structural Precast Elements:
• Precast Double Tees
• Precast Girders
• Precast Slabs
• Rooftop parking for 250 cars.
• 52,400 square feet of precast concrete walls.
• 121,400 square feet of precast concrete double tees.
• Gold LEED certification.
• R-14 insulation provided via 3 inches of expanded polystyrene (EPS) rigid foam between two wythes of concrete.
• Total-precast concrete structural system provided sufficient support that separate parking structure was unnecessary, saving time, money, space and material.
• Required 50-year service life was achieved with precast concrete’s mass and durability.
• Fabricating components under factory-controlled conditions improved quality assurance, speeded up construction, and minimized site congestion.
• R-14 value achieved with insulated precast panels, eliminating steps to insulate once façade was in place.
• Fast construction of shell allowed interior trades inside to run utilities before building was completely dried in.
• Support for 15-foot-wide double tees used a combination pocket/corbel in the load bearing CarbonCast walls. The pockets penetrated the 4-inch back wythe without disturbing the insulation layer. Corbels extended from 7” to 8” depending on the design.
• Precast concrete helped achieve Gold LEED certification by providing energy efficiency, using local materials for manufacturing,
fabricating components near the site, minimizing construction waste, and other attributes.
Bus Facility with CarbonCast Enclosure Rides to LEED Gold
Energy efficiency was a key driver the in the selection of CarbonCast High Performance Insulated Wall Panels for the new William B. Hurd Transit Facility in Alexandria, Va. The heated garage was to provide maintenance as well as storage space for 96 DASH buses, and thermally efficient walls were a clear requirement. But when the design team determined that the fully structurally composite panels would support a level of parking, they were delighted to get on board with a CarbonCast solution from High Concrete Group.
The City of Alexandria anticipated the challenges of fitting the operation into the tight, urban brownfield site. The authority’s design-build RFP project included a number of mandatory programmatic conditions, including LEED Silver Certification, and suggested a separate precast parking structure for employee cars. Winning architect Michael Baker Jr., Inc. and general contractor Hensel Phelps Construction Company were able to recycle more in construction to get the additional points needed for LEED Gold Certification.
“We considered a steel structured building for the bus storage area,” says project architect Peter Schmidt, “but in order to provide rooftop parking, precast provided the ideal solution.” He emphasizes the City’s tight timeframe for the project. “Because of the limited maximum duration in the RFP, this was a fast track project.” The site was prepared while High Concrete Group was fabricating the precast in the factory. “We would still be building it if we had used CMU,” he says.
By integrating the parking with the roof of the bus facility in place of a separate structure, the design-build team was able to save significantly on the already-tight budget and bring the project in ahead of schedule. The strategy also allowed room for 36 additional buses in a future expansion, mandated in the RFP but for which it didn’t allow space.
Building Details — Fast Track Schedule Rolls Out a Durable Design
Durability was a primary concern of the design-build team. ‘The RFP called for a 50-year longevity for this building,” says Schmidt. “That was another key driver in our decision to recommend a precast solution.” He notes that precast assured a high level of quality in materials for long-term durability. “And we didn’t have to sacrifice schedule. High Concrete Group began producing the structural building elements while the site work and foundations were going in, which facilitated speed of construction.”
Phase I of the project consists of a fully precast concrete structure including a one-story bus maintenance area and adjacent two-story wing. “Precast saved us time,” says Schmidt. “We were able to run the utilities before the building was completely dried in.” There are ten maintenance shops on grade, with cassette-based in-ground lifts in all service and PM bays. Administrative offices and operations spaces are located over the maintenance area on the second floor immediately adjacent to the rooftop employee parking. The second floor received a steel truss roof, the only structural steel used in the building.
Support for the 15’-wide double tees was accomplished using a combination pocket/corbel in the load bearing CarbonCast walls. The pockets penetrated the 4” back wythe without disturbing the insulation layer. Corbels extended from 7” to 8” depending on the design.
Insulation value of R-14 minimum was accomplished with 3” of expanded polystyrene (EPS) rigid foam; panels containing 5” of EPS foam were supplied for selected areas. The foam is sandwiched between two precast concrete wythes, meeting the requirements for edge-to-edge continuous insulation (“ci”) defined by the ASHRAE 90.1 energy code. The concrete wythes are connected with C-GRID carbon fiber shear trusses, which provide a fully structurally composite structure and thermal transfer that is virtually nil.
Accessed via a two-way ramp from Roth St., the roof has 260 parking spaces including 100 spaces for other nearby City agencies. The deck is composed of 15’-wide double tees with a 4” pretopped flange and 26” legs; 3” flanges were supplied for double tees that were to be field-topped. Direct access to the bus parking on the lower level is provided from the operator’s room. Visitor parking is on street level outside the administrative entrance. A total of 98 on-grade spaces are shared between the visitors’ lot and limited access service parking.
Located in downtown Alexandria next to the rail line, the site is fully utilized. It was formerly a rail yard, and had to be remediated prior to construction for known hazardous materials and stormwater management concerns. These were among the salient design features that led to the facility achieving LEED Gold. Further significant LEED contributions were in the areas heat island reduction, proximity to mass transportation, alternative transportation options, energy efficient building envelope and mechanical systems, including heat recovery, exemplary water use reduction, and daylight and views.
Aesthetics — Art Deco Flair Softens Façade
The CarbonCast panels were cast in a standard buff color in a medium sandblast finish. A light sandblast was used in the office area where a bold fluted vertical detail was cast at the panel edges. Topped with a rectangular accent, the detail helps disguise the caulk joints and evokes an art deco flair that helps integrate the building with its neighbors, several of which date from the mid- to late-1800s.
Reveals in the panels on the two-story section help to break up the mass of the building and create subtle shadow lines. The reveals follow the lines of the windows on the second floor, and help frame the windows within the panel on the first floor.
The panels were stained in the factory to match the DASH livery. A band of yellow surrounds the building providing easy identification. A wide, blue field tops the yellow band on the two-story section The panels have punch windows with LOE glass in painted blue frames; entry and bus doors are also blue. An aluminum canopy greets visitors at the main entrance, and a DASH illuminated logo sign is mounted on the top of the façade at the corner.