Design and construction of 7 World Trade Center

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7 World Trade Center

7 World Trade Center was built in the mid 1980s, above a two-story Con Edison substation. The 47-story building was located to the north of the World Trade Center twin towers and other buildings that were constructed in the 1970s. 7 World Trade Center was developed by Larry Silverstein. As a Port Authority project, 7 World Trade Center received an abatement in city taxes; Instead, the city received payments in lieu of taxes.

Construction of 7 World Trade Center was completed in 1987. Each floor of the building provided 47,000 square feet of rentable space - considerably larger than most other buildings. In all, the building had 1.86 million square feet of office space. WTC7 had overall dimensions of approximately 330 ft long, 140 ft wide, and 610 ft high.[1]

Design

The original 7 World Trade Center was a 47-story building, designed by Emery Roth & Sons, with a red granite façade. The building was 610 feet (185.9 m) tall, with a trapezoid-shaped footprint that was 330 ft (100.6 m) long and 140 ft (42.7 m) wide.[2][3] Tishman Realty & Construction managed construction of the building, which began in 1984.[2] In March 1987, the building opened, to become the seventh structure of the World Trade Center.

The building was constructed above a Con Edison substation, which had been on the site since 1967.[4] The substation had a caisson foundation designed to carry the weight of a future building on the site of 25 stories containing 600,000 sq ft (55,700 m²).[5] The final design for 7 World Trade Center was for a much larger building covering a larger footprint than originally planned when the substation was built.[6]

The structural design of 7 World Trade Center included numerous features to allow a larger building than originally planned to be constructed. A system of gravity column transfer trusses and girders was located between floors 5 and 7 to transfer loads to the smaller foundation.[4] Existing caissons installed in 1967 were used, along with additional new caissons, to accommodate the building. The 5th floor functioned as a structural diaphragm, providing lateral stability and distribution of loads between the new and old caissons. Above the 7th floor, the building's structure was a typical tube-frame design, with columns in the core and on the perimeter and lateral loads resisted by perimeter moment frames.[5]

A shipping/receiving ramp, which served the entire World Trade Center complex, occupied the eastern quarter of the 7 World Trade Center footprint. The building was open below the 3rd floor, providing space for truck clearance on the shipping ramp.[5] The spray-on fireproofing for structural steel elements was generally less than an inch thick.[7] The material was a gypsum-based Monokote which had a 2-hour fire rating for steel beams, girders and truss, and a 3-hour rating for columns.[8]

Mechanical equipment, including 12 transformers on the 5th floor, was installed on floors 4 through 7. Several generators in the building were used by the Office of Emergency Management, Salomon Smith Barney and others.[8] Storage tanks contained 24,000 gallons (91,000 L) of diesel fuel to supply the generators.[9] Fuel oil distribution components were located at ground level, up to the 9th floor.[10] The roof of the building included a small west penthouse and a larger east mechanical penthouse.[4]

Each floor had 47,000 sq ft (4,366 m²) of rentable office space which made the building's floor plans considerably larger than most office buildings in the City.[11] In all, 7 World Trade Center had 1,868,000 sq ft (174,000 m²) of office space.[8] Two pedestrian bridges connected the main World Trade Center complex, across Vesey Street, to the 3rd floor of 7 World Trade Center. The lobby of 7 World Trade Center had three murals by artist Al Held: The Third Circle, Pan North XII, and Vorces VII.[12][13] A sculpture by Alexander Calder, called WTC Stabile (also known by other names, The Cockeyed Propeller and Three Wings) was on a plaza in front of the building.[14]

Tenants

In June 1986, before construction was completed, Silverstein signed Drexel Burnham Lambert as a tenant to lease the entire 7 World Trade Center building for $3 billion over a term of 30 years.[15] In December 1986, after the Boesky insider-trading scandal, Drexel Burnham Lambert canceled the lease leaving Silverstein to find other tenants.[16] Spicer & Oppenheim agreed to lease 14 percent of the space but for more than a year, as Black Monday and other factors adversely affected the Lower Manhattan real estate market, Silverstein was unable to find tenants for the remaining space. By April 1988, Silverstein had lowered the rent and made other concessions.[17]

In November 1988, Salomon Brothers withdrew from plans to build a large new complex at Columbus Circle in Midtown and agreed to a 20-year lease for the top 19 floors of 7 World Trade Center.[18] The building was extensively renovated in 1989 to accommodate the needs of Salomon Brothers.[19] Most of three existing floors were removed as tenants continued to occupy other floors, and more than 350 tons (U.S.) of steel were added to construct three double-height trading floors. Nine diesel generators were installed on the 5th floor as part of a backup power station. "Essentially, Salomon is constructing a building within a building - and it's an occupied building, which complicates the situation," said a district manager of Silverstein Properties. The unusual task was possible, said Larry Silverstein, because it was designed to allow for "entire portions of floors to be removed without affecting the building's structural integrity, on the assumption that someone might need double-height floors."[19]

