World Trade Center

The original World Trade Center was a complex of seven buildings featuring landmark twin towers in Lower Manhattan, New York City. The complex opened on April 4, 1973, and was destroyed in 2001 during the September 11 attacks. The site is being rebuilt with five new skyscrapers and a memorial to the casualties of the attacks. At the time of their completion, 1 and 2 World Trade Center were the tallest buildings in the world, surpassing the Empire State Building, also in Manhattan.

The cost for the construction was $400 million ($0 as of 2011). The complex was designed by Minoru Yamasaki in the early 1960s using a tube-frame structural design for the twin 110-story towers. To gain 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 (1) was completed in December 1972 and the South Tower (2) was finished in July 1973. The construction project involved excavating a large amount of material, which was later used as landfill to build Battery Park City on the west side of Lower Manhattan.

The complex 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. The Windows on the World restaurant was located on the 106th and 107th floors of 1 World Trade Center (the North Tower) while the Top of the World observation deck was located on the 107th floor of 2 World Trade Center (the South Tower). Other World Trade Center buildings included the Marriott World Trade Center; 4 World Trade Center; 5 World Trade Center; 6 World Trade Center, which housed the United States Customs. All of these buildings were built between 1975 and 1981. The final building constructed was 7 World Trade Center, which was built in 1985. The second King Kong was filmed in 1976 with some scenes mentioning and seeing the World Trade Center. 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.

On the morning of September 11, 2001, Al-Qaeda-affiliated hijackers flew two 767 jets into the complex, one into each tower, in a coordinated terrorist attack. After burning for 56 minutes, the South Tower (2) collapsed, followed a half-hour later by the North Tower (1), with the attacks on the World Trade Center resulting in 2,752 deaths. 7 World Trade Center collapsed later in the day and the other buildings, although they did not collapse, had to be demolished because they were damaged beyond repair. 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 1776 ft One World Trade Center, three office towers along Church Street and a memorial designed by Michael Arad.

Planning and construction
The idea of establishing a World Trade Center in New York City was first proposed in 1943. The New York State Legislature passed a bill authorizing New York Governor Thomas E. Dewey to begin developing plans for the project but the plans were put on hold in 1949. 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.

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

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. 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. With the new location and Port Authority acquisition of the H&M Railroad, New Jersey, agreed to support the World Trade Center project.

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. In subsequent years, the payments would rise as the real estate tax rate increased.

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. Yamasaki devised the plan to incorporate twin towers; Yamasaki's original plan called for the towers to be 80 stories tall. To meet the Port Authority's requirement for 10000000 sqft) of office space, the buildings would each have to be 110 stories tall.



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. Yamasaki and the engineers decided to use a new system with two "sky lobbies"—floors where people could switch from a large-capacity express elevator to a local elevator that goes to each floor in a section. This allowed the design to stack local elevators 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 elevator shafts. Altogether, the World Trade Center had 95 express and local elevators. 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.

Yamasaki's design for the World Trade Center, unveiled to the public on January 18, 1964, called for a square plan approximately 208 ft in dimension on each side. The buildings were designed with narrow office windows 18 in wide, which reflected Yamasaki's fear of heights as well as his desire to make building occupants feel secure. Yamasaki's design included building facades sheathed in aluminum-alloy. 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.

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 acre superblock.

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. 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. The tube-frame design, earlier introduced by Fazlur Khan, was a new approach that allowed more open floor plans than the traditional design that distributed columns throughout the interior to support building loads. The World Trade Center towers used 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. The spandrel plates were welded to the columns to create the modular pieces off-site at the fabrication shop. 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 core of the towers housed the elevator and utility shafts, restrooms, three stairwells, and other support spaces. The core –a combined steel and concrete structure– 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. The floors consisted of 4 in 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 that helped reduce the amount of sway felt by building occupants.

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; it 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.

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 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 comfortably tolerate, however, many subjects experienced dizziness and other ill effects. 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.

