Innovative Applications of Concrete and Steel

by Katherine S. Robertson

A Manhattan neighborhood that had been on the urban frontier for most residential developers is now becoming a hot spot for conversions and new luxury housing projects.

Concrete and steel are the bedrock of New York regional construction efforts, but new ways to form, apply, and maximize the attributes of these two core materials are always afoot. Here are three brief examples:

Self Consolidating Concrete

Self-consolidating concrete has been used in the greater New York area for pours in congested spaces but industry representatives are lobbying for it to get another look in high-quality architectural concrete finishes.

Flowing by gravity instead of settling through vibration, self-consolidating concrete fills in nooks and crannies, thereby eliminating virtually all air pockets and providing a tight finish, said Anthony Turchiarelli, who works in the New York office of PERI Formwork Systems, a German-based concrete form and scaffolding technology company. "It's a flowable product that gets wherever it wants to go," he said.

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A contractor doesn't have to vibrate the forms to eliminate air from the mix, said Michael Mota, New York regional structural engineer for the Portland Cement Association.

The technology has yet to gain a foothold for use in architectural concrete in the region, Turchiarelli said. "It's been used all over the world," he added. "It's only a matter of time before it takes hold here."

Steel Plate Shear Walls

The Sept. 11, 2001 terrorist attacks and the destruction of the Twin Towers in New York opened a strong debate over the construction of building cores, giving a boost to proponents of reinforced concrete, which is being used to shield cores in several high-profile projects such as 7 World Trade Center and the Freedom Tower in Manhattan. While heavier than steel, concrete began to win favor for its ability to resist seismic loading and to mitigate the impacts of seismic movements and man-made blasts.

The steel industry's response has been a push to promote steel plate shear walls, a system in use in other parts of the world for 30 years but rare in greater New York.

The system of thin steel plates or bracing "hardens" and supports core building columns while protecting exit staircases, internal ductwork, and elevator shafts, said Gary Higbee, director of industry development for the Steel Institute of New York. The plates can absorb bucking caused by a blast or seismic event on that single plane, taking such pressure away from the rest of the frame.

Higbee said the systems can provide stiffness, seismic resistance, and load-bearing capacity without the weight or bulk of reinforced concrete. A steel core is up to 20 percent lighter, potentially saving on foundation costs and avoiding the need for deep digs into a subsurface infrastructure in congested areas such as Manhattan, he added.

"You can give yourself more leeway in placing those piles," Higbee said.

He added that a reinforced concrete wall has a minimum thickness of 28 in., while steel plate shear walls are available as thin as 18 in.

"That's a major advantage in a place like New York City where space is so dear," Higbee said.

Steel Girders for Precast Planks

The Girder-Slab system aims to mix the best of concrete and steel with its patented asymmetrical interior structural steel girders or D-beams, which support precast, prestressed hollow core slabs on their bottom flanges. Intended for mid- and high-rise residential construction, the system allows for low floor-to-floor ratios and fireproofing attributes similar to concrete flat plate construction, said Daniel Fisher Sr., managing partner of Girder Slab Technologies, a company based in Cherry Hill, N.J.

Fisher said the technology offers time efficiencies for contractors, because it requires no shoring - allowing trades to begin work on the lower floors while construction goes on above them.

A core element of the system is a D-beam - an inverted tee impregnated in the profile of the plank like a spine, Fisher added. The flanges of the girder support the 8-in. or 10-in. concrete slab, which accommodates low floor-ceiling heights.

"It's a flush beam system," said Thomas Faraone, senior engineer for the American Institute for Steel Construction's Northeast regional office, who works out of Patchogue, N.Y. "You don't have beams dropping beneath the slab, which is very appealing to designers."

Girder Slab actually sells the patent on the beam and composite action, not the product itself, meaning that builders can hire their own subcontractors and maintain control over cost, construction, and quality.

The technology has not been used widely around New York. It is part of the residential component of a $93 million student housing program at the University of Medicine and Dentistry of New Jersey in Newark.