2 World Trade Center (1971–2001)

In 1961, the Port Authority of New York and New Jersey agreed to build the World Trade Center on the site of the Hudson Terminal in Lower Manhattan, New York City.^[10] On September 20, 1962, the Port Authority announced the selection of Minoru Yamasaki as lead architect and Emery Roth & Sons as associate architects.^[11] Yamasaki devised the plan to incorporate twin towers. His original plan called for the towers to be 80 stories tall,^[12] but to meet the Port Authority's requirement for 10,000,000 square feet (930,000 m²) of office space, the buildings would each have to be 110 stories tall.^[13] 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.^[12][14]

In March 1965, the Port Authority began acquiring property at the World Trade Center site.^[15] Demolition work began on March 21, 1966,^[16] and groundbreaking for the construction of the World Trade Center took place on August 5, 1966.^[17] In January 1967, the Port Authority awarded $74 million in contracts to various steel suppliers.^[18] Construction on the South Tower was under way by January 1969.^[19] The topping out ceremony for 2 WTC (the South Tower) occurred on July 19, 1971.^[19] The South Tower began accepting tenants in January 1972,^[20] and a ribbon cutting ceremony took place on April 4, 1973.^[21]

In 1981, the Port Authority announced a $45 million plan to install sprinklers throughout the World Trade Center, following a major fire that occurred at the North Tower in 1975.^[22]

On February 26, 1993, a Ryder truck filled with 1,500 pounds (680 kg) of explosives (planted by Ramzi Yousef) detonated in the North Tower's underground garage.^[23] 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 skyscrapers.^[24] Six people were killed and 1,042 others were injured in the attacks.^[25][26]

In February 2001, the Port Authority leased the entire World Trade Center complex to Vornado Realty Trust.^[27] However, Vornado insisted on last minute changes to the deal,^[28] and the next-highest bidder, Silverstein Properties, signed a lease for the complex on July 24, 2001.^[29]

A 360° panoramic view of New York City and surrounding areas from the Top of the World observation deck, circa August 2001. North is near center and the North Tower is left.

Although most of the space in the World Trade Center complex was off-limits to the public, the South Tower featured a public glass-enclosed observation deck on the 107th floor called Top of the World and an open-air deck with the height of 110 stories.^[30][31][32] The observation deck opened in December 1975 and operated from 9:30 a.m. to 11:30 p.m. (June to August) and from 9:30 a.m. – 9:30 p.m. (September to May).^[33][34] After paying an entrance fee in the second floor, visitors were required to pass through security checks added after the 1993 World Trade Center bombing.^[35] They were then sent to the 107th-floor indoor observatory at a height of 1,310 feet (400 m) by a dedicated express elevator, which could be only accessed by entering the core.^[31] The exterior columns were narrowed to allow 28 inches of window width between them. In 1995, the Port Authority leased operation of the observatory to Ogden Entertainment, which decided to renovate it.^[36] On April 30, 1997, the Top of the World tour reopened after renovations were finished.^[37] Dellmont Leisure Design, a La Crescenta firm led by former Disney Imagineer David Schweninger, carried out the renovations.^[38] Attractions added to the observation deck included 24 video monitors, which provided descriptions of 44 points of interest in six languages; a theater showing a film of a simulated helicopter tour around the city called "Manhattan Magic";^[39][31][40] a model of Manhattan with 750 buildings; a Kodak photo booth and two gift shops.^[39] The 107th-floor also featured a subway-themed food court that featured Sbarro Street Station and Nathan's Famous Hot Dogs with a dining area that simulated Central Park.^[41][42]

Weather permitting, visitors could ride two short escalators up from the 107th-floor viewing area to an outdoor platform at a height of 1,377 ft (420 m).^[43][31][44] On a clear day, visitors could see up to 50 miles (80 km).^[41] 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. This left the view unobstructed, unlike the observation deck of the Empire State Building.^[42]

At 9:03 a.m. EDT on September 11, 2001, five terrorists crashed United Airlines Flight 175 into the southern face of the South Tower.^[45][46] Three buildings in the World Trade Center complex, including 2 WTC, collapsed due to fire-induced structural failure.^[47] The light construction and hollow nature of the structures allowed the jet fuel to penetrate far inside the towers, igniting many large fires simultaneously over a wide area of the impacted floors. The fuel from the planes burned at most for a few minutes, but the contents of the buildings burned over the next hour to hour and a half.^[48]

The fires might not have been as centrally positioned, nor as intense, had traditionally heavy high-rise construction been standing in the way of the aircraft. Debris and fuel would likely have remained mostly outside the buildings or concentrated in more peripheral areas away from the building cores, which would then not have become unique failure points. In this scenario, the towers might have stood far longer, perhaps indefinitely.^[49][50] The fires were hot enough to weaken the columns and cause floors to sag, pulling perimeter columns inward and reducing their ability to support the mass of the building above.^[51] The South Tower collapsed at 9:59 a.m. after burning for 56 minutes in the fire caused by the impact of United Airlines Flight 175 and the explosion of its fuel.^[47]

The Twin Towers' facades were made of high-strength, load-bearing perimeter steel columns which acted as Vierendeel trusses.^[59][60] Although the columns themselves were lightweight, they were spaced closely together, forming a strong, rigid wall structure.^[61][62] There were 59 perimeter columns, narrowly spaced, on each side of the building.^[63][60] In all, the perimeter walls measured 210 feet (64 m) long on each side, and the corners were beveled.^[64][62] The perimeter structure was constructed of prefabricated modular pieces connected by spandrel plates.^[62] From the 7th floor to the ground level, and down to the foundation, the columns were spaced 10 feet (3.0 m) apart to accommodate doorways.^[65][60] All columns were placed on bedrock 65–85 feet (20–26 m) below the surface.^[66]

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.^[67] The South Tower's structural core was oriented with the long axis north to south.^[68] The core columns supported about half the towers' weight.^[68] All elevators were located in the core. Each building had three stairwells, also in the core, except on the mechanical floors.^[63] The large, column-free space between the perimeter and core was bridged by prefabricated floor trusses, which connected to the perimeter columns.^[69]

Hat trusses (or "outrigger truss") located from the 107th floor to the top of the North and South towers were designed to support a tall communication antenna on top of each building.^[69] The South Tower never had an antenna fitted.^[70] The framed-tube 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.^[71] In designing the World Trade Center, Leslie Robertson considered the scenario of the impact of a jet airliner crashing into the building.^[72] The National Institute of Standards and Technology (NIST) found a three-page white paper that mentioned another aircraft impact analysis, involving impact of a jet at 600 mph (970 km/h), was indeed considered, but NIST could not locate the documentary evidence of the aircraft impact analysis.^[73]

Sprayed-fire resistant materials (SFRMs), gypsum wallboard, and vermiculite were used to provide fireproofing to the interiors.^[63] More fireproofing was added after a fire in February 1975,^[74] but after the 1993 bombing, inspections found fireproofing to be deficient.^[75] The 1968 New York City building codes did not require sprinklers for high-rise buildings, except for underground spaces,^[76] but the entire complex was retrofitted by 2001.^[77]