Construction
In March 1965, the Port Authority began acquiring property at the World Trade Center site. 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. Groundbreaking for the construction of the World Trade Center took place on August 5, 1966.

The site of the World Trade Center was located on landfill with the bedrock located 65 ft below. To construct the World Trade Center, it was necessary to build a "bathtub" with a slurry wall around the West Street side of the site, to keep water from the Hudson River out. The slurry method selected 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 and water, which plugged holes and kept groundwater 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. The 1.2 million cubic yards (917,000 m3) of material excavated were used (along with other fill and dredge material) to expand the Manhattan shoreline across West Street to form Battery Park City.

In January 1967, the Port Authority awarded $74 million in contracts to various steel suppliers, and Karl Koch was hired to erect the steel. Tishman Realty & Construction was hired in February 1967 to oversee construction of the project. Construction work began on the North Tower in August 1968; construction on the South Tower was underway by January 1969. 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.

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. The first tenants moved into the North Tower in December 1970; the South Tower accepted tenants in January 1972. When the World Trade Center twin towers were completed, the total costs to the Port Authority had reached $900 million. The ribbon cutting ceremony was on April 4, 1973.

Criticism
Plans to build the World Trade Center were controversial. The site for the World Trade Center was the location of Radio Row, home to hundreds of commercial and industrial tenants, property owners, small businesses, and approximately 100 residents, many of whom fiercely resisted forced relocation. A group of small businesses affected filed an injunction challenging the Port Authority's power of eminent domain. The case made its way through the court system to the United States Supreme Court; the Court refused to accept the case.

Private real estate developers and members of the Real Estate Board of New York, led by Empire State Building owner Lawrence A. Wien, expressed concerns about this much "subsidized" office space going on the open market, competing with the private sector when there was already a glut of vacancies. Others questioned whether the Port Authority really ought to take on a project described by some as a "mistaken social priority."

The World Trade Center design brought criticism of its aesthetics from the American Institute of Architects and other groups. Lewis Mumford, author of The City in History and other works on urban planning, criticized the project and described it and other new skyscrapers as "just glass-and-metal filing cabinets." The twin towers' narrow office windows, only 18 in wide, were disliked by many for impairing the view from the buildings.

The trade center's "superblock," replacing a more traditional, dense neighborhood, was regarded by some critics as an inhospitable environment that disrupted the complicated traffic network typical of Manhattan. For example, in his book The Pentagon of Power, Lewis Mumford denounced the center as an "example of the purposeless giantism and technological exhibitionism that are now eviscerating the living tissue of every great city."

For many years, the immense Austin J. Tobin Plaza was often beset by brisk winds at ground level. In 1999, the outdoor plaza reopened after undergoing $12 million renovations, which involved replacing marble pavers with gray and pink granite stones, adding new benches, planters, new restaurants, food kiosks and outdoor dining areas.

North and South towers


With the construction of 7 World Trade Center in the 1980s, the World Trade Center had a total of seven buildings, but the most notable were the main two towers, which each were 110 stories tall, stood over 1350 ft high, and occupied about one acre (43,560 square feet) of the total 16 acre 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."

When completed in 1972, 1 World Trade Center (the North Tower) became the tallest building in the world for two years, surpassing the Empire State Building after a 40-year reign. The North Tower stood 1368 ft tall and featured a telecommunications antenna or mast that was added at the top of the roof in 1978 and stood 360 ft tall. With the 360 ft-tall antenna/mast, the highest point of the North Tower reached 1728 ft. 2 World Trade Center (the South Tower) became the second tallest building in the world when completed in 1973. The South Tower's rooftop observation deck was 1362 ft high and its indoor observation deck was 1310 ft high. The World Trade Center towers held the height record only briefly: Chicago's Sears Tower, finished in May 1973, reached 1450 ft at the rooftop.

Of the 110 stories, eight were set aside for technical services in mechanical floors Level B5/B6 (floors 7/8, 41/42, 75/76, and 108/109), which are four two-floor areas evenly spaced up the building. All the remaining floors were free for open-plan offices. Each floor of the towers had 40000 sqft of space for occupancy. Each tower had 3800000 sqft of office space. Altogether the entire complex of seven buildings had 11200000 sqft) of space.



