WSAAF - Barracks Expansion
Four barracks were constructed using precast concrete hollowcore slabs at Fort Drum, near Watertown, New York, housing the Army’s 10th Mountain Division Aviation Brigade.
"The key to minimizing progressive collapse is to create a cantilevered design that does not rely on the outer wall for support", says David Wan, chief engineer for Oldcastle Building Systems of South Bethlehem, New York, which supplied the components.
The first bay of precast concrete slabs was designed as a 10’-8” backspan with a 10-ft-long cantilever rather than as a 20’- 8” simple span. In the event of a blast in which the end wall (which normally supports the slabs) is destroyed, the precast slabs will cantilever 10 ft from the interior bearing wall and not collapse.
The 8-in.-thick precast concrete hollowcore slabs will continue to be supported by an interior wall 10 ft inside the outer wall. The 10-ft overhang will have almost 3 in. of deflection, Wan says. “But the goal here is prevention of progressive collapse for the safety of individuals, not later serviceability.” To protect against an interior blast, “the use of a relatively heavy and dense material like hollowcore slabs naturally offers good protection against a reversed vertical load, such as with an upward explosion.” The slabs are prestressed, adding four 1/2-in. top strands in addition to the standard six 1/2-in. bottom strands. “The new Department of Defense design requirements also require that the floor system be designed for potential load reversal or an upward load caused by an internal blast,” Wan adds.
Ned M. Cleland, president of Blue Ridge Design Inc. in Winchester, Virginia, noted that, “In the early development of multistory buildings with large panels, the primary loads that were considered in the design process were gravity and wind. In today’s world the focus is on explosive bombings. Loading problems related to terrorist attacks and bombings have been conceived generally as a vulnerability to progressive collapse.”
Economy, Speed Cited
Precast concrete wins plaudits from the contractor on the first two of the four barracks to be constructed at Fort Drum, each of which used about 81,000 sq ft. “For economy and speed of construction, and structural requirements, precast was the natural choice for this sort of building,” says Steve Maslen, project executive for Clark Construction Group in Bethesda, Maryland.
“Previously constructed barracks at Fort Drum were all two stories, extending from a hub with two long wings, and they weren’t subject to blast rules,” Maslen explains. “But once the design added a third story, as we did with these new barracks, the force-protection rules applied, and we went to the cantilever method.”
“The design team on this project showed that precast, prestressed concrete hollowcore slabs can be successfully integrated with other building materials, such as steel and masonry, to meet new DoD anti-terrorism requirements,” says Wan. “The use of a precast-plank floor system also met tough budget limits and a fast-track construction schedule.” The masonry and precast work were completed in January and February of 2006 in upstate New York, where the winter can be harsh on construction activity, he notes.