Building the World Trade Center

[Jeannette Latoria]

 Arrangement of World Trade Center buildings at the site pre-9/11.

[Jeannette Latoria]

Original architectural/engineering model of the World Trade Center, on exhibit at the Skyscraper Museum in New York City.

[Jeannette Latoria]

Structural design

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 Port Authority required architects and structural engineers to follow the New York City building codes. At the time when the World Trade Center was planned, new building codes were being devised to replace the 1938 version that was still in place. The structural engineers ended up following draft versions of the new 1968 building codes, which incorporated "advanced techniques" in building design.

The World Trade Center towers included many structural engineering innovations in skyscraper design and construction, which allowed the buildings to reach new heights and become the tallest in the world. Traditionally, skyscrapers used a skeleton of columns distributed throughout the interior to support building loads, with interior columns disrupting the floor space. The tube-frame concept was a major innovation, allowing open floor plans and more space to rent. The buildings used high-strength, load-bearing perimeter steel columns called Vierendeel trusses that were spaced closely together to form a strong, rigid wall structure. There were 59 perimeter columns, narrowly spaced on each side of the buildings. These were designed to provide support for virtually all lateral loads (such as wind loads) and to share the gravity loads with the core columns. Structural analysis of major portions of the World Trade Center were computed on an IBM 1620.

[Jeannette Latoria]

The perimeter structure was constructed with extensive use of prefabricated modular pieces, which consisted of three columns, three stories tall, connected by spandrel plates. The perimeter columns had a square cross section, 14 inches (36 cm) on a side, and were constructed of welded steel plate. The thickness of the plates and grade of structural steel varied over the height of the tower, ranging from 36,000 to 100,000 pounds per square inch (260 to 670 MPa). The strength of the steel and thickness of the steel plates decreased with height because they were required to support lesser amounts of building mass on higher floors. The tube-frame design required 40 percent less structural steel than conventional building designs. From the 7th floor to the ground level, and down to the foundation, the columns were spaced 10 feet (3 m) apart. All columns were placed on bedrock, which, unlike that in Midtown Manhattan, where the bedrock is shallow, is at 65–85 feet (20–26 m) below the surface.

The spandrel plates were welded to the columns to create the modular pieces off-site 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. 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.

The building's core 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 columns tapered with height, and consisted of welded box-sections at lower floors and rolled wide-flange sections at upper floors. The structural core in 1 WTC was oriented with the long axis east to west, while that of 2 WTC was oriented north to south. All elevators were located in the core. Each building had three stairwells, also in the core, except on the mechanical floors where they were located outside the core.

[Jeannette Latoria]

[Jeannette Latoria]

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, provided lateral stability to the exterior walls, and distributed wind loads among the exterior walls. 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 had a span of 60 feet (18.2 m) in the long-span areas and 35 feet (11 m) in the short span area. 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 thick (10 cm) lightweight concrete floor slab, with shear connections for composite action.

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. Only 1 WTC (north tower) actually had an antenna fitted, which was added in 1978. 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.

[Jeannette Latoria]

Typical WTC architectural floor plan

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Schematic of composite floor truss system.

[Jeannette Latoria]

[Jeannette Latoria]

Wind effects

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. During the design process, wind tunnel tests were done at Colorado State University and at the National Physical Laboratory in the U.K. to establish design wind pressures that the World Trade Center towers could be subjected to and structural response to those forces. Experiments also were done to evaluate how much sway occupants could tolerate. Subjects were recruited for "free eye exams," while the real purpose of the experiment was to subject them to simulated building sway and find out how much they could comfortably tolerate. Many subjects did not respond well, experiencing dizziness and other ill effects. The chief engineer 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.

[Jeannette Latoria]

Fire protection

Sprayed-fire resistant materials (SFRMs) were used to protect some structural steel elements in the towers, including all floor trusses and beams. Gypsum wallboard in combination with SFRMs, or in some cases gypsum wallboard alone, was used to protect core columns. Vermiculite plaster was used on the interior-side and SFRMs on the other three sides of the perimeter columns for fire protection. The 1968 New York City building codes were more lenient in some aspects of fire protection, such as allowing three exit stairwells in the World Trade Center towers, instead of six as required under older building codes.

The 1968 New York City building codes did not require sprinklers for high-rise buildings, except for underground spaces. In accordance with building codes, sprinklers were originally installed only in the underground parking structures of the World Trade Center. Following a major fire in February 1975, the Port Authority decided to start installing sprinklers throughout the buildings. By 1993, nearly all of Tower 2 [South Tower] and 85 percent of Tower 1 had sprinklers installed, and the entire complex was retrofitted by 2001.