At the time of the September 11, 2001 attacks, Salomon Smith Barney was by far the largest tenant in 7 World Trade Center, occupying 1,202,900 sq ft (111,750 m²) (64 percent of the building) which included floors 28–45.[20][8] Other major tenants included ITT Hartford Insurance Group (122,590 sq ft/11,400 m²), American Express Bank International (106,117 sq ft/9,900 m²), Standard Chartered Bank (111,398 sq ft/10,350 m²), and the Securities and Exchange Commission (106,117 sq ft/9,850 m²).[20] Smaller tenants included the Internal Revenue Service Regional Council (90,430 sq ft/8,400 m²) and the United States Secret Service (85,343 sq ft/7,900 m²).[20] The smallest tenants included the New York City Office of Emergency Management, NAIC Securities, Federal Home Loan Bank, First State Management Group Inc., Provident Financial Management, and the Immigration and Naturalization Service.[20] The Department of Defense (DOD) and Central Intelligence Agency (CIA) shared the 25th floor with the IRS.[8] Floors 46–47 were mechanical floors, as well as the bottom six floors and part of the 7th floor.[8][21]

Design

7 World Trade Center was built on top of a Con Edison substation, but the footprint of the building was considerably larger than the substation and the originally built foundation. Between floors 5 and 7, the building had a system of transfer trusses and girders to transfer load to the smaller-sized foundation.[4]

Construction

  • Owner/developer: Larry Silverstein
  • Construction manager: Tishman Construction Company of New York
  • Design architect: Emery, Roth & Sons
  • Structural consultant: The Office of Irwin G. Cantor, P.C.
  • Mechanical/electrical consultant: Syska & Hennessy, P.C.
  • Structural consultant (Con Ed substation): Leslie E. Robertson Associates

Construction difficulties

The building was constructed over an existing Con Edison power substation, which was built at the time that the twin towers were built. The substation was built with a foundation that could support a future building atop it of the same dimensions. However, the final design for WTC7 was for a building covering a significantly larger footprint.

In addition to design challenges for the structural engineers, the power substation led to many difficulties during construction. Caissons had to be installed through a utility power substation without tripping a blackout, and alongside a busy WTC service ramp and a subway.[22]

The most trying work was through the substation (along the 350-ft side). After breaking open the roof, workers had to cut holes through two floor slabs, near active transformers. Then, using a crane with a hammer attachment, the caisson shells were "pounded," not drilled, to rock to minimize vibrations. "If vibrations had reached a certain level," says Robert J. Washlick, Tichman project manager, they would have tripped switch gear, causing a lower-Manhattan blackout," he says. When possible, work went on during off hours.[22]

"We mobilized for four conditions on site -- the substation, truck ramp, subway and open area," says Washlick. "It was an expensive foundation, no two ways about it."[22]

The first seven floors were also difficult. The building's 350-ft side cantilevers 7 ft over the substation from the seventh floor to the roof. Eight 55-ton, 60-ft-long girders that taper from 9 to 3 ft deep had to be lifted over the substation and placed under seventh-floor framing to carry cantilever loads to the core. The weight and reach involved prompted the use of two cranes and a derrick to lift each girder.[22]

Structural design challenges

From level seven to the top, the structural-steel building has a perimeter moment-resisting frame. Two two-story-high belt trusses, one at the 22nd story and one at the seventh, reduce deflection during high winds.[22]

The first seven stories are more complicated. To resist lateral loads, the structural engineer designed a seven-story braced core linked through floor diaphragms to seven-story-high wind truss systems on the shorter sides of the building. "The end frames create channel action to stiffen the building," says Tamboli. Horizontal bracing in the fifth and seventh-story floor slabs transfers lateral loads from the moment frame to the braced core.[22]

For the 1.8-million-sq-ft building, caissons had to be used that were intended for a 1-million-sq-ft building. The location of new caissons was limited, mostly by the two-story substation and the ramp.Problems lining up old caissons and new columns prompted use of three reinforced-concrete caps as big as 64 x 6 ft to spread column loads to caissons.[22]

In the braced core, however, lateral loads are transferred to a braced foundation-slab diaphragm and into existing and new caissons, some of which are clustered and battered. The steel bracing in the diaphragm connects the caissons and redistributes their loads, says Tamboli. Like a tug of war, "the moment one caisson starts to lift, bracing goes into action and transfers extra load to another not being used to full advantage," he says.[22]

For project construction manager Tishman Construction, the biggest problem was logistics on site -- an irregular trapezoid 154 ft on its short sides and 350 and 250 ft on its long sides. Of 50 new caissons, their steel cores as long as 110 ft, fewer than half went into the excavated area, which is itself crossed by utility lines and required continuous dewatering. Others went through the substation and alongside the ramp and subway.[22]

Con Ed substation

Floors 8-47, above the Con Ed substation, contained rentable office space. The substation was built in 1967, with future plans in mind to build an office tower above the substation. Caissons were installed with the substation that would be able to bear the weight of a future building.

