On the morning of 11 September 2001, terrorists targeted the World Trade Center in Manhattan, first crashing a hijacked commercial jetliner into the upper levels of One World Trade Center, one of its twin 110-story iconic skyscrapers. A few minutes later a second hijacked aircraft sliced through the middle levels of Two World Trade Center, the other tower. (A third airliner crashed into the Pentagon outside Washington, D.C., while a fourth crash landed in a field in Pennsylvania, its intended target undetermined.) The effects were predictably devastating: both buildings burned fiercely before totally collapsing in clouds of dust and rubble that darkened the sky above Manhattan. A third building in the complex, a forty-seven-story office block (Seven World Trade Center), damaged by flying debris, followed soon after.
In an earlier raid in February 1993 Arab terrorists exploded a 1,200-pound (550-kilogram) truck bomb in the Center’s parking garage, creating a 150-foot diameter (46-meter) crater. Six people died, and over 1,000 were injured. Floors were destroyed for three levels below the point of detonation, but because of the load-bearing exterior walls, the structural stability of the building was largely unaffected. Tenants returned to their offices by the end of March. The cost of repairs was $250 million. The World Trade Center occupied a 16-acre (6.5 hectare) site a few blocks from Wall Street at the southwestern tip of Manhattan Island, near the bank of the Hudson River. Designed by Minoru Yamasaki and Associates and supervised by Emery Roth and Sons, it was the core of an urban renewal scheme sponsored by the Port Authority of New York and New Jersey to attract international firms to downtown Manhattan.
The surviving parts of the complex are a twenty-two story, 818-room hotel (Three World Trade Center); two nine-story office buildings (Four and Five World Trade Center); and an eight-story Customs House (Six World Trade Center). With the destroyed buildings, they were grouped around the 5-acre (2 hectare) landscaped Austin J. Tobin Plaza. Beneath it is The Mall, with about sixty specialty shops, banks, restaurants, and function spaces. Before the tragedy, about 500 international companies were located in the center, employing 50,000 people. It had its own subway stations and its own zip code. In March 1999 U.S. construction executives named the World Trade Center among the top ten construction achievements of the twentieth century.
For a short while the One and Two World Trade Center towers, at around 1,353 feet (411 meters), were the world’s tallest buildings, but they were superseded in 1974 by the 1,442-foot (450-meter) Sears Tower in Chicago. In 1998 the Petronas Twin Towers in Kuala Lumpur, Malaysia, reached 1,483 feet (452 meters). Even higher buildings have been projected: for example, the Taipei Financial Center, to be completed in August 2002, will stand 1,660 feet (508 meters) tall and Hong Kong’s Kowloon MTR Tower will be 1,903 feet (580 meters). As technically demanding as it is, great height does not qualify a building as an architectural feat. It was their structural system and the consequent creation of usable space that made the New York World Trade Center’s towers remarkable. Ironically, it also was a contributor to their collapse.
Yamasaki’s team was selected over a dozen other American architects. During the preliminary design phase, more than 100 proposals were reviewed, ranging from a single 150-story tower (its scale was far too large) to a series of lower towers (which “looked too much like a housing project”). Following Yamasaki’s recommendation, the Port Authority decided on the twin towers as the focus for the complex. The final plan was produced, subject to some minor changes, in 1964 and construction began in 1966. The project, administered by the Tishman Realty and Construction Co., entailed over 700 contracts.
The site was first isolated by building a 3-foot (0.9-meter) thick, 70-foot-high (21.5-meter) wall, keyed into the bedrock. A six-level basement was built in the foundation excavation, and the spoil was used to create over 23 acres (9.3 hectares) of new land in adjacent Battery Park. The towers, each 208 feet (63.6 meters) square in plan, started to rise in March 1969.
The structural engineering firm of Worthington, Skilling, Helle and Jackson designed the world’s highest load-bearing walls as vertical cantilevered steel tubes. Fourteen-inch square (0.36-meter), aluminum-clad box columns at 39-inch (1-meter) centers around the perimeter of the towers had spandrels welded to them at each floor, effectively making them into huge trusses. The facades were in effect steel lattices, a light, economical structure that provided efficient wind bracing at the outside surface of the building. Less than a third of the towers’ surface area was glass; the slot-like windows were narrower than the columns between them. Thirty-three-inch- deep (0.84-meter) prefabricated steel trusses, spanning 60 feet (18.4 meters) to the towers’ central service cores, carried the floors. Installation of 200,000 structural steel components—they were prefabricated in Seattle, St. Louis, and Los Angeles—presented a major logistical problem, because of the tight urban building site. Delivery and fixing was managed