Initially conceived as a complex dedicated to companies and organizations directly taking part in "world trade," they at first failed to attract the expected clientele. During the early years, various governmental organizations became key tenants of the World Trade Center including the State of New York. It was not until the 1980s that the city's perilous financial state eased, after which an increasing number of private companies—mostly financial firms tied to Wall Street—became tenants. During the 1990s, approximately 500 companies had offices in the complex including many financial companies such as Morgan Stanley, Aon Corporation, Salomon Brothers and the Port Authority itself. The basement concourse of the World Trade Center included The Mall at the World Trade Center along with a PATH station. The North Tower became the home of the corporate headquarters of Cantor Fitzgerald, and it also became the headquarters of the Port Authority of New York and New Jersey.

Electrical service to the towers was supplied by Consolidated Edison (ConEd) at 13,800 volts. This service passed through the World Trade Center Primary Distribution Center (PDC) and sent up through the core of the building to electrical substations located on the mechanical floors. The substations "stepped" the 13,800 primary voltage down to 480/277 volt secondary power and further to 120/208 volt general power and lighting service. The complex also was served by emergency generators located in the sublevels of the towers and on the roof of 5 WTC.

The 110th floor of 1 World Trade Center (the North Tower) housed radio and television transmission equipment. The roof of 1 WTC contained a vast array of transmission antennas including the 360 ft (approx 110 m) center antenna mast, rebuilt in 1999 by Dielectric Inc. to accommodate DTV. 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 from the WTC Operations Control Center (OCC) located in the B1 level of 2 WTC.

Top of the World observation deck


Although most of the space in the World Trade Center complex was off-limits to the public, the South Tower featured an indoor and outdoor public observation area called Top of the World Trade Center Observatories on its 107th and 110th floors. Visitors would pass through security checks added after the 1993 World Trade Center bombing, then were whisked to the 107th floor indoor observatory at a height of 1310 ft. The columns on each face of the building were narrowed on this level to allow 28 inches of glass between them. The Port Authority renovated the observatory in 1995, then leased it to Ogden Entertainment to operate. Attractions added to the observation deck included a simulated helicopter ride around the city. The 107th floor food court was designed with a subway car theme and featured Sbarro and Nathan's Famous Hot Dogs. Weather permitting, visitors could take two short escalator rides up from the 107th floor viewing area to an outdoor viewing platform on the 110th floor at a height of 1377 ft. On a clear day, visitors could see up to 50 mi. An anti-suicide fence was placed on the roof itself, with the viewing platform set back and elevated above it, requiring only an ordinary railing and leaving the view unobstructed, unlike the observation deck of the Empire State Building.

Windows on the World restaurant
The North Tower had a restaurant on its 106th and 107th floors called Windows on the World, which opened in April 1976. The restaurant was developed by Joe Baum at a cost of more than $17 million. Aside from the main restaurant, two offshoots were located at the top of the North Tower: "Hors d'Oeuvrerie" (offered a Danish smorgasbord during the day and sushi in the evening) and "Cellar in the Sky" (a small wine bar). Windows on the World also had a wine school program run by Kevin Zraly. Windows on the World was closed following the 1993 World Trade Center bombing. Upon reopening in 1996, Hors d'Oeuvrerie and Cellar in the Sky were replaced with the "Greatest Bar on Earth" and "Wild Blue." In 2000, its last full year of operation, Windows on the World reported revenues of $37 million, making it the highest-grossing restaurant in the United States.

