September 12, 2005
At the dawn of a marketing blitz for
tenants to join him in the first office tower to rise from
Ground Zero, Seven World Trade Center developer Larry Silverstein
is all gab about the souped-up-for-more-safety features of
the 52-story replacement for the tower that collapsed on Sept.
11, 2001, after sustaining collateral damage from attacks
on the WTCs twin towers. But to the builders of the
new 1.7-million-sq-ft high-rise, the job stands out for a
completely different reason. By building the towers
steel frame ahead of its structural concrete core, the team
managed to beat its own predictions for a reduced schedule
by a month. By demanding cooperation among trades, the approach
breaks the norm for construction in New York City in an "extraordinarily"
positive way, say sources. And, that is no mean accomplishment.
Steel ahead of concrete turned out to
be so successful that, after much initial resistance, Silverstein
Properties Inc. gave the green light to use the same approach
on its planned 1,776-ft-tall Freedom Tower across Vesey Street.
"The construction method for 7 is exactly what well
be using on the Freedom Tower," says Carl Galioto, technical
partner in the New York City office of the architect for both
7 and the Freedom Tower, Skidmore, Owings & Merrill.
Rebuilt. New 7 WTC (top right),
replacing original (above), is first tower at Ground Zero.
Photo top right by Michael Goodman for ENR
During planning for 7, the construction manager predicted
the strategy would shorten the schedule by three to four months.
"There was a lot of skepticism within our ranks about
the steel-first method and the client was not sure it was
the best way to go," says Elio Cettina, supervisor of
general superintendents for Tishman Construction Corp. of
New York. The CM also is building the Freedom Tower.
The proof is in the pudding, says Cettina. "We were able
to maintain four-day-per-floor cycles for all operations"
and beat the clock by a month beyond the prediction, he says.
Steel topped out last October instead of in November. Substantial
completion is targeted for early November. Silverstein plans
to move into its space in March.
The steel-first approach requires meticulous planning and
added doses of coordination and communication among the major
contractors. It also requires buy-in by the trades, especially
ironworkers. "It really took a big team effort,"
which is a switch from the usual adversarial atmosphere, says
Dominick DAntonio, chief engineer for the steel contractor,
Helmark Steel Inc., Wilmington, Del.
The structural engineer still thinks concrete first is the
better way to go. There is no need for erection columns and
other temporary steel to stabilize the frame laterally until
the core, which provides the structures permanent lateral
stability, catches up, says Silvian Marcus, CEO for WSP Cantor
Seinuk, which engineered the original 7 and also is designing
the Freedom Tower. With concrete first, there also is no need
for extra temporary protection.
The methodologys advantages far outweigh any disadvantages,
claims Mel Ruffini, Tishmans project executive. Steel
first allows the concrete to be cast inside a protective frame.
It is considered safer, and thus acceptable to New York City
ironworkers, who will not allow other work overhead. Steel
first also allows the curtain wall to start sooner. And it
is a more sustainable approach because the self-climbing forms
used for the concrete core are reusable, says Tishman.
The concrete contractor credits the self-climbers for much of
the success of the method. It would have taken almost twice
as long to complete a floor with a hand-set system, says William
Kell, chief estimator-project manager for the concrete contractor,
Sorbara Construction Corp., Lynbrook, N.Y. That would have had
concrete work lagging too far behind steel work. Tishman says
the self-climbers have not been used in New York City for an
Self-Climber. Formwork kept
core from lagging too far behind steel.
Photo Courtesy Tishman Construction Corporation
The steel-first approach in New York City is not new. The
engineer had used it first in the late 1960s on two office
towers and recently on two more. But none of the buildings
used self-climbers. On 7, "they literally had to design
the building around the forming system," says Kell.
The glass-clad office tower, 213 x 171 ft in plan, sits on
a utility power substation, framed in concretealso a
replacement. Only the tower core and lobby "corridor"
penetrate the substation. Tower mechanical floors begin 78
ft above grade. The first office floor is 125 ft above grade.
The new substation and tower jobs, designed and managed by
the same firms, share foundations. Tower core walls take loads
to caissons 15 to 20 ft below the water table. The engineer
was able to reuse 30% of the original 7s caissons.
Work on the office tower could not proceed until the substation
was topped out in October 2003. Work was complicated and delayed
by obstructions encountered during foundation work.
In essence, workers initially built two independent structures.
The first was a parallelepiped-shaped steel frame, like a
warped doughnut. The second was a rectilinear core, 108 ft
on side, which filled the doughnut hole.
Sequence. All operations, starting
with steel erection, followed a four-day-per-floor cycle
at 7 WTC. Sequence required meticulous planning.
Image Courtesy Tishman Construction Corporation
Steel erection for 7 had to follow the
substation frame. Helmark and Falcon Steel Co., its Forth
Worth, Texas-based erector, had never done a steel-first,
hybrid-system building. Despite a learning curve, the system
"proved out," says DAntonio.
The doughnuts lack of lateral stability was compounded
by forces imposed by two tower cranes and other construction
equipment. To counter this, Helmark temporarily braced 12
bays. Bracing overlapped the concrete operation by four floors
to facilitate the transfer of lateral loads into the permanent
system, says DAntonio. There also was a temporary floor
system of steel joists and deck panels to cover the core void,
and safety nets to protect workers.
