The tower's footprint is a 93.5-ft square, an efficient size for residential were it not for the height. Because of the height, structure and services eat up a great deal of livable space, says Gloria Glas, a partner of executive architect SLCE Architects LLP.
That reality pressured the designers to maximize saleable floor space. For example, the architect introduced a double-switchback core stairway because it takes up less room than a single switchback. The move freed up 200 sq ft—enough for another bathroom.
The switch also increased floor-to-floor heights to 15.5 ft from 12 ft. "It's a luxury condominium, so the developer can sell the extra floor space to compensate for the cost of the extra height," says Herr, adding that a bonus of the double switchback is 12.5-ft ceilings.
For building services, the strategy was to break the tower into five stacked, 12-story zones, each served by a two-story mechanical level. "We pull in fresh air at the mechanical levels, instead of at the street," says Gary Pomerantz, executive vice president for WSP USA Building Systems. "That reduced riser size."
WSP also devised a system to reduce the size of the electrical conduits. A basement transformer boosts to 5,000 volts the utility's incoming 460-volt service. Then, at the mechanical floors, the electricity is reduced to 208 volts. "This reduced a 100-square-foot riser to 10 square feet and cut the amount of copper wire by 90%," says Pomerantz.
The structural engineer also minimized member sizes by using up to 14,000-psi concrete and up to 97-ksi, large-dia rebar. The building has 2,915 tons of 97-ksi vertical rebar, which is the most extensive use in a high-rise anywhere, says Silvian Marcus, director of WSP USA's Building Structures unit.
The 2.5-in.-dia, 97-ksi rebar brought its own challenges. For example, it required a template for accuracy in positioning. And at large diameters, the code does not permit splicing, only mechanical connections; the threaded 97-ksi bars are connected by sleeves.
In terms of space savers, Marcus says the exposed structure and consequent absence of cladding added 8 or 9 in. to the usable floor plate and eliminated a cost. But to achieve the specified 44-in.-wide columns and 44-in.-deep spandrel beams, WSP had to increase depths. At lower floors, columns are as deep as 6.5 ft.
The primary concrete structure is a square tube—the central core—within the expressed perimeter tube, which has seven columns on a side, 16 ft on center. Flat-plate floor slabs are typically 10 in. thick.
The foundations consist of spread footings, with tie-down rods into rock to resist overturning. Only the core sits on a mat.
The skinny tower's ratio of width to height, called the slenderness or aspect ratio, is nearly 1:15. "The New York City code considers anything in excess of 1:7 [as being] slender," says Marcus.
Wind-tunnel tests at RWDI revealed unacceptable dynamic motions and acceleration when the engineer followed the building code's standard stress requirements (see p. 26). "Initially, it was quite a lively building," says Derek Kelly, RWDI's project manager.