Other buildings
Five smaller buildings stood around the 16 acre block. One was the 22-floor hotel, which opened in 1981 as the Vista Hotel, and in 1995 became the Marriott World Trade Center (3 WTC) at the southwest corner of the site. Three low-rise buildings (4 WTC, 5 WTC, and 6 WTC) in the same hollow tube design as the towers also stood around the plaza. 6 World Trade Center, at the northwest corner, housed the United States Customs Service and the U.S. Commodities Exchange. 5 World Trade Center was located at the northeast corner above the PATH station and 4 World Trade Center was at the southeast corner. In 1987, a 47-floor office building called 7 WTC was built north of the block. Beneath the World Trade Center complex was an underground shopping mall, which in turn had connections to various mass transit facilities including the New York City Subway system and the Port Authority's own PATH trains connecting Manhattan to Jersey City, Hoboken, and Newark.

One of the world's largest gold depositories was stored underneath the World Trade Center, owned by a group of commercial banks. The 1993 bomb detonated close by the vault held. Seven weeks after the September 11 attacks, $230 million in precious metals was removed from basement vaults of 4 WTC, which included 3,800 100-Troy-ounce registered gold bars and 30,000 1,000-ounce silver bars.

Life and events
On a typical weekday 50,000 people worked in the towers with another 200,000 passing through as visitors. The complex was so large that it had its own zip code: 10048. The towers offered expansive views from the observation deck atop the South Tower and the Windows on the World restaurant on top of the North Tower. The Twin Towers became known worldwide, appearing in numerous movies and television shows as well as on postcards and other merchandise, and became seen as a New York icon, in the same league as the Empire State Building, Chrysler Building and the Statue of Liberty.

French high wire acrobatic performer Philippe Petit walked between the towers on a tightrope in 1974, as shown in the documentary film Man on Wire. Brooklyn toymaker George Willig scaled the south tower in 1977.

In 1983, on Memorial Day, high-rise firefighting and rescue advocate Dan Goodwin successfully climbed the outside of the WTC's North Tower. His stunt was meant to call attention to the inability to rescue people potentially trapped in the upper floors of skyscrapers.

The 1995 PCA world chess championship was played on the 107th floor of the South Tower.

In January 1998, Mafia member Ralph Guarino, who had gained maintenance access to the World Trade Center, arranged a three-man crew for a heist that netted over $2 million from a Brinks delivery to the eleventh floor of the World Trade Center.

February 13, 1975 fire
On February 13, 1975, a three-alarm fire broke out on the 11th floor of the North Tower. Fire spread through the core to the 9th and 14th floors by igniting the insulation of telephone cables in a utility shaft that ran vertically between floors. Areas at the furthest extent of the fire were extinguished almost immediately and the original fire was put out in a few hours. Most of the damage was concentrated on the 11th floor, fueled by cabinets filled with paper, alcohol-based fluid for office machines, and other office equipment. Fireproofing protected the steel from melting and there was no structural damage to the tower. Other than the damage caused by the fire, a few floors below suffered water damage from the extinguishing of the fires above. At that time, the World Trade Center had no Fire sprinkler systems.

February 26, 1993 bombing
On February 26, 1993, at 12:17 p.m., a Ryder truck filled with 1500 lb of explosives, planted by Ramzi Yousef, detonated in the underground garage of the North Tower. The blast opened a 100 foot (30 m) hole through five sublevels with the greatest damage occurring on levels B1 and B2 and significant structural damage on level B3. Six people were killed and 50,000 other workers and visitors were left gasping for air within the 110 story towers. Many people inside the North Tower were forced to walk down darkened stairwells that contained no emergency lighting, some taking two hours or more to reach safety. Yousef fled to Pakistan after the bombing but was arrested in Islamabad in February 1995, and was extradited back to the United States to face trial. Sheikh Omar Abdel Rahman was convicted in 1996 for involvement in the bombing and other plots. Yousef and Eyad Ismoil were convicted in November 1997 for their carrying out the bombing. Four others had been convicted in May 1994 for their involvement in the 1993 bombing. According to a presiding judge, the conspirators' chief aim at the time of the attack was to destabilize the north tower and send it crashing into the south tower, toppling both landmarks.

