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 WTC’s 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 tower’s 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 we’ll 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.

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 D’Antonio, chief engineer for the steel contractor, Helmark Steel Inc., Wilmington, Del.

Click here for slide show

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 structure’s 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 methodology’s advantages far outweigh any disadvantages, claims Mel Ruffini, Tishman’s 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.

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.

Power Pedestal

The glass-clad office tower, 213 x 171 ft in plan, sits on a utility power substation, framed in concrete–also 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 7’s 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 D’Antonio.

The doughnut’s 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 D’Antonio. 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 D’Antonio.

D’Antonio 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 D’Antonio. "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.

Four-Day Floors

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 steel’s 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 form’s 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 city’s transit authority. "Everyone worked in a checkerboard fashion, leapfrogging around each other," says Ruffini.

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.

Core in Core. Emergency access core within Freedom Tower’s 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 emergency services.

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 Manhattan. 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. "We’re eager to proceed."

Nadine M. Post , Engineering News-Record