World Trade Center: Rise & Fall of an Icon

[Jeannette Latoria]

World Trade Center



The twin towers of the World Trade Center in March 2001.




World Trade Center were the world's tallest buildings from 1972 to 1973.*
Preceded by Empire State Building
Surpassed by Sears Tower
Information
Location New York City, NY, U.S.
Status Destroyed on September 11, 2001
Constructed 1966-1973
Height
Antenna/Spire 1,727 ft (526.3 m)
Roof 1,368 ft (417.0 m)
Top floor 1,355 ft (413.0 m)
Technical details
Floor count 110
Floor area 8.6 million sq ft
800,000 m² (1 & 2)
Elevator count 198 (1 & 2)
Companies
Architect Minoru Yamasaki
Emery Roth & Sons
Structural
Engineer Leslie E. Robertson Associates
Contractor Tishman Realty & Construction Company
Owner Port Authority of New York and New Jersey

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*Fully habitable, self-supported, from main entrance to highest structural or architectural top; see the list of tallest buildings in the world for other listings.

[Jeannette Latoria]

The World Trade Center was a seven-building complex in Lower Manhattan that was destroyed in 2001 in the September 11 attacks. The site is currently being rebuilt with six new skyscrapers and a memorial to the casualties of the attacks.

The original World Trade Center was designed by Minoru Yamasaki in the early 1960s using a tube-frame structural design for the twin 110-story towers. In gaining approval for the project, the Port Authority of New York and New Jersey agreed to take over the Hudson & Manhattan Railroad which became the Port Authority Trans-Hudson (PATH). Groundbreaking for the World Trade Center took place on August 5, 1966. The North Tower was completed in December 1970 and the South Tower was finished in July 1971. Construction of the World Trade Center involved excavating a large amount of material which was used in making Battery Park City on the west side of Lower Manhattan.

The complex (sometimes abbreviated WTC and informally called the "trade center" or the Twin Towers) was located in the heart of New York City's downtown financial district and contained 13.4 million square feet (1.24 million m²) of office space.[1] The Windows on the World restaurant was located on the 106th and 107th floors of the North Tower, while the Top of the World observation deck was located on the 107th floor of the South Tower. Other World Trade Center buildings included the Marriott World Trade Center; 6 World Trade Center, which housed the United States Customs; and 7 World Trade Center, which was built in the 1980s. The World Trade Center experienced a fire on February 13, 1975 and a bombing on February 26, 1993. In 1998, the Port Authority decided to privatize the World Trade Center, leasing the buildings to a private company to manage, and awarded the lease to Silverstein Properties in July 2001.

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On the morning of September 11, 2001, al-Qaeda-affiliated hijackers flew two Boeing 767 jets into the complex, one into each tower, in a coordinated suicide attack. After burning for 59 minutes, the South Tower collapsed, followed a half-hour later by the North Tower, with the attacks on World Trade Center resulting in 2,750 deaths. 7 World Trade Center collapsed later in the day and the other buildings in the complex were also destroyed. The process of cleanup and recovery at the World Trade Center site took eight months. The first new building at the site was 7 World Trade Center which opened in May 2006. The Lower Manhattan Development Corporation (LMDC), established in November 2001 to oversee the rebuilding process, organized competitions to select a site plan and memorial design. Memory Foundations, designed by Daniel Libeskind, was selected as the master plan, which included the 1,368 ft Freedom Tower, three office towers along Church Street and a memorial designed by Michael Arad.

[Jeannette Latoria]

Planning and construction

The idea of establishing a world trade center in New York City was first proposed in 1946. The New York State Legislature passed a bill authorizing New York Governor Thomas E. Dewey to begin developing plans for the project[2] but the plans were put on hold in 1949.[3] During the late 1940s and 1950s, economic growth in New York City was concentrated in Midtown Manhattan, while Lower Manhattan was left out. To help stimulate urban renewal, David Rockefeller suggested that the Port Authority build a World Trade Center in Lower Manhattan.[4]

Initial plans, made public in 1961, identified a site along the East River for the World Trade Center.[5] As a bi-state agency, the Port Authority required approval from both the governors of New York and New Jersey in order to undertake new projects. New Jersey Governor Robert B. Meyner objected to New York getting a $335 million project.[6] Toward the end of 1961, negotiations with outgoing New Jersey Governor Meyner reached a stalemate.[7]