Following the bombing, floors that were blown out needed to be repaired to restore the structural support they provided to columns. The slurry wall was in peril following the bombing and loss of the floor slabs that provided lateral support against pressure from Hudson River water on the other side. The refrigeration plant on sublevel B5, which provided air conditioning to the entire World Trade Center complex, was heavily damaged. Subsequent to the bombing, the Port Authority installed photoluminescent markings in the stairwells. The fire alarm system for the entire complex needed to be replaced because critical wiring and signaling in the original system was destroyed. As a memorial to the victims of the bombing of the tower, a reflecting pool was installed with the names of those who had been killed in the blast. However, the memorial was destroyed following the September 11 attacks. A new memorial is planned to be built honoring the victims of the bombing together with the 9/11 attacks on the new World Trade Center site.

Lease
In 1998, the Port Authority approved plans to privatize the World Trade Center. In 2001, the Port Authority sought to lease the World Trade Center to a private entity. Bids for the lease came from Vornado Realty Trust, a joint bid between Brookfield Properties Corporation and Boston Properties, and a joint bid by Silverstein Properties and The Westfield Group. By privatizing the World Trade Center, it would be added to the city's tax rolls and provide funds for other Port Authority projects. On February 15, 2001, the Port Authority announced that Vornado Realty Trust had won the lease for the World Trade Center, paying $3.25 billion for the 99-year lease. Vornado outbid Silverstein by $600 million though Silverstein upped his offer to $3.22 billion. However, Vornado insisted on last minute changes to the deal, including a shorter 39-year lease, which the Port Authority considered nonnegotiable. Vornado later withdrew and Silverstein's bid for the lease to the World Trade Center was accepted on April 26, 2001, and closed on July 24, 2001.

Destruction
On September 11, 2001, terrorists hijacked American Airlines Flight 11 and crashed it into the northern facade of the north tower at 08:46, impacting between the 93rd and 99th floors. Seventeen minutes later, a second team of terrorists crashed the similarly hijacked United Airlines Flight 175 into the south tower, impacting between the 77th and 85th floors. The damage caused to the north tower by Flight 11 destroyed any means of escape from above the impact zone, trapping 1,344 people. Flight 175 had a much more off-centered impact compared to Flight 11, and a single stairwell was left intact; however, only a few people managed to successfully pass through it before the tower collapsed. Although the south tower's floors of impact were lower, a smaller number, less than 700, were killed instantly or trapped. At 9:59 a.m., the south tower collapsed after burning for approximately 56 minutes due to fire, which caused steel structural elements, already weakened from the plane impact, to fail. The north tower collapsed at 10:28 a.m., after burning for approximately 102 minutes.

At 5:20 p.m. on September 11, 2001, 7 World Trade Center started to collapse with the crumble of the east penthouse, and it collapsed completely at 5:21 p.m. due to uncontrolled fires causing structural failure. 3 World Trade Center, a Marriott hotel, was destroyed during the collapse of the two towers. The three remaining buildings in the WTC plaza sustained heavy damage from debris and were ultimately demolished. The Deutsche Bank Building across Liberty Street from the World Trade Center complex was later condemned due to the uninhabitable toxic conditions inside; it was deconstructed, with work completed in early 2011. The Borough of Manhattan Community College's Fiterman Hall at 30 West Broadway was also condemned due to extensive damage in the attacks and is slated for deconstruction.

