Since the 9/11 attacks on the World Trade Center, there has been increased focus in New York City on egress in emergency situations occurring in multi-story buildings as well as on the ability of emergency personnel to access fires and other crises on higher floors.

A prime example of this new approach to high-rise construction was seen in late June when New York City Mayor Michael Bloomberg signed a law upgrading the safety of such buildings. Based on recommendations of the New York City Mayoral Commission, the measure bans the use of open-web steel joists for structures more than 75 ft. tall because fire-proofing used to protect these floor-framing systems is inadequate. The law effectively eliminates one of two steel floor systems that compete with concrete. Additionally, new buildings will have to comply with blast-resistant construction requirements. According to Portland Cement Association's regional structural engineer Mike Mota, who also serves on the Mayoral Commission that recommended the changes, that provision will force more steel buildings to rely on concrete shear walls and cores, making composite steel-concrete buildings more competitive.

Composite high-rise buildings in New York consume an average of 20 tons of cement per 1,000 sq. ft. of floor space. For example, the recently announced 2.6 million-sq.-ft. Freedom Tower, anchoring the redeveloped WTC complex known in the interim as Ground Zero, will be a composite office building projected to consume about 52,000 tons of cement.

‘THE SAFEST BUILDING’

The new, 52-story 7 World Trade Center was conceived immediately after the destruction of the original 47-floor structure by the terrorist acts of 9/11. The building was not directly hit by the two planes that took down the larger World Trade Center towers, and there were no known casualties due to its collapse; yet, the performance of the original WTC 7 is significant because it appears its destruction was due primarily to fire, started by some combination of debris falling from WTC 1 and a fuel-distribution system needed to run the building's generators. The resulting fire burned for about seven hours before its collapse at 5:20 the afternoon of Sept. 11.

According to the Federal Emergency Management Agency/American Society of Civil Engineers' WTC Building Performance Study, prior to September 11, 2001, there was little, if any, record of fire-induced collapse of large, fire-protected steel buildings.

Built atop an existing Con Edison electrical substation that supplied power to lower Manhattan, the original WTC 7, completed in 1985, stood just over 620 ft. tall and featured a core made up of multiple layers of gypsum wallboard. That design detail was borrowed from the Twin Towers (WTC 1 and 2), whose construction 15 years prior marked the debut of such “cavity shaft wall”-type systems. WTC 7's two main exit stairways discharged to the exterior at ground level; were approximately 4 ft. 10 in. wide; and, like the core, built of fire-rated gypsum wallboard.

Construction on the new WTC 7 began in May 2002. The following January, Larry Silverstein, president of Silverstein Properties, the owner/developer of the property, told the The New York Times that the project must be “the safest building” in the city as one of the first high-rise structures built from the ground up after 9/11. Reflecting on a Silverstein planning meeting at which he was a guest, Times reporter David Dunlap noted: “They [project principals] described a building that would exceed current code requirements, and it was intended to meet or exceed possible future standards. Most fundamentally, the building will have a concrete core rather than a steel-frame core with drywall partitions.”

Much of the original foundation is used to support the new WTC 7 super-structure. However, due to distinct differences between the old and new buildings, additional foundations were required. These consisted primarily of 36-in.-diameter concrete caissons with wide-flange steel cores extending down to and socketed into bedrock.

From the foundation through the first 80 ft., the structure is 100 percent cast-in-place reinforced concrete. Above, the structure consists of a cast-in-place concrete core and steel framing around the perimeter. The new WTC 7 contains many of the same program elements as the original, but on a significantly smaller footprint to allow a reopened Greenwich Street, closed for the initial WTC complex, to run through the site and continue downtown. As a result, the building rises to 750 ft.-130 ft. taller than its predecessor — in order to provide 1.6 million sq. ft. of office space. The cellar and the first five levels are dedicated to the Con Edison substation and building mechanical services. Though the first office level is 115 ft. above grade, the lobby is still at ground level with the loading dock and 10 transformer vaults of the substation.

