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The World Trade Center was originally planned to be built on the east side of Lower Manhattan, but the New Jersey and New York state governments, which oversee the Port Authority, could not agree on this location. The Port Authority hired architect Minoru Yamasaki , who came up with the specific idea for twin towers.

The towers were designed as framed tube structures , which provided tenants with open floor plans, uninterrupted by columns or walls. This was accomplished using numerous closely spaced perimeter columns to provide much of the strength to the structure, along with gravity load shared with the core columns.

The elevator system, which made use of sky lobbies and a system of express and local elevators, allowed substantial floor space to be freed up for use as office space by making the structural core smaller. The design and construction of the World Trade Center, most centrally its twin towers, involved many other innovative techniques, such as the slurry wall for digging the foundation , and wind tunnel experiments.

Extensive use of prefabricated components helped to speed up the construction process. Four other low-level buildings were constructed as part of the World Trade Center in the early s, and the complex was mostly complete by A seventh building, 7 World Trade Center , was opened in In , Austin J. Tobin became the Executive Director of the Port Authority , beginning a year career during which he oversaw the planning and development of the World Trade Center.

In , a year after the war formally ended, the New York State Legislature passed a bill that called for a "world trade center" to be established. This trade center would increase New York City's role in transatlantic trade. Dewey to develop plans for the project. Meanwhile, the Financial District of Lower Manhattan was left out of the economic boom of financial industries there.

The writer Paul Goldberger states that the Financial District, in particular, was devoid of "almost any kind of urban amenity", including entertainment, cultural hubs, or housing. David Rockefeller suggested that the Port Authority would be a logical choice for taking on the project [14] because it had experience with similar large engineering projects, and also because the Port Authority, rather than Rockefeller, would be paying for the complex's construction.

Tobin remarked that the proposed project should be the World Trade Center, and not just a generic "world trade center". The report recommended building a trade center along the waterfront to facilitate commerce within the Port of New York.

The States of New York and New Jersey also needed to approve the project, given their control and oversight role of the Port Authority. Even though Nelson Rockefeller had been warned of the hastiness of the deal, owing to the fact that it highlighted the price disparity between the two agreements, the New York state legislature approved the bill anyway. After taking a walk around the Radio Row neighborhood, Schachter concluded that the site could be used for the World Trade Center.

This would carry multiple benefits: The new location not only was closer to New Jersey, but also was able to utilize the unused air rights above the Hudson Terminal: Tozzoli to lead the new office.

The new site constituted a mostly trapezoidal plot of land, with an extension on the north that resembled a "cork". This would become the site of 7 World Trade Center , although that building was not added until much later. The two blocks to the west and east of the "cork" were occupied by the New York Telephone Company Building and the Federal Building, respectively. The site for the World Trade Center was the location of Radio Row, which was home to commercial or industrial tenants, over one thousand offices, many small businesses, and approximately residents.

Small Businessman" in a mock funeral. The Port Authority opened an office to assist tenants with relocation; although this offer was accepted by some tenants, it was protested by others. Nadel believed that this would allow the Port Authority to convince individual tenants to move out, thus causing an exodus of merchants from the area.

However, when the meeting did occur in mid-June , Nadel immediately rejected Tobin's proposal to give him a storefront space in the World Trade Center. The dispute with local business owners went through the court system to the New York State Court of Appeals.

Although several lower courts refused to hear the merchants' cases, they also ruled that the World Trade Center did not serve a "public purpose" and that the plan had cause harm to Radio Row merchants, contrary to what Tobin had claimed. However, in public, he acted as though the judges had already ruled in his favor on the constitutional question.

Under the state law, the Port Authority was required to assist business owners in relocating, something it had been doing since First, the agency approached the United States Customs Service because the Customs Service had been dissatisfied with its current headquarters, the Alexander Hamilton U. Custom House on the tip of Lower Manhattan. The Port Authority then solicited the New York state government, since there were too few private companies willing to move to the World Trade Center.

After considering these facts, Tobin and Tozzoli determined that the only way to make the World Trade Center appealing to private entities was to make the trade center the world's largest. Fearing controversy, Tobin kept this plan secret from the public. Tozzoli dismissed the three planners who had worked on the original World Trade Center study, due to their lack of creative designs for the World Trade Center complex. Tozzoli had a general idea of what should be in the trade center, but had not yet devised the specific details of the design.

Yamasaki remarked that the "obvious alternative, a group of several large buildings, would have looked like a housing project.

At the time, it was not considered economical to build an office building with more than 80 floors. Another major limiting factor in the buildings' designs was elevators. As the building got taller, more elevators were needed to service the building, which required more elevator banks that in turn took up space. Thus, the local elevators can be stacked within the same elevator shaft. Yamasaki's final design for the World Trade Center was unveiled to the public on January 18, , with an eight-foot model.

The plaza was modelled after Mecca , incorporating features such as a vast delineated square, a fountain , and a radial circular pattern. Yamasaki described the plaza as "a mecca, a great relief from the narrow streets and sidewalks of the Wall Street area.

The World Trade Center design brought criticism of its aesthetics from the American Institute of Architects and other groups. Monti, the Port Authority's Chief Engineer, oversaw the project. 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 version that was still in place.

The structural engineers ended up following draft versions of the new 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 buildings used high-strength, load-bearing perimeter steel columns called Vierendeel trusses. Although the columns themselves were lightweight, they were spaced closely together, forming a strong, rigid wall structure.

The perimeter columns were designed to provide support for virtually all lateral loads such as wind loads and to share 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 by spandrel plates.

The spandrel plates were welded to the columns to create the modular pieces off-site at the fabrication shop. 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 North Tower's structural core was oriented with the long axis east to west, while the South Tower's 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 the two outside stairwells temporarily left the core in order to avoid the express elevator machine rooms, and then rejoined the core by means of a transfer corridor.

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 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. Hat trusses or "outrigger truss" located from the th floor to the top of the buildings were designed to support a tall communication antenna on top of each building.

This truss system allowed some load redistribution between the perimeter and core columns and supported the transmission tower. 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.

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.

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. The structural engineers on the project also considered the possibility that an aircraft could crash into the building.

In July , a B bomber that was lost in the fog had crashed into the 78th and 79th floors of the Empire State Building. A year later, another airplane crashed into the 40 Wall Street building, and there was another close call at the Empire State Building.

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.

More fireproofing was added after a fire in February that spread to six floors before being extinguished. The Port Authority was in the process of replacing it, but replacement had been completed on only 18 floors in the North Tower, including all the floors affected by the aircraft impact and fires on September 11, [] and on 13 floors in the South Tower, although only three of these floors 77, 78, and 85 were directly affected by the aircraft impact.

The 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.

By , nearly all of the South Tower and 85 percent of the North Tower had sprinklers installed [] and the entire complex was retrofitted by Even once the agreement between the states of New Jersey, New York, and the Port Authority had been finalized in , the World Trade Center plan faced continued controversy.

By , by which time the intended scale of the scheme had been made public with plans for twin story towers, private real estate developers and members of the Real Estate Board of New York also expressed concerns about the World Trade Center's much-"subsidized" office space going on the open market, competing with the many vacancies in the private sector.

The city and developers used a myriad of arguments to stall construction for two years. Wagner's successor, John Lindsay , and the New York City Council raised concerns about the limited extent that the Port Authority involved the city in the negotiations and deliberations.

Negotiations between the New York City government and the Port Authority were centered on tax issues: This dispute went on for two years, during which time the projected price of the World Trade Center increased sharply.

By July , neither the city or the Port Authority were able to come to an agreement.