In the aftermath of the attacks, media reports suggested that tens of thousands might have been killed in the attacks, as on any given day upwards of 50,000 people could be inside the towers. Ultimately, 2,752 death certificates were filed relating to the 9/11 attacks, including one filed for Felicia Dunn-Jones, who was added to the official death toll in May 2007; Dunn-Jones died five months later from a lung condition linked to exposure to dust during the collapse of the World Trade Center. Two other victims were then added to the official death toll by the city medical examiner's office: Dr. Sneha Anne Philip, who was last seen the day before the attacks, and Leon Heyward, a man who developed lymphoma and subsequently died in 2008 as a result of dust ingestion during the events following the attacks to the Twin Towers. Cantor Fitzgerald L.P., an investment bank on the 101st–105th floors of One World Trade Center, lost 658 employees, considerably more than any other employer, while Marsh & McLennan Companies, located immediately below Cantor Fitzgerald on floors 93–101 (the location of Flight 11's impact), lost 295 employees, and 175 employees of Aon Corporation were killed. As well, 343 deaths were New York City firefighters, 84 were Port Authority employees, of whom 37 were members of the Port Authority Police Department, and another 23 were New York City Police Department officers. Ten years after the attacks, only 1,629 victims have been positively identified. Of all the people who were still in the towers when they collapsed, only 20 were pulled out alive, PAPD Officers William Jimeno and Sgt. John McLoughlin were survivors 18 and 19.

Rebuilding


The process of cleanup and recovery continued 24 hours a day over a period of eight months. Debris was transported from the World Trade Center site to Fresh Kills on Staten Island, where it was further sifted. On May 30, 2002, a ceremony was held to officially mark the end of the cleanup efforts. In 2002, ground was broken on construction of a new 7WTC building located just to the north of the main World Trade Center site. Since it was not part of the site master plan, Larry Silverstein was able to proceed without delay on the rebuilding of 7 World Trade Center, which was completed and officially opened in May 2006; this had been considered a priority since restoring the Consolidated Edison Cos. electrical substation in the building's lower floors was necessary to meet power demands of Lower Manhattan. While 7 World Trade Center was not part of the master plan for the Twin Towers site, Silverstein and Con Edison recognized that the rebuilding of 7 World Trade Center would have to be consistent with the master plan which was expected to re-open the street grid which had been blocked by the original World Trade Center super-block. As a result, the design for the new 7 World Trade Center allowed for the re-opening of Greenwich Street, which had been blocked by the original 7 World Trade Center. A temporary PATH station at the World Trade Center opened in November 2003; it will be replaced by a permanent station designed by Santiago Calatrava.

With the main World Trade Center site, numerous stakeholders were involved including Silverstein and the Port Authority, which in turn meant the Governor of New York State, George Pataki, had some authority. In addition, the victims' families, people in the surrounding neighborhoods, Mayor Michael Bloomberg, and others wanted input. Governor Pataki established the Lower Manhattan Development Corporation (LMDC) in November 2001 as an official commission to oversee the rebuilding process. The LMDC held a competition to solicit possible designs for the site. The Memory Foundations design by Daniel Libeskind was chosen as the master plan for the World Trade Center site. The plan included the 1776 ft Freedom Tower (now known as One World Trade Center) as well as a memorial and a number of other office towers. Out of the World Trade Center Site Memorial Competition, a design by Michael Arad and Peter Walker entitled Reflecting Absence was selected in January 2004.

On March 13, 2006, workers arrived at the World Trade Center site to remove remaining debris and start surveying work. This marked the official start of construction of the National September 11 Memorial & Museum, though not without controversy and concerns from some family members. In April 2006, the Port Authority and Larry Silverstein reached an agreement in which Silverstein ceded rights to develop the Freedom Tower and Tower Five in exchange for financing with Liberty Bonds for Towers Two, Three, and Four. On April 27, 2006, a ground-breaking ceremony was held for the Freedom Tower.

In May 2006, architects Richard Rogers and Fumihiko Maki were announced as the architects for Towers Three and Four, respectively. The final designs for Towers Two, Three and Four were unveiled on September 7, 2006. Tower Two, or 200 Greenwich Street, will have a roof height of 1254 ft and a 96 ft tripod spire for a total of 1350 ft. Tower Three, or 175 Greenwich Street will have a roof height of 1,155 feet (352 m) and an antenna height reaching 1255 ft. Tower Four, or 150 Greenwich Street, will have an overall height of 946 ft. On June 22, 2007, the Port Authority of New York and New Jersey announced that JP Morgan Chase will build Tower 5, a 42-story building on Site 5 currently occupied by the Deutsche Bank Building, and Kohn Pedersen Fox was selected as the architect for the building. Four renowned architects, including Spanish architect Santiago Calatrava, who designed the transit hub, One WTC designer David Childs of Skidmore, Owings and Merrill, and the famed British architect Norman Foster of Foster and Partners designed Tower Two and masterminded the iconic diamond design, will greatly enhance the street-level atmosphere of the rebuilt site. The majority of the projects will be complete between early 2013 to mid 2014.