Foundation plan of 7 World Trade Center
Foundation plan of 7 World Trade Center


Foundation

The building's footprint covered a larger area than originally assumed when the substation was built. Existing and new caissons were used to build the foundation for 7 World Trade Center.

Tenants

Silverstein initially signed on Drexel Burnham Lambert Inc. to lease the entire building. Being at the center of the Savings & Loan scandals, Drexel backed out of the lease and Silverstein eventually found other tenants for 7 World Trade Center.

At the time of the 9/11 attacks, Salomon Smith Barney occupied the top 18 floors of 7 World Trade Center, with a mix of financial/insurance firms and government tenants on other floors.

See also: List of 7 World Trade Center tenants

References

  1. ↑ NIST NCSTAR1-1, page xxxvii
  2. ↑ 2.0 2.1 Lew, H.S., Richard W. Bukowski, Nicholas J. Carino (September 2005). Design, Construction, and Maintenance of Structural and Life Safety Systems (NCSTAR 1-1). National Institute of Standards and Technology, p. 13. 
  3. ↑ Seven World Trade Center (pre-9/11). Emporis.com. Retrieved on 2006-05-07.
  4. ↑ 4.0 4.1 4.2 4.3 Interim Report on WTC 7 (pdf). Appendix L. National Institute of Standards and Technology (2004). Retrieved on 2007-08-20.
  5. ↑ 5.0 5.1 5.2 Template:Cite conference
  6. ↑ Lew, H.S., et al (September 2005). "NIST NCSTAR 1-1: Design, Construction, and Maintenance of Structural and Life Safety Systems", Final Reports of the Federal Building and Fire Investigation of the World Trade Center Disaster (pdf), NIST, p. xxxvii. 
  7. ↑ Glanz, James. "Engineers Have a Culprit in the Strange Collapse of 7 World Trade Center: Diesel Fuel", The New York Times, November 29, 2001.
  8. ↑ 8.0 8.1 8.2 8.3 8.4 8.5 Gilsanz, Ramon, Edward M. DePaola, Christopher Marrion, and Harold "Bud" Nelson (May 2002). "WTC7 (Chapter 5), pdf", World Trade Center Building Performance Study. FEMA. 
  9. ↑ Milke, James. "Study of Building Performance in the WTC Disaster", Fire Protection Engineering, Spring 2003.
  10. ↑ Grill, Raymond A., Duane A. Johnson (September 2005). "NIST NCSTAR 1-1J: Documentation of the Fuel System for Emergency Power in World Trade Center 7", Final Reports of the Federal Building and Fire Investigation of the World Trade Center Disaster. NIST. 
  11. ↑ Horsley, Carter B.. "Lower Manhattan Luring Office Developers", The New York Times, October 25, 1981.
  12. ↑ Al Held. National Gallery of Australia. Retrieved on 2007-05-29.
  13. ↑ Plagens, Peter. "Is Bigger Necessarily Better?", Newsweek, April 17, 1989.
  14. ↑ Wenegrat, Saul. "Public Art at the World Trade Center", International Foundation for Art Research, February 28, 2002. Retrieved on 2007-07-27.
  15. ↑ Scardino, Albert. "A Realty Gambler's Big Coup", The New York Times, July 11, 1986.
  16. ↑ Scardino, Albert. "$3 Billion Office Pact Canceled by Drexel", The New York Times, December 3, 1986.
  17. ↑ Berg, Eric N.. "Talking Deals; Developer Plays A Waiting Game", The New York Times, April 7, 1988.
  18. ↑ Berkowitz, Harry. "Salomon to Move Downtown", Newsday, November 29, 1988.
  19. ↑ 19.0 19.1 McCain, Mark. "The Salomon Solution; A Building Within a Building, at a Cost of $200 Million", The New York Times, February 19, 1989.
  20. ↑ 20.0 20.1 20.2 20.3 "7 World Trade Center tenants", CNN.
  21. ↑ "CIA Lost Office In WTC: A secret office operated by the CIA was destroyed in the terrorist attacks on the World Trade Center, seriously disrupting intelligence operations'", CBSNews.com / AP, 2001, November 5, 2001.
  22. ↑ 22.0 22.1 22.2 22.3 22.4 22.5 22.6 22.7 22.8 Location is good, location is bad for high-rise;Good real estate offers nightmare site Engineering News-Record November 28, 1985

Other references

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