Experts report that about 80 million Americans go on a fitness plan each year. These people know that reducing weight while strengthening their muscles will improve the way their bodies function.
The same can be said for pre-cast concrete, specifically the double-tees used in parking structures. If producers can cast tees thinner, and thereby lighter, without losing any strength, architects can immediately see significant benefits. These elements would use less concrete. They would weigh less, which could reduce the substructure needs. And erectors could use smaller cranes for installation. All of these factors would reduce costs without diminishing a parking structure's performance.
But the ability to create thin double-tee flanges has been elusive. The main design constraint has historically been the thickness of cover required to protect the steel mesh reinforcement in the flange from water penetration and corrosion.
But the future is now. Recent studies have reported that the innovative use of carbon fiber grid as a replacement for steel mesh reinforcement can enable engineers to reduce the required cover in the flange without sacrificing a double-tee's performance.
Simply put, double-tees are being put on a much-needed diet, with very appetizing results.
Reducing a double-tee's weight contributes to a variety of benefits throughout the structure. “Thinning the flange is where it's at,” says Harry Gleich, vice president of engineering at Metromont, a major pre-caster in the Southeast. “When you take out ¾ inch of concrete in the flange, you're reducing the tee's weight by 10 pounds per square foot, which reduces loads on the beams, columns, and foundations.”
That's critical because the structure's overall dead load is used to calculate seismic forces. “In general, the heavier the dead load, the greater the seismic force,” says Ken Baur, director of research and development, High Concrete Group LLP, a wide-width producer of precast parking garages.
Within the last decade, the International Building Code widened its seismic requirements to include most parts of the country. To comply, engineers must design their structures for an earthquake during a 100-year horizon, says Baur. Lighter tees allow engineers to meet the new seismic codes while reducing the superstructure.
Producers also share in the benefits. Lighter double-tees also could require less reinforcement in the stem to support the lighter load, which can sometimes save strands. Lighter tees require less fuel for transportation to the jobsite. And smaller cranes can be used to lift the element into place.
The mighty enabler
In conventional double-tees, flanges are generally at least 4 inches thick. American Concrete Institute requirements force engineers to incorporate at least 2 inches of concrete cover into the flange to protect the steel mesh from water and corrosive elements.