World Trade Center

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World Trade Center

The World Trade Center was built by the Port Authority of New York and New Jersey (PANYNJ) in the late 1960s and early 1970s. The buildings were dedicated in April 1973. For a brief period, the twin towers were the tallest buildings in the world, soon surpassed by the Sears Tower in Chicago.

Background

Main article: Design and construction of the World Trade Center

Austin J. Tobin and the World Trade Center

World Trade Center
World Trade Center

In 1942, Austin J. Tobin became the Executive Director of the Port Authority and would be its chief executive through 1972. In the post-World War II period, the Port Authority expanded its operations to include airports, and marine terminals, with projects including Newark Liberty International Airport, Port Newark, and Port Elizabeth. During this period, the United States thrived economically, with increasing international trade, and the concept of establishing a "world trade center" was conceived. At the time, economic growth was concentrated in Midtown Manhattan, with Lower Manhattan left out. One exception was the construction of Chase Manhattan Bank Tower in the Financial District, by David Rockefeller who led urban renewal efforts in Lower Manhattan.[1]

In initial plans made public in 1961, the World Trade Center was slated to be built on a site along the East River. Objections to the plan came from New Jersey Governor Robert B. Meyner, who resented that New York would be getting this $335 million project.[1] Meanwhile, New Jersey's Hudson and Manhattan Railroad (H&M) was facing bankruptcy. Port Authority executive director, Austin J. Tobin agreed to take over control of the H&M Railroad, in exchange for support from New Jersey for the World Trade Center project. As part of this acquisition, the Port Authority would rehabilitate the Downtown and Uptown Hudson Tubes. The Port Authority would also obtain the Hudson Terminal, and decrepit buildings located above the terminal in Lower Manhattan. The Port Authority decided to demolish these buildings, and use this site along the Hudson River for the World Trade Center.

Even once the agreement between the states of New Jersey, New York, and the Port Authority was finalized, the World Trade Center plan faced continued controversy. New York City mayor Robert Wagner raised concerns about the limited extent that the Port Authority involved the city in the negotiations and deliberations. The site was the location of Radio Row electronics businesses, and the World Trade Center plans involved evicting hundreds of commercial and industrial tenants, property owners, small businesses, and approximately 100 residents, some of whom fiercely protested the forced relocation.[1]

In 1964, Minoru Yamasaki was hired by the Port Authority as architect, who came up with the idea of twin towers. To meet the Port Authority's requirement to build 10 million square feet of office space, the towers would each be 110-stories tall. The size of the project raised ire from the owner of the Empire State Building, which would lose its title of tallest building in the world. Other critics objected to the idea of this much "subsidized" office space going on the open market, competing with the private sector. Others questioned the cost of the project, which in 1966 had risen to $575 million.[1] Final negotiations between The City of New York and the Port Authority centered on tax issues. A final agreement was made that the Port Authority would make annual payments in lieu of taxes, for the 40 percent of the World Trade Center leased to private tenants. The remaining space was to be occupied by state and federal government agencies. In 1962, the Port Authority had signed up the United States Customs Service as a tenant, and in 1964 they inked a deal with the State of New York to locate government offices at the World Trade Center.

Construction

Groundbreaking for the World Trade Center was on August 5, 1966, with construction on the North Tower beginning in August 1968.[2] Construction on the south tower beginning in January 1969.[3]. Construction of World Trade Center 1 (North Tower) was completed in 1970, with World Trade Center 2 (South Tower) completed in 1972. When the World Trade Center twin towers were completed, the total costs to the Port Authority had reached $900 million.[4] The buildings were dedicated on April 4, 1973, with Tobin, who resigned the year before, absent from the ceremonies.[5]

The architect was Minoru Yamasaki, with Emery Roth & Sons as associate architects, and Skilling, Helle, Christiansen and Robertson responsible for the structural design. The towers were designed as framed tube structures, providing tenants with open floor plans. The World Trade Center had a steel facade, with 59 perimeter columns on each side of the building, that were designed to resist lateral and gravity forces. The interior core was designed only to carry gravity load.