Temporary steel was erected along with the permanent frame.
The operation was conventional except for moving the void
cover and netting; jumping 14-story steel stair towers, also
temporary; and removing and relocating temporary bracing as
the steel went up.
There were as many as 16 to 18 floors of temporary bracing.
"We erected, unerected and re-erected temporary steel
three times," says DAntonio.
DAntonio does not know whether there was a time penalty
for erecting and moving the 1,000 tons of temporary steel
within the 12,000-ton frame.
There could be no fewer than eight and no more than 12 floors
from the steel work floor to the top-most poured and cured
core section. "We had to stay on a very tight schedule
with the concrete contractor," says DAntonio. "We
could not go too fast or too slowly."
After the substation was topped out, ironworkers mobilized
cranes and erected embedded steel columns around the core
void. Then, the steel operation demobilized for about a month
so that Sorbara could mobilize the self-climbers. It was the
only interruption in the flow.
After the formwork was in, the four-day-per-floor cycle began
with steel erection. Metal deck followed, with its concrete
topping. Workers then cast core shear walls and slabs. Next
came the steels sprayed-on fireproofing. The remaining
trades followed. This included curtain-wall work, mechanical,
electrical and plumbing work, elevators and interior work.
The shear-wall operation began its first cycle after the
steel frame was eight floors up and concrete had been placed
on the metal deck a few floors overhead. First, the inside
face of the form was fixed off the climbing work platform.
Workers installed bulkheads off of the inside form and frame
reinforcing steel. The forms outside face was raised
with chain falls and the forms were closed. Then, concrete
was cast. Workers stripped forms the next day. The self-climber
platform was jacked and inside forms reset. Once the trailing
platforms hanging from the self-climbers were a few floors
above, floor slabs were conventionally constructed.
The construction of the original 7 was described as a logistical
nightmare by its designers and builders (ENR 11/28/85 p. 30).
The 9/11 terrorist attacks ended up causing another nightmare.
But according to the new team, which includes the same structural
engineer and CM, the design and construction of the new 7
was made simpler because of the opportunity to integrate the
substation design and construction with the tower above it.
Both jobs were equally difficult because major utilities in
all surrounding streets ripped open because of post-9/11 work.
Work also was complicated by neighboring projects to restore
buildings damaged on 9/11, and nearby road work. To handle
the congestion, access problems and staging limitations, Tishman
had weekly coordination meetings with the power utility, the
phone company, city and state transportation departments and
the citys transit authority. "Everyone worked in
a checkerboard fashion, leapfrogging around each other,"
Larry Silverstein boasts that 7 will be the safest commercial
office building in the U.S., until the Freedom Tower is built.
"Miraculously, the structure of 7 and the life-safety
enhancements parallel the recommendations" made in June
by the National Institute of Standards and Technology in its
$16-million report on the World Trade Center, says Silverstein.
"We anticipated this by about three years," he says.
Each subsequent building in the WTC redevelopment will be designed
using the same principles, Silverstein says. Plans for the Freedom
Tower go further. It will have a concrete-enclosed emergency
access core within the concrete core.
Core in Core. Emergency access
core within Freedom Towers main core will provide
dedicated and protected elevators for firefighters, and
more. Image Courtesy of Skidore Owings and Merrill LLP
The EAC is designed to contain five service elevators, protected
so that equipment will be able to resist water. The service
lobby would be pressurized to mitigate smoke intrusion, either
on the floor or in hoistways. The elevators and pressurized
stairs would be dedicated for use by emergency responders.
If an incident is on the 50th floor, firefighters will take
the elevator to 48 and walk up, says Galioto. Elevators also
would be used to evacuate those with disabilities. The EAC
would contain the electrical and com-munications closets for
The Freedom Tower project lost about eight months when the
city police came up with new security rules for standoff distances
after New York Gov. George E. Pataki decided West Street alongside
the WTC site would remain a surface street. That meant going
back to the drawing board. A new design concept was released
in June (ENR 7/11 p. 10).
SOM plans to complete schematic design by year-end. Foundation
packages should be issued before that so construction can
commence in the first quarter of next year, says Galioto.
That should "maintain the governor's...schedule"
for occupancy of the building in the spring of 2010, he says.
Towering Symbol. 1,776-ft-tall
replacement for twin, 110-story towers to anchor lower
Rendering Courtesy Skidmore, Owings & Merrill LLP
Silverstein calls the need to redesign a "frustrating"
experience. He says his biggest regret in the last year was
the "lack of coordination relative to the New York Police
Dept. and security issues on the site. We lost time and encountered
a good deal of additional cost," he says. Silverstein
has not yet tallied the toll, but probably will seek reimbursement
from the city or state.
For Tishman, it was not a total loss. Some of the planning
and all of the relationships developed, especially with the
Port Authority of New York and New Jersey, which owns the
land and runs the PATH subway that runs under the site. But
the team had to throw out lots of work.
Silverstein and SOM think the new design beats the first.
"It responds to an even greater variety of issues,"
says Galioto. "Were eager to proceed."