[Jeannette Latoria]

World Trade Center in 1973

[Jeannette Latoria]

Construction

In March 1965, the Port Authority began acquiring property at the World Trade Center site. The Ajax Wrecking and Lumber Corporation was hired for the demolition work, which began on March 21, 1966 to clear the site for construction of the World Trade Center.

Groundbreaking was on August 5, 1966, marking the beginning of construction of the World Trade Center's foundations. The site of the World Trade Center was located on landfill, with the bedrock located 65 feet (20 m) below. 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, to keep water from the Hudson River out. This method was used in place of conventional dewatering methods because lowering the groundwater table would cause large settlements of nearby buildings not built on deep foundations. The slurry method involves digging a trench, and as excavation proceeds, filling the space with a "slurry" mixture, composed 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. Towards the end of 1966, work began on building the slurry wall, led by Montreal-based Icanda, a subsidiary of an Italian engineering firm, Impresa Costruzioni Opere Specializzate (I.C.O.S.). It took fourteen months for the slurry wall to be completed, which was necessary before excavation of material from the interior of the site could begin. The original Hudson Tubes, which carried PATH trains into Hudson Terminal, remained in service as elevated tunnels until 1971 when a new PATH station was built.

Construction work began on the North Tower in August 1968 with construction beginning on the South Tower by January 1969. In January 1967, $74 million in contracts were awarded to the Pacific Car and Foundry Company, Laclede Steel Company, Granite City Steel Company, and Karl Koch Erecting Company to supply steel for the project. The Port Authority chose to use many different steel suppliers, bidding on smaller portions of steel, rather than buy larger amounts from a single source such as Bethlehem Steel or U.S. Steel as a cost-saving measure. Karl Koch was also hired to do all the work of erecting the steel, and a contract for work on the aluminum facade was awarded to the Aluminum Company of America. Tishman Realty & Construction was hired in February 1967 to oversee construction of the project.

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Extensive use of prefabricated parts for the perimeter framing and floor truss systems helped speed up the construction process and reduce costs, while providing greater quality control. Steel components were freighted into a Penn Central yard in Jersey City. From there, they were brought in early morning hours through the Holland Tunnel to the construction site, and lifted into place by a crane. Larger pieces were brought to the construction site by tugboats. A special type of crane, suitable for constructing such tall buildings, that used hydraulics to lift components and provided its own power was used in construction of the World Trade Center. The Favco Standard 2700 Crane, manufactured by Favelle Mort Ltd. of New South Wales, Australia was informally called a "kangaroo crane".

In 1970, tugboat workers went on strike, halting the transport of material to the construction site. The Port Authority attempted other means of transporting material, including via helicopter. When this method was tried, the helicopter lost its load of steel into the Kill Van Kull. Some other mishaps occurred during the construction process, including disruption of telephone service in Lower Manhattan when telephone cables were crushed by pile drivers. On March 16, 1970, a propane gas explosion injured six workers when a truck hit a propane tank. In all, 60 workers were killed in construction accidents while the World Trade Center was being built.

[Jeannette Latoria]

Construction of 1 World Trade Center (North Tower) was completed on December 23, 1970, with 2 World Trade Center (South Tower) completed on July 19, 1971. The first tenants moved into the North Tower in December 1970, and into the South Tower in January 1972.[84] The buildings were dedicated on April 4, 1973; Tobin, who had resigned the year before, was absent from the ceremonies.

Building the World Trade Center involved excavating 1.2 million cubic yards (917,000 m³) of material. Rather than transporting this material at great costs out to sea or to landfills in New Jersey, the fill material was used to expand the Manhattan shoreline across West Street. Work to demolish the piers began on January 5, 1967, including Pier 7 to Pier 11 which were all constructed around 1910. The demolition work moved forward, despite conflicts between David Rockefeller, Governor Nelson Rockefeller and Mayor John Lindsay regarding plans for Battery Park City. Landfill material from the World Trade Center was used to add land, and a cellular cofferdam was constructed to retain the material. The result was a 700 foot (210 m) extension into the Hudson River, running six blocks or 1,484 feet (450 m). This land was a "gift" to New York City, allowing more tax-generating developments in Battery Park City.

The original estimates put forth by the Port Authority had the costs for construction of the World Trade Center at $350 million—an optimistic figure. In December 1966, the Port Authority announced increased cost estimates, bringing the estimated total to $575 million. This announcement brought criticism of the project from private real estate developers, The New York Times, and others in New York City. The critics charged that the Port Authority figure was an unrealistically low estimate, and they estimated the project would end up costing $750 million. When the World Trade Center twin towers were completed, the total costs to the Port Authority had reached $900 million. The project was financed through tax-exempt bonds issued by the Port Authority.