To ensure maximum strength (about 14,000 psi) for the foundation in the shortest amount of time, the contractor Urban Foundations used a mix design that included about 1,750 yd. of Grancem, a St. Lawrence Cement/Holcim (US) Inc. slag cement product. New York's Ferrara Bros. supplied the foundation concrete. Tishman Construction Corp. is the company in charge of reconstruction of the building, with Sorbara Construction Corp. acting as the concrete subcontractor and The Cantor Seinuk Group Inc. working as structural engineers for the project, scheduled for completion in late 2005 or early 2006.

The choice of the structural systems was driven by a variety of factors, the first being time. With the substation phase scheduled for a May 2004 completion, it was imperative that the lower levels of the building be completed as quickly as possible so that Con Edison could start to provide power downtown for the hot summer months. Because concrete does not require the lead time of structural steel, construction could begin almost immediately after the design was complete. Therefore, it was decided that the lower levels of the building would be built entirely of concrete.

Another driving force was safety. The owner wanted to go beyond the minimum building code requirements and create perhaps the safest building of its size from the standpoint of fire and terrorist hazards. The building's core consists of concrete shear walls as thick as 2 ft. 3 in., which surround the elevator shafts, stairways and transfer corridors. The walls will help to mitigate potential damage and injury in the event of an explosion. The robust shell enclosing the stairs also will help keep fire exits open in an evacuation.

As with any real estate development, economics is always a factor. Though the shear walls provide the added benefit of strengthening the vertical circulation areas, first and foremost, they are also an efficient lateral system. Also, because of differing space requirements between the office building above and the substation below, essentially every perimeter column had to be transferred. A concrete wall/ring beam was used for this purpose and allowed the flexibility to provide several openings of varying size and to provide redundant load paths for additional safety.

Overall, the foundation of 7 World Trade Center required 17,000 yds. of concrete, and the building's superstructure used 16,000 yds. The floor slabs are 5,000 psi from 14 to 8 in. thick. The walls are 12,000 psi from 27 to 12 in. thick, and 8,000 psi from 19 to 12 in. thick.

The building code at the time the new WTC 7 was designed would have allowed for the two stairwells to be closer to each other and the core, but the architects at New York-based Skidmore, Owings & Merrill (who also designed the Freedom Tower to be built across the street) placed the stairs at opposite ends of the building, hopefully minimizing the chance of both being damaged in an attack similar to that upon the World Trade Center towers and keeping them away from the core. Building owner Silverstein Properties also insisted that the fire stairs be 11 inches wider than code dictated, making then 66 in., enough room, the developer says, for two-way traffic: firefighters climbing up and tenants going down. Other safety elements involving sprinkler systems, water-tank capacity, emergency generators, fuel storage, antennas for emergency communications and secondary means of egress have also been built into the building's design.

PROMOTING SAFETY

PCA's Mota believes that the design elements of WTC 7, as well as Mayor Bloomberg's banning the use of open-web steel joists for structures more than 75 ft., is a direct result of the efforts of several local and regional concrete promotion groups, including the New York City Concrete Promotion Council, which even before 9/11 was driving home the point with architects, builders and engineers that fireproofing was inadequate in many high rises. “As a result of these efforts,” says Mota, “many builders will probably go beyond what the code requires.”

Patrick Reardon, executive director of the Northeast Cement Shippers Association, says that the promotion council was in the process just before 9/11 of forming segment committees to concentrate on areas such as concrete homes, flowable fill, pavement and high-rise work. “After 9/11, we immediately got the high-rise group off the ground with Mota as chairman,” he says. “The group focused on areas such as post-tensioning, a major component in commercial high-rise building.”

Working with the local Concrete Alliance and other promotion groups, everyone from contractors and unions to legislators and lobbyists came to the table to learn about concrete and the benefits of composite buildings. “We covered everything from fire resistance, better egress time, greater structural integrity, cost savings, construction cycle times and flexibility when design changes happen,” Reardon adds. “We also discussed the concerns regarding steel costs and availability, and the over-reliance on sprinkler systems. We want to do what we can to minimize the chances that someone could attack a light steel-framed building with a weak core. We have a better way.”