One World Trade Centers Crews Get Royal Treatment
Tishman tracks everything, down to each piece of steel, without the help of a construction building information model because, when the project started, BIM was not very advanced. But DCM used a steel BIM to determine tonnage. The project will contain 48,000 tons of structural steel.
Other quantities are equally mind-blowing. The 215,000 cu yd of concrete would fill a string of concrete mixers stretching from New York City to Providence, R.I. The nearly 47,000 tons of rebar, combined with structural steel, adds up to 13 Eiffel Towers.
Memory is a theme of 1 WTC, which was designed by the local office of architect Skidmore, Owings & Merrill in an early collaboration with the local office of Studio Daniel Libeskind. The tower's mechanical podium has a 205-ft-square footprint to recall those of the original 110-story twin towers. The upper 6 ft of the 33-ft-tall roof parapet is 1,368 ft above grade to mark the height of the original 1 WTC. The bottom of the parapet is 1,362 ft above grade to mark the height of 2 WTC. The stainless-steel-clad corners of the new, 104-story tower are chamfered like the twin towers.
Unlike the originals, the new tower tapers and “twists.” The twist is created by “rotating” each floor plate slightly, for a total of 45 degrees. Floors shrink one foot per story, creating a perfect octagon in plan at midheight. The roof is nearly a 150-ft square, with clipped corners. The first office floor is 42,000 sq ft; the 90th is 25,000 sq ft. “We wouldn't, in good conscience, carry the World Trade Center 'square' up” because too much of the 42,000 sq ft is taken up by the core at great heights, says Kenneth A. Lewis, SOM's project manager.
The tower is likely one of the world's most terrorist-resistant. “We can't talk about the security aspect too much, [but] we can say that the building will behave equal to or better than U.S. embassies,” says Ahmad Rahimian, the principal in charge for the project's local structural engineer, WSP Cantor Seinuk (WSPCS).
According to SOM, the life-safety systems encased in the core's 3-ft-thick walls will exceed local code requirements. There is structural redundancy. Cementitious fireproofing is dense and highly adhesive. The air supply system has biological and chemical filters. The core has extra-wide, pressurized stairwells. There are multiple backup systems for emergency lighting and concrete protection for fire sprinklers. Emergency risers are provided. There are interconnected, redundant exits, additional stair exit locations at all adjacent streets and direct exits from tower stairs to the street. The tower also has a dedicated stair for use by firefighters and an elevator to evacuate people with disabilities.
Even the curtain wall is hardened. During blast tests, the mock-up wall moved only 3.5 in. Only two lights of glass broke, says Lewis.
WSPCS selected a hybrid structure to reap the benefits of both steel and concrete. The moment frame's perimeter tube is connected to a concrete spine of shear walls through a system of steel beams rigidly connected via shear studs to composite floor diaphragms of the concrete-on-metal deck. Foundations, 20 ft into rock, consist of spread footings under columns and strip footings under the core. Outriggers near the roof will connect the tube and the core.
The core, up to 110 ft square in plan, is as “big as many residential buildings,” says Rahimian. The core is stiffer than the tube, so it resists most of the wind and seismic load.
The taper and twist break up the wind, which minimizes the negative sail effect. The building is designed for a 1,000-year hurricane event.
The 441-ft-tall spire's tapered mast will consist of stacked steel rings made from solid-plate or lattice steel, joined by cast-steel elements. The mast is designed to support antennas and satellite dishes, but due to changes in the broadcast industry, there are no tenants to date. However, the system meets broadcast-industry criteria, including limits on lateral movement. Near the tower's tip, deflection is kept to 0.5 degrees under a sustained 50-mph wind, says Christian Rieser, an associate in the local office of spire structural engineer, Schlaich Bergermann.
Made from a synthetic fiber that does not interfere with broadcast signals, guy cables will anchor the mast to the roof slab. A radio-frequency “transparent” material will cover it. Fins on the cover will mitigate vortex shedding, which creates whirlpool wind eddies that induce movement, which can cause metal fatigue.
The skyscraper's core contains up to 14,000-psi concrete. Previously, the highest strength used in New York City was 12,000 psi. The pumped concrete mix, which had to meet the engineer's strength and stiffness requirements, presented flow challenges. The mix selected replaces 50% to 75% of the cement with slag, which decreases the heat of hydration produced as the concrete cures. Temperature requirements for the core walls are more typical of mass concrete for dams, says Juan Estevez, a Tishman project manager.
Collavino, which has a $237-million contract for above-grade concrete, is using a climbing-form system, Crews must follow guidelines to handle differential shortening between steel and concrete during construction. To keep slabs level, each floor is installed at a slightly higher elevation. Early core work “was tough, but it's gone remarkably well,” says Estevez.