In September 2003, federal, state, and local officials broke ground for the new U.S. Census Bureau Headquarters in Suitland, Maryland. The new 1.5 million-square-foot complex, a $331 million investment, will feature two eight-story, curvilinear office buildings, connected by a large glass atrium that makes them appear to be one continuous building. The combination of these two sleek buildings will house 6000 Census employees who previously had been located among several buildings in Suitland and surrounding areas.
Composite metal deck construction was chosen for the elevated floors in the complex. This type of decking takes advantage of the high tensile strength of steel and the high compressive strength of concrete to handle gravitational and lateral loads in the deck during construction and in service. This is especially important because a composite metal deck is usually put into service a day or two after the concrete has been placed. Some of the heaviest loads these floors will ever encounter will be during the construction phase—quantities of dry wall, a pallet of CMUs, or innocent-appearing but potentially damaging scissor-lifts and industrial lift trucks. Due to these loads and other stresses, cracks can form so it is important to contain or reduce these cracks.
Because of the risk of these superimposed loads to the Census Bureau project and the fact that close to 1 million square feet of concrete was to be used in the decking, it was critical to understand the stresses that this decking system would be subject to and the potential for cracking. The cracking can occur because of two reasons: flexing of the metal deck or restraint of movement of the concrete. The normal method used to control cracks in concrete over metal deck assemblies is shrinkage and temperature reinforcement so as to distribute cracks uniformly and hold them tightly closed.
The Steel Deck Institute (SDI) recognizes traditional steel such as welded wire fabric for use as shrinkage and temperature reinforcement. In May of 2003, SDI expanded this to include the use of steel fibers meeting a performance requirement of at least 80 psi average residual strength when tested in accordance with ASTM C 1399.
Armed with the SDI recognition, UL approval, and knowledge of the added performance benefits, the contractor, with the approval of the structural engineer, decided to use Novomesh 850, an engineered blend of steel and polypropylene fibers manufactured by SI Concrete Systems. As with traditional construction practice, supplementary steel reinforcement was used to provide continuity over areas of negative moments. The synthetic fibers worked to reduce the formation of plastic shrinkage and settlement cracks, allowing the concrete to develop to its optimum long-term integrity. The steel fibers reduce drying shrinkage cracking that may occur over time, resulting in redistribution of stresses to larger areas of the concrete and more tightly held cracks. And because the fibers were distributed evenly throughout the concrete, this multi-dimensional secondary reinforcement is ensured to always be positioned correctly, providing 100% positive placement.
While performance is key in composite metal deck construction, the Novomesh 850 fibers minimized other challenges that the contractor faced while working on the elevated floors of the Census Bureau project—worker safety and timeline. Fibers eliminated the hazards inherent in cutting and placing reinforcement as well as tripping hazards which are a major concern in elevated construction. Novomesh 850 was simply added directly to the concrete mix at the onsite Superior Concrete batch plant requiring no extra installation. Nothing had to be hoisted or lifted; the fibers were simply pumped directly onto the decking as a component of the concrete mix and finished with standard techniques. And because there was no extra installation, Skanska was able to cut several days from its placement schedule, speeding up the overall construction timeline of the Census Bureau project.
The first phase of the Census Bureau project will be completed in Spring 2006, with employees scheduled to move into the first building by September 2006. Phase two of construction will include an additional 729,722 square feet and have 1506 parking spaces. Occupation of the second portion is set for March 2007.