Original building (1970–2001)
The first One World Trade Center under construction in 1971
The construction of the World Trade Center, of which the Twin Towers (One and Two World Trade Center) were the centerpieces, was conceived as an urban renewal project and spearheaded by David Rockefeller. The project was intended to help revitalize Lower Manhattan. The project was planned by the Port Authority of New York and New Jersey, which hired architect Minoru Yamasaki. Yamasaki came up with the idea of building twin towers. After extensive negotiations, the New Jersey and New York State governments, which supervise the Port Authority, consented to the construction of the World Trade Center at the Radio Row site, located in the lower-west area of Manhattan. To satisfy the New Jersey government, the Port Authority agreed to buy the bankrupt Hudson & Manhattan Railroad (renamed to Port Authority Trans-Hudson), which transported commuters from New Jersey to Lower Manhattan.
The towers were designed as framed tube structures, giving tenants open floor plans, unobstructed by columns or walls. This design was accomplished by using many closely spaced perimeter columns, providing much of the structure's strength, with the 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 used for office purposes by making the structural core smaller. The design and construction of the towers involved many other innovative techniques, such as wind tunnel experiments and the slurry wall for digging the foundation.
Construction of the North Tower (One World Trade Center) began in August 1966; extensive use of prefabricated components sped up the construction process. The first tenants moved into the North Tower in December 1970.
In the 1970s, four other low-level buildings were built as part of the World Trade Center complex. A seventh building was built in the mid-1980s.
Specifications and operations
After Seven World Trade Center was built in the 1980s, the World Trade Center complex had a total of seven buildings; however, the most notable ones were the main Twin Towers built in the 1970s—One World Trade Center was the North Tower, and Two World Trade Center was the South Tower. Each tower was over 1,350 feet (410 m) high, and occupied about 1 acre (0.40 ha) of the total 16 acres (6.5 ha) of the site's land. During a press conference in 1973, Yamasaki was asked, "Why two 110-story buildings? Why not one 220-story building?" His response was, "I didn't want to lose the human scale."
The original World Trade Center complex in March 2001; the original 1 WTC is the tower on the left with the spire
When it was topped out on December 4, 1970, One World Trade Center became the tallest building in the world, surpassing the Empire State Building, which had held the record for 40 years. The North Tower was 1,368 feet (417 m) tall, and in 1978, a telecommunications antenna was added to the top of the roof; by itself, the antenna was 360 feet (110 m) tall. With the 360-foot (110 m)-tall antenna, the highest point of the North Tower reached 1,728 ft (527 m). However, the tower only held its record until May 1973, when Chicago's Sears Tower (now Willis Tower), which was 1,450 feet (440 m) tall at the rooftop, was completed. At 110 floors, the World Trade Center towers had more floors than any other building at that time. This number was not surpassed until the construction of the Burj Khalifa (163 floors), which opened in 2010.
Of the 110 stories, eight were set aside as mechanical floors (floors 7/8, 41/42, 75/76, and 108/109), which were four two-floor areas that were spaced up the building in even intervals. All the remaining floors were open for tenants. Each floor of the tower had 40,000 square feet (3,700 m2) of available space. The North and South tower had 3,800,000 square feet (350,000 m2) of total office space. The entire complex of seven buildings had a combined total of 13,400,000 square feet (1,240,000 m2) of office space.
Lobby of Tower 1, looking south along the east side of the building, August 19, 2000
The complex initially failed to attract the expected clientele. During the early years, various governmental organizations became key tenants of the World Trade Center, such as the State of New York. In the 1980s, the city's perilous financial condition eased, after which an increasing number of private companies—mostly financial firms related to Wall Street—became tenants. During the 1990s, approximately 500 companies had offices in the complex, including financial companies such as Morgan Stanley, Aon Corporation, and Salomon Brothers. The basement concourse of the World Trade Center included The Mall at the World Trade Center, and a PATH station. The North Tower became the main corporate headquarters of Cantor Fitzgerald, and it also became the headquarters of the Port Authority of New York and New Jersey.
The tower's electrical service was supplied by Consolidated Edison (ConEd) at 13,800 volts. The electricity passed through the World Trade Center Primary Distribution Center (PDC), and was then sent up the building's core to electrical substations located on the mechanical floors. The substations lowered the 13,800 primary voltage to 480/277 volts, and the voltage was then further lowered to 208/120 volts for general power and lighting services. The complex was also served by emergency generators located in the sub-levels of the towers and on the roof of Five World Trade Center.