In all, the towers had 10,000,000 square feet of usable space, and approximately one acre (or 31,000 sq ft) of space on each floor.[6][7] Atop the North Tower was a 360 ft transmission antenna mast that was added in 1978.[8]

Structural design

Perimeter columns

The perimeter columns supported virtually all lateral loads (such as wind loads), and shared the gravity loads (50%) with the core columns. The perimeter structure was constructed with extensive use of prefabricated modular pieces, which consisted of three columns, three stories tall, connected together by spandrel plates. Above the seventh floor there were 59 perimeter columns along each face of the building. The perimeter columns had a square cross section, 14 inches on a side (36 cm), and were constructed of welded steel plate.[9] The thickness of the plates and grade of steel were varied over the height of the tower, ranging from 36 ksi to 100 ksi, with the steel strength and plate thickness decreasing with height.[9] The perimeter columns supported virtually all lateral loads such as wind loads and shared the gravity loads with the core columns. The perimeter structure was constructed with extensive use of prefabricated modular pieces, which consisted of three columns, three stories tall, connected together by spandrel plates. The spandrel plates were welded to the columns at the fabrication shop. The modular pieces were typically 52 inches (1.3 m) deep, and extended for two full floors and half of two more floors.[9] Adjacent modules were bolted together, with the splices occurring at mid-span of the columns and spandrels. The spandrel plates were located at each floor, and served to transmit shear flow between columns, thus allowing them to work together in resisting lateral loads. The joints between modules were staggered vertically, so the column splices between adjacent modules were not at the same floor.[9]

Core

The building's core housed the elevator and utility shafts, restrooms, three stairwells, and other support spaces.

Floor system

Floor truss and perimeter wall structure, under construction
Floor truss and perimeter wall structure, under construction

The floors supported their own weight, as well as live loads, provided lateral stability to the exterior walls, and distributed wind loads among the exterior walls. The floors consisted of 4 inch thick lightweight concrete slabs laid on a fluted steel deck. A grid of lightweight bridging trusses and main trusses supported the floors. The floors were connected to the perimeter spandrel plates with viscoelastic dampers, which helped reduce the amount of sway felt by building occupants.

Hat trusses

Hat trusses located from the 107th floor to the top of the buildings were designed to support a tall communications antenna on top of each building. Though, only WTC1 (north tower) actually had an antenna.

Design innovations

The tube-frame was a major innovation in skyscraper design, allowing open floor plans and more rentable space.

Other innovations included:

  • Two-way truss framed floors
  • Prefabricated columns and spandrel plates
  • Sprayed fire-resistant material
  • Use of viscoelastic dampers - reduced oscillation of the building

Elevators

The World Trade Center used a series of local and express elevators. This allowed occupants to ride express elevators up to skylobbies at the 44th and 78th floors, then transfer to local elevators to their floor. There were 24 local elevators serving each group of floors, 8 express elevators between the ground level and 44th floor, and 11 express elevators between the ground level and 78th floor. There were two additional express elevators to the observation level and Windows on the World at the top of the buildings. In all, each building had 99 passenger and 7 freight elevators.[10]

Slurry wall

The World Trade Center site is located along the Hudson River, with some of the land comprised of fill material, that was added to expand the Manhattan shoreline. Unlike Midtown Manhattan, were the bedrock is shallow, with the World Trade Center site, bedrock is 65 feet below the surface. In order to construct the World Trade Center, it was necessary to build the "bathtub", with the slurry wall along the West Street side of the site, which serves the purpose of keeping water from the Hudson River out.

The slurry method involves digging a trench, and as excavation proceeds, filling the space with a "slurry" mixture, comprised of bentonite which plugs holes and keeps water out. When the trench was dug out, a steel cage was inserted, with concrete poured in, forcing the "slurry" out. The "slurry" method was devised by Port Authority chief engineer, John M. Kyle, Jr.

The complex

WTC complex and nearby buildings.(click to enlarge)
WTC complex and nearby buildings.
(click to enlarge)

In addition to the twin towers, the World Trade Center complex included several other buildings:

  • WTC3 (Marriott Hotel) - 22 floors
  • WTC4 (South Plaza building) - 9 floors
  • WTC5 (North Plaza building) - 9 floors
  • WTC6 (U.S. Customs House) - 8 floors
  • WTC7 - 47 floors

In addition to these buildings which were all destroyed, the 40-story Bankers Trust (Deutsche Bank) building (to the south of the WTC site) was heavily damaged and as of 2006, is being deconstructed. The St. Nicholas Greek Orthodox Church building, to the south of WTC2 and next to the Bankers Trust building, was also destroyed. Borough of Manhattan Community College's Fiterman Hall, at 30 West Broadway, was heavily damaged in the attacks, and is slated to be demolished and rebuilt.

Facts

World Trade Center 2 (WTC2) - South tower

  • 1,362 feet (415 m) tall
  • 360 foot high TV antenna - added in 1978
  • Observation decks
    • Indoor observation deck - 107th floor
    • Outdoor deck - above the 110th floor
    • Hours: 9:30 a.m. - 9:30 p.m.

World Trade Center 1 (WTC1) - North tower

  • 1,368 feet (417 m) tall
  • Windows on the World
  • No observation deck

The buildings used a network of express and local elevators, with skylobbies located on the 44th and 78th floor of both buildings.

  • More than 50,000 people worked in the World Trade Center, with another 80,000 visitors daily.
  • Each floor spanned more than 40,000 square feet.

History

1975 World Trade Center Fire

On February 13, 1975, a three-alarm fire broke out on the 11th floor of the North Tower, with fire spreading through the core from the 9th to 14th floors where electrical wiring was damaged. The fire ignited the insulation of telephone cables in a utility shaft that ran vertically between floors. Most of the damage was concentrated on the 11th floor, with the fire fueled by cabinets filled with paper, alcohol-based fluid for office machines, and other office equipment. At that time, the World Trade Center was not equipped with sprinkler systems, and being owned by the Port Authority, the buildings were not subject to local safety codes.

The location of the fire, only the 11th floor (versus 75+ floors up on 9/11) made it much easier for the fire department to deal with this fire Firefighters typically carry at least 50 pounds of equipment (sometimes up to 100 pounds, with special gear that may be needed). With elevators out of service, firefighters need to haul themselves and their gear up the stairs which takes time, and even for firefighters that are in good shape, it is very exhausting and difficult.[11]

9/11 attacks

Aircraft entry hole on the north side of WTC 1, photographed 30 seconds after impact of American Airlines Flight 11. (Copyright: John F. Davis)
Aircraft entry hole on the north side of WTC 1, photographed 30 seconds after impact of American Airlines Flight 11. (Copyright: John F. Davis)

North tower

By 8:45 a.m., approximately 14,000 people normally would have arrived for their jobs in the twin towers.[12] At 8:46 a.m. on 9/11, American Airlines Flight 11 (a Boeing 767-200ER series) crashed into the North Tower (WTC1). The aircraft, traveling at 470 mph, hit the building at the 94-98th floors. The aircraft was carrying approximately 10,000 gallons of jet fuel, which is less than half of capacity.[13] At impact, the aircraft was banked 25° to the left, cutting a gash that was over half the width of the building and extended from the 93 to 99th floor.[13] 15-18 perimeter and 5-6 core columns were severed at the 95th and 96th floors.[14] A large section of the 96th floor slab, 40 ft wide and 80 ft in depth, was destroyed upon impact.[14] Insulation was knocked off nearly all the core columns and 40 foot portion of the floor trusses on the south side of the tower.[14] Twelve perimeter columns were severed on the 97th floor.[15] In all, 35 perimeter and six core columns were severed.[15] Fireproofing insulation was stripped off of 43 out of 47 core columns, as well as floor trusses covering 60,000 ft2 of area.[16]

When the plane crashed into the building, approximately 30% of the jet fuel burned off immediately in the fireballs, and roughly half of the jet fuel remained unburned beyond the initial impact and fires. Jet fuel was sent pouring down elevator and utility shafts, causing fires and injuries on lower floors. Port Authority police and FDNY members on the scene saw people seriously burned and enveloped in flames, from a fireball at concourse level, as well as through the lobby of the North Tower. Jet fuel also started fires on the B4 level of the North Tower, when a fireball came down the elevator shaft to the basement levels.[17] At 8:47 a.m., WTC security reported a fire on the 22nd floor.[18]

South tower

Flight 175, just prior to the plane impact with the South tower
Flight 175, just prior to the plane impact with the South tower

United Airlines Flight 175, also a Boeing 767-200ER series, travelling at 590 miles per hour crashed into the South Tower (WTC2) at 9:03 a.m, hitting the building at the 78-84th floors. With the impact of the plane, jet fuel was shot down elevator and utility shafts. There were also jet fuel fires on the 51st floor.[19]

Calls from people trapped in the upper floors to 9-1-1 relayed information about conditions. At 9:37, an occupant on a floor in the 90s, reported that a floor had collapsed.[20]

Deteriorating conditions were also reported by the NYPD helicopters.[21]

  • At 9:52 a.m., the NYPD aviation unit report over the radio that "large pieces may be falling from the top of WTC 2. Large pieces are hanging up there"
  • At 9:58 a.m., the NYPD aviation unit reports that the South Tower is coming down.

After 56 minutes, the South tower collapsed at 9:59 a.m.

In this series of images from a video shot from outside Trinity Church, the columns can be seen bending and bowing inward in the moments before the collapse occurred.








North tower collapses

NYPD helicopters report deteriorating conditions of the North Tower.[21]

  • At 10:20 a.m., the NYPD aviation unit reports that the top of the tower might be leaning.
  • At 10:21 a.m., the NYPD aviation unit reports that the North Tower is buckling on the southwest corner and leaning to the south.
  • At 10:27 a.m., the NYPD aviation unit reports that the roof is going to come down very shortly.
  • At 10:28 a.m., the NYPD reports that the tower is collapsing.

At 10:29 a.m., the North tower collapsed.

Collapse

Main article: Collapse of the World Trade Center
  • The South Tower collapsed at 9:56 a.m. -- 56 minutes after Flight 175 crashed into it
  • The North Tower collapsed at 10:29 a.m. -- 102 minutes after Flight 11 crashed into it.

See also

References

  1. ↑ 1.0 1.1 1.2 1.3 Gillespie, Angus K. (1999) Twin Towers: The Life of New York City's World Trade Center, Chapter 1, Rutgers University Press
  2. ↑ Federal Emergency Management Agency (May 2002), World Trade Center Building Performance Study, Chapter 1
  3. ↑ Timeline: World Trade Center chronology, PBS - American Experience
  4. ↑ Cudahy, Brian J. (2002) Rails Under the Mighty Hudson: The Story of the Hudson Tubes, the Pennsy Tunnels, and Manhattan Transfer, Chapter 3, Fordham University Press.
  5. ↑ Darton, Eric (1999) Divided We Stand: A Biography of New York's World Trade Center, Chapter 6, Basic Books.
  6. ↑ Domel, Jr., August (November 2001) World Trade Center Disaster: Structural Engineers at Ground Zero
  7. ↑ NISTNCSTAR1-1, page xxxvi
  8. ↑ NIST NCSTAR 1, WTC Investigation Report, Chapter 1
  9. ↑ 9.0 9.1 9.2 9.3 NIST NCSTAR 1, WTC Investigation Report, Chapter 1
  10. ↑ NIST NCSTAR 1-1, page xxxvii
  11. ↑ NIST NCSTAR1-8, p. 41-42
  12. ↑ Dwyer, Jim, 102 Minutes, p. 2
  13. ↑ 13.0 13.1 NIST NCSTAR 1, WTC Investigation, Final Report, p. 20
  14. ↑ 14.0 14.1 14.2 NIST NCSTAR 1, WTC Investigation, Final Report, p. 21
  15. ↑ 15.0 15.1 NIST NCSTAR 1, WTC Investigation, Final Report, p. 22
  16. ↑ NIST NCSTAR 1, WTC Investigation, Final Report, p. 23
  17. ↑ NIST NCSTAR 1-8, p. 43-44
  18. ↑ NIST NCSTAR1-8, p. 36
  19. ↑ NIST NCSTAR 1-8, p. 36
  20. ↑ FDNY McKinsey Report - Executive Summary. FDNY / McKinsey & Company (August 2002).
  21. ↑ 21.0 21.1 Lawson, J. Randall, Robert L. Vettori (September 2005). NIST NCSTAR 1-8 - The Emergency Response p. 37. National Institute of Standards and Technology.

Design and construction

Firefighting

  • Cerreta, Kenneth (1993). Stopping the Smoke. New York City Fire Department. 
  • Dunn, Vincent (1998). Collapse of Burning Buildings, A Guide to Fireground Safety. Penn Well Publishing Company. 
  • Hall, Richard and Barbara Adams (2000). Essentials of Fire Fighting. Oklahoma State University. 
  • Isner, Michael S. and Thomas J. Klem (1993). Explosion and Fire Disrupt World Trade Center. National Fire Protection Assocition. 
  • O'Hagan, John T. (1977). High Rise/Fire and Life Safety. Fire Engineering. 

9/11 Commission testimony

Articles

  • Breasted, Mary (February 15, 1975) "Sprinklers urged for Trade Center", New York Times.
  • "Trade Center hit by 6-floor fire". New York Times, February 14, 1975.
  • Ramabhushanam, Ennala, and Marjorie Lynch. (Nov 1994) "Structural assessment of bomb damage for World Trade Center", Journal of Performance of Constructed Facilities, 8(4).
  • Tarricone, Paul (May 1993) "After the blast. (World Trade Center bombing)", Civil Engineering, 63(5)
  • Davenport, Alan G. (December 2002), "Past, present and future of wind engineering", Journal of Wind Engineering and Industrial Aerodynamics, 90(12), p. 1371-1380.
  • Cermak, J.E. (1979) "Applications of Wind Tunnels to Investigation of Wind-Engineering Problems", AIAA Journal, 17(7), p. 679-690.


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