Skidmore, Owings & Merrill
New York, New York
A controversial tower rises at ground zero
By Joann Gonchar, AIA
There is no denying that One World Trade Center (WTC), the 104-story
tower now rising at the northern end of the Ground Zero site, is a
tremendously ambitious commercial real estate venture. The building,
owned by the Port Authority of New York and New Jersey with the
developer Durst Organization holding a 10 percent stake, will contain
3.1 million square feet of office space when completed in late 2013.
Below grade, connected to the WTC site’s vast underground transportation
infrastructure, there will be 55,000 square feet of retail, and near
the top, the tower will include a two-level observation deck and a
restaurant. But when the designers of the $3.19 billion project describe
the building, they generally focus first on its potential as a symbol:
“It will serve as the marker of the 9/11 memorial on the skyline,” says
David Childs, consulting design partner to Skidmore, Owings &
Merrill (SOM).
This hybrid of a civic emblem and a speculative office building,
which now has framing approaching the 80th floor, will have a stainless
steel capped glass parapet whose edges mark 1,362 feet and 1,368 feet.
The elevations are the heights of the original Yamasaki &
Associates–designed Twin Towers completed in 1972. With the help of a
spire that extends 408 feet above the parapet, One WTC will reach 1,776 feet — a height set in Studio Daniel Libeskind’s
2002 Ground Zero master plan, making it the tallest building in the Western Hemisphere.
The building should be immediately identifiable, and
not only because of its height: The scheme has a simple, iconic form,
much like that of the Washington Monument, says Childs. It includes an
84-story shaft rising from a base that, in plan, measures 187 feet tall
and 200 feet wide. Above this almost cubic pedestal, which contains
mechanical floors stacked on top of a 50-foot-tall lobby, One WTC’s
corners are chamfered back, creating progressively smaller floor plates
that gradually shift from a square above the podium to an octagon at
midsection. At the top, it is once again square, but 150 feet on a side
and rotated 45 degrees from the base.
Although the “twisting” of the floor plates produces a
tapering tower with a complex geometry, it is a geometry that is much
more straightforward than that of an earlier version designed by SOM in
collaboration with Libeskind. Then referred to with the emotionally
charged name the “Freedom Tower,” the building had a torqued cable-net
exterior and an off-center spire intended to evoke the torch of the
Statue of Liberty. This proposal was scuttled in 2005 and the site
shifted, after New York City police objected to its proximity to the
busy roadway at the western edge of Ground Zero. Both the abandoned
scheme and the tower under construction were commissioned by developer
Larry Silverstein, who ceded control of the project to the Port
Authority in April 2006 (see
time line).
Despite One WTC’s adjusted location, some still regard
it as an obvious terrorist target. However, its designers point out that
it will have enhanced security and life-safety systems, including a
robust, reinforced concrete shear-wall core surrounded by a steel moment
frame spanning 45 feet to perimeter columns. The building is part of a
post–September 11 trend for New York City office towers to move away
from all-steel structures toward composite systems. And although
concrete shear-wall construction is just one way to design and harden a
core, in the case of One WTC, the approach was deemed the most
appropriate due to the tower’s height and slenderness, according to
Ahmad Rahimian, CEO of WSP Cantor Seinuk, the project’s structural
engineer. “For tall and supertall buildings, a reinforced concrete core
provides excellent resistance to wind, earthquake, and gravitational
loads by virtue of its mass, strength, stiffness, and fire-protection
properties,” he says.
If the tower, with its composite structure, were under
construction in another locale, pouring of the concrete core would
likely have advanced ahead of the surrounding moment frame’s erection.
But the sequence at One WTC is reversed, with steel work leading core
construction by about 10 stories on any given day due to “union
jurisdictional issues” peculiar to New York City, says Mel Ruffini,
executive vice president at Tishman Construction. The company, part of
AECOM, is the project’s construction manager.
Pouring of the slabs on the floors’ metal deck lags
about eight levels behind the top of steel erection, with other trades
following core forming, including application of cementitious
fireproofing and curtain-wall installation.
When designing the structure, engineers took this
sequence into account, incorporating a steel ring beam into the core.
Although the surrounding steel superstructure ties into this beam, its
primary purpose is to serve as anchorage for erection steel and for the
self-climbing concrete formwork that contractors “jump” within the core
void using hydraulic jacks. “We needed to consider both temporary and
permanent conditions,” says Rahimian.
Surrounding the moment frame, cladding operations are
well under way, with installation of the curtain wall’s low-iron
insulated glazing units (IGUs) already extending from the top of the
cubic podium past the 50th floor. The IGUs are 5 feet wide and 13 feet 4
inches tall — the largest production IGUs available. They have thicker
than typical outer lites (3⁄8-inch thick versus the more standard 1⁄4
inches) and laminated inner lites whose thickness varies depending on
location (some are thicker than others due to security concerns).
Although the IGUs are heavier than standard units, which complicates
installation, the panels’ size allows them to span the full
floor-to-floor height without intermediate mullions or spandrels. The
thickness of the outer lites, along with inner lites’ lamination, should
also prevent “oil canning,” or pillowing of the glass panels, says SOM
managing director Kenneth Lewis. The goal is to create a “uniform and
crystalline” surface — or one that appears to be “shaved and carved,”
adds Childs, again alluding to the form of the Washington Monument.
At the ground, each side of the building will have a
cable-net-supported glazed portal, 60 feet tall, although the lobby will
be otherwise surrounded in concrete. The architects had planned to
camouflage the podium with tempered and laminated prismatic glass panels
that would reflect, refract, and transmit light, in order to “establish
a relationship with the water cascading in the memorial pools,” says
Childs. However, after discovering that tempering caused the panels to
bow, interfering with the laminating process, the project team devised
another solution. The alternate, which sources describe as glass fins
projecting from the facade at various angles, is now in the bidding
phase but has yet to be finalized.
Many of the life-safety strategies being implemented at
One WTC were pioneered at 7 WTC — the 52-story office building that
sits just beyond the edge of the Ground Zero site, catercorner to One
WTC. Completed in 2006, by an almost identical design and construction
team — including SOM, WSP Cantor Seinuk, and Tishman — it has features
that exceed the requirements of the New York City Building Code, which
are also included in the newer tower. For example, air intakes are
located well above grade, which make it difficult to introduce
biological or chemical contaminants into the ventilation system. Both
towers also have widened egress stairwells for emergency evacuation with
doors positioned so that anyone entering easily merges with the flow of
exiting people. In addition, the stair landings have designated “areas
of refuge” where occupants who are unable to descend on their own can
wait for assistance.
The tower’s life-safety features new to New York City
skyscraper design include a stair for use only by first responders and a
“fireman’s lift” — an elevator used as a service elevator during the
building’s day-to-day operations but equipped with water-resistant
controls and a second door that opens onto a pressurized, dedicated
fireman’s lobby. The other service elevators have water-resistant
controls as well, and are contained within a smoke-resistant enclosure,
allowing first responders to press them into service if needed.
Tenants have begun to make commitments for space in the
building. In May, the Port Authority finalized a deal with magazine
publisher Condé Nast for one million square feet on 21 floors. (Last
year, the Chinese real estate company Vantone signed a lease for 200,000
square feet on floors 64 though 69.) It’s not clear whether the appeal
is in the attractive lease terms, the advanced security features, or the
finally visible iconic geometry of the structure. Whatever the lure,
New York’s tallest skyscraper is beginning to fill up, a sign of upturn
for downtown.