The 110th floor of One World Trade Center (the North Tower) housed radio and television transmission equipment. The roof of the North Tower contained a vast array of transmission antennas, including the 360 feet (110 m) center antenna mast, rebuilt by Dielectric Inc. to support DTV in 1999. The center mast contained the television signals for almost all NYC television broadcasters: WCBS-TV 2, WNBC-TV 4, WNYW 5, WABC-TV 7, WWOR-TV 9 Secaucus, WPIX 11, WNET 13 Newark, WPXN-TV 31 and WNJU 47 Linden. It also had four NYC FM broadcasters: WPAT-FM 93.1, WNYC 93.9, WKCR 89.9, and WKTU 103.5. Access to the roof was controlled by the WTC Operations Control Center (OCC), located in the B1 level of the South Tower. After the September 11 attacks of 2001, the broadcasting equipment for the radio and television stations was moved to the Empire State Building.
On a typical weekday, a combined total of 50,000 people worked in the North and South Towers, with another 140,000 passing through as visitors. The complex was so large that it had its own zip code: 10048. The Windows on the World restaurant, located on top of the North Tower, reported revenues of $37 million in 2000, making it the highest-grossing restaurant in the United States. The Twin Towers became known worldwide, appearing in movies, television shows, postcards, and other merchandise. The towers came to be seen as a New York City icon, much like the Empire State Building, the Chrysler Building, and the Statue of Liberty.
On February 13, 1975, a three-alarm fire broke out on the 11th floor of the North Tower. The fire spread through the core of the building to the 9th and 14th floors, as the insulation for telephone cables, located in a utility shaft that ran vertically between floors, had been ignited. Areas most affected by the fire were extinguished almost immediately, and the original fire was put out in a few hours. Most of the damage was on the 11th floor, where the fire was fueled by cabinets filled with paper, alcohol-based fluid for office machines, and other office equipment. Fireproofing protected the steel, and there was no structural damage to the tower. In addition to the fire damage on the 9th and 14th floors, water used to extinguish the fire damaged a few floors below. At the time, the World Trade Center complex had no fire sprinkler systems.
Underground damage from the 1993 bombing
The first terrorist attack on the World Trade Center occurred on February 26, 1993, at 12:17 p.m., when a Ryder truck filled with 1,500 pounds (680 kg) of explosives, planted by Ramzi Yousef, detonated in the underground garage of the North Tower. The blast resulted in a 100-foot (30 m) hole through five sublevels. The greatest damage was on levels B1 and B2, with significant structural damage on level B3. Six people were killed, and more than a thousand were injured, as 50,000 workers and visitors were inside the tower at the time. Many people inside the North Tower were forced to walk down darkened stairwells that had no emergency lighting, and some took two hours or more to reach safety.
September 11 attacks
Impact locations on One and Two World Trade Center
At 8:46 a.m. (EDT) on September 11, 2001, five hijackers affiliated with al-Qaeda crashed American Airlines Flight 11 into the northern facade of the North Tower between the 93rd and 99th floors. Seventeen minutes later, at 9:03 a.m. (EDT), a second group of terrorists crashed the hijacked United Airlines Flight 175 into the southern facade of the South Tower, striking between the 77th and 85th floors.
By 9:59 a.m. (EDT), the South Tower collapsed after burning for approximately 56 minutes. After burning for 102 minutes, the North Tower collapsed due to structural failure at 10:28 a.m. (EDT). When the North Tower collapsed, debris fell on the nearby 7 World Trade Center, damaging it and starting fires. The fires burned for hours, compromising the building's structural integrity. Seven World Trade Center collapsed at 5:21 p.m. (EDT).
Together with a simultaneous attack on the Pentagon in Arlington, Virginia, and a failed plane hijacking that resulted in a plane crash in Shanksville, Pennsylvania, the attacks resulted in the deaths of 2,996 people (2,507 civilians, 343 firefighters, 72 law enforcement officers, 55 military personnel, and the 19 hijackers). More than 90% of the workers and visitors who died in the towers had been at or above the points of impact. In the North Tower, 1,355 people at or above the point of impact were trapped, and died of smoke inhalation, fell, jumped from the tower to escape the smoke and flames, or were killed when the building eventually collapsed. One stairwell in the South Tower, Stairwell A, somehow avoided complete destruction, unlike the rest of the building. When Flight 11 hit, all three staircases in the North Tower above the impact zone were destroyed, thus making it impossible for anyone above the impact zone to escape. 107 people below the point of impact also died.