As of August 2011, One World Trade Center is at 80 stories with glass up to the 54th floor, Tower Four is up around 38 stories with glass up to the 15th floor, and the former Deutsche Bank Building has been completely dismantled and the Port Authority is working on their Vehicle Security Center. The World Trade Center Transportation Hub's PATH hall is nearly complete, and the National September 11 Memorial & Museum is on track to open the plaza on the tenth anniversary of the attacks, on September 11, 2011. World Trade Center Tower Three and Two's foundations are becoming visible, and Tower Three will be completed in mid 2014 if Silverstein Properties can meet requirements set by the Port Authority, as they likely will. Tower Two will rise only to street level, and the actual Skyscraper will be completed when the economy improves. However construction probably will be complete on Tower Two by 2020, given that numerous American and Chinese companies are "very interested" in leasing space at the WTC. Publishing Giant Conde Nast has agreed to move its headquarters to One World Trade Center, and with this shift many more are expected to follow.

Controversy
The construction of 1 World Trade Center has been met with criticism, ranging from the design itself to the name change. Governor of New York George Pataki stated in 2003 that, "The Freedom Tower isn't going to be One World Trade Center, it's going to be the Freedom Tower."

WTC American flags
A small number of American flags which flew on or near the WTC during the attacks were recovered during the clean-up efforts. One was found at 5:30 a.m. on the morning of September 12, 2001, by New York City Police Sergeant Gerald Kane and Detective Peter Friscia. While assisting rescue teams at "Ground Zero," they noticed that the large American flag that once flew in front of the WTC at Church Street had been blown off the flagpole during the collapse of the buildings and was tangled upside down on a streetlight several feet away. The two men recruited a number of soldiers and firefighters in the area who hoisted a ladder to the top of the streetlight. Detective Friscia climbed the rungs of the ladder to the top, untangled and retrieved the flag and brought it down to the ground. Kerik later released the flag to NASA officials and it was transported aboard the space shuttle Endeavour (STS-108) as part of its December 5–17, 2001, mission to the International Space Station. On Flag Day, June 14, 2002, the American flag was returned to the people of New York City by Sean O'Keefe of NASA and Commander Dom Gorie and the crew members of the Endeavour, in a ceremony at the Rose Center at the American Museum of Natural History. The flag is secured and maintained by New York City's Commissioner of Records and is part of the annual 9/11 ceremony at Ground Zero.



Another flag, which had originally flown atop one of the towers, was recovered three days after the attacks and, owing to its poor condition, was turned over to the National Guard for its ceremonial destruction. The National Guard then discovered from a label that the flag was the property of the Port Authority, and instead of retiring the flag, returned it. It was then used during the memorial service of several first responders, and was flown during both the 2001 World Series and Super Bowl XXXVI. The flag was also used, controversially, in the Opening Ceremony of the 2002 Winter Olympics held in Salt Lake City. Initially the International Olympic Committee (IOC) refused to allow the flag to be used in ceremony, fearing it would be seen as too pro-American, overly patriotic, and create complications during future ceremonies. But an agreement was eventually reached between the event organizers and the IOC, which allowed the flag to be carried into Rice-Eccles Olympic Stadium by both U.S. athletes and 9/11 first responders. The flag was carried into the stadium in silence, after which the flag bearers paused in front of the Mormon Tabernacle Choir, allowing them to sing The Star-Spangled Banner, while a separate American flag was raised.

In popular culture
The World Trade Center was an iconic structure and has been featured in numerous films as well as appearing in many television shows, cartoons, comic books, video games and music videos.