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At the time, ridership on New Jersey's Hudson and Manhattan Railroad (H&M) had declined substantially from a high of 113 million riders in 1927 to 26 million in 1958 after new automobile tunnels and bridges had opened across the Hudson River.[8] In a December 1961 meeting between Port Authority director Austin J. Tobin and newly elected New Jersey Governor Richard J. Hughes, the Port Authority offered to take over the Hudson & Manhattan Railroad to have it become the Port Authority Trans-Hudson (PATH). The Port Authority also decided to move the World Trade Center project to the Hudson Terminal building site on the west side of Lower Manhattan, a more convenient location for New Jersey commuters arriving via PATH.[7] With the new location and Port Authority acquisition of the H&M Railroad, New Jersey agreed to support the World Trade Center project.[9]

Approval was also needed from New York City Mayor John Lindsay and the New York City Council. Disagreements with the city centered on tax issues. On August 3, 1966, an agreement was reached that the Port Authority would make annual payments to the City in lieu of taxes for the portion of the World Trade Center leased to private tenants.[10] In subsequent years, the payments would rise as the real estate tax rate increased.[11]

[Jeannette Latoria]

Architectural design

On September 20, 1962, the Port Authority announced the selection of Minoru Yamasaki as lead architect and Emery Roth & Sons as associate architects.[12] Yamasaki devised the plan to incorporate twin towers; Yamasaki's original plan called for the towers to be 80 stories tall.[13] In order to meet the Port Authority's requirement to build 10 million square feet (930,000 m²) of office space, the buildings would each need to be 110 stories tall.[14]

A major limiting factor in building height is the issue of elevators; the taller the building, the more elevators are needed to service the building, requiring more space-consuming elevator banks.[14] Yamasaki and the engineers decided to use a new system with sky lobbies; floors where people could switch from a large-capacity express elevator which serves the sky lobbies, to a local elevator that goes to each floor in a section. This allowed the local elevators to be stacked within the same elevator shaft. Located on the 44th and 78th floors of each tower, the sky lobbies enabled the elevators to be used efficiently, increasing the amount of usable space on each floor from 62 to 75 percent by reducing the number of required elevator shafts.[15].[16] Altogether, the World Trade Center had 95 express and local elevators.[17] This system was inspired by the New York City Subway system whose lines include local stations where local trains stop and express stations where all trains stop.[18]

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Yamasaki's design for the World Trade Center, unveiled to the public on January 18, 1964, called for a square plan approximately 207 feet (63 m) in dimension on each side.[13][19] The buildings were designed with narrow office windows 18 inches (45 cm) wide, which reflected Yamasaki's fear of heights as well as his desire to make building occupants feel secure.[20] Yamasaki's design included building facades sheathed in aluminum-alloy.[21] The World Trade Center was one of the most striking American implementations of the architectural ethic of Le Corbusier and it was the seminal expression of Yamasaki's gothic modernist tendencies.[22]

In addition to the twin towers, the plan for the World Trade Center complex included four other low-rise buildings which were built in the early 1970s. The 47-story 7 World Trade Center building was added in the 1980s to the north of the main complex. Altogether, the main World Trade Center complex occupied a 16 acres (65,000 m2) superblock.[23]

[Jeannette Latoria]

A typical floor layout and elevator arrangement of the WTC towers

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Structural design

The structural engineering firm Worthington, Skilling, Helle & Jackson worked to implement Yamasaki's design, developing the tube-frame structural system used in the twin towers. The Port Authority's Engineering Department served as foundation engineers, Joseph R. Loring & Associates as electrical engineers, and Jaros, Baum & Bolles as mechanical engineers. Tishman Realty & Construction Company was the general contractor on the World Trade Center project. Guy F. Tozzoli, director of the World Trade Department at the Port Authority, and Rino M. Monti, the Port Authority's Chief Engineer, oversaw the project.[24] As an interstate agency, the Port Authority was not subject to local laws and regulations of the City of New York including building codes. Nonetheless, the structural engineers of the World Trade Center ended up following draft versions of the new 1968 building codes.[25]

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The tube-frame design, earlier introduced by Fazlur Khan, was a new approach which allowed open floor plans rather than columns distributed throughout the interior to support building loads as had traditionally been done. The World Trade Center towers utilized high-strength, load-bearing perimeter steel columns called Vierendeel trusses that were spaced closely together to form a strong, rigid wall structure, supporting virtually all lateral loads such as wind loads, and sharing the gravity load with the core columns. The perimeter structure containing 59 columns per side was constructed with extensive use of prefabricated modular pieces each consisting of three columns, three stories tall, connected by spandrel plates.[25] The spandrel plates were welded to the columns to create the modular pieces off-site at the fabrication shop.[26] 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, transmitting shear stress between columns, 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.[25]

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The core of the towers housed the elevator and utility shafts, restrooms, three stairwells, and other support spaces. The core of each tower was a rectangular area 87 by 135 feet (27 by 41 m) and contained 47 steel columns running from the bedrock to the top of the tower. The large, column-free space between the perimeter and core was bridged by prefabricated floor trusses. The floors supported their own weight as well as live loads, providing lateral stability to the exterior walls and distributing wind loads among the exterior walls.[27] The floors consisted of 4 inch (10 cm) thick lightweight concrete slabs laid on a fluted steel deck. A grid of lightweight bridging trusses and main trusses supported the floors. The trusses connected to the perimeter at alternate columns and were on 6 foot 8 inch (2.03 m) centers. The top chords of the trusses were bolted to seats welded to the spandrels on the exterior side and a channel welded to the core columns on the interior side. The floors were connected to the perimeter spandrel plates with viscoelastic dampers which helped reduce the amount of sway felt by building occupants. The trusses supported a 4-inch (100 mm) thick lightweight concrete floor slab with shear connections for composite action.[28]

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Hat trusses (or "outrigger truss") located from the 107th floor to the top of the buildings were designed to support a tall communication antenna on top of each building.[29] Only 1 WTC (north tower) actually had an antenna fitted; it was added in 1978.[30] The truss system consisted of six trusses along the long axis of the core and four along the short axis. This truss system allowed some load redistribution between the perimeter and core columns and supported the transmission tower.[31]

The tube frame design using steel core and perimeter columns protected with sprayed-on fire resistant material created a relatively lightweight structure that would sway more in response to the wind compared to traditional structures such as the Empire State Building that have thick, heavy masonry for fireproofing of steel structural elements.[32] During the design process, wind tunnel tests were done to establish design wind pressures that the World Trade Center towers could be subjected to and structural response to those forces.[33] Experiments also were done to evaluate how much sway occupants could comfortably tolerate, however, many subjects experienced dizziness and other ill effects.[34] One of the chief engineers Leslie Robertson worked with Canadian engineer Alan G. Davenport to develop viscoelastic dampers to absorb some of the sway. These viscoelastic dampers, used throughout the structures at the joints between floor trusses and perimeter columns along with some other structural modifications, reduced the building sway to an acceptable level.[35]

[Jeannette Latoria]

Fill excavated from the site of the World Trade Center, seen here during construction in 1971, later provided the foundation for the World Financial Center and Battery Park City, which rose to the left of the World Trade Center in this image, on the other side of West St.

[Jeannette Latoria]

Construction

In March 1965, the Port Authority began acquiring property at the World Trade Center site.[36] Demolition work began on March 21, 1966 to clear thirteen square blocks of low rise buildings in Radio Row for construction of the World Trade Center.[37] Groundbreaking for the construction of the World Trade Center took place on August 5, 1966.[38]

The site of the World Trade Center was located on landfill with the bedrock located 65 feet (20 m) below.[39] In order to construct the World Trade Center, it was necessary to build the "bathtub" with a slurry wall along the West Street side of the site, serving to keep water from the Hudson River out.[40] The slurry method devised by Port Authority’s chief engineer, John M. Kyle, Jr., involved digging a trench, and as excavation proceeded, filling the space with a "slurry" mixture composed of bentonite which plugged holes and kept water out. When the trench was dug out, a steel cage was inserted and concrete was poured in, forcing the "slurry" out. It took fourteen months for the slurry wall to be completed; it was necessary before excavation of material from the interior of the site could begin.[41] The 1.2 million cubic yards (917,000 m³) of material excavated were used to expand the Manhattan shoreline across West Street to form Battery Park City (along with other fill and dredge material.)[42][43]

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In January 1967, the Port Authority awarded $74 million in contracts to various steel suppliers, and Karl Koch was hired to erect the steel.[44] Tishman Realty & Construction was hired in February 1967 to oversee construction of the project.[45] Construction work began on the North Tower in August 1968; construction on the South Tower was underway by January 1969.[46] The original Hudson Tubes, carrying PATH trains into Hudson Terminal, remained in service as elevated tunnels during the construction process until 1971 when a new PATH station opened.[47]

The topping out ceremony of 1 WTC (North Tower) took place on December 23, 1970, while 2 WTC's ceremony (South Tower) occurred later on July 19, 1971.[46] The first tenants moved into the North Tower in December 1970; the South Tower accepted tenants in January 1972.[48] When the World Trade Center twin towers were completed, the total costs to the Port Authority had reached $900 million.[49] The ribbon cutting ceremony was on April 4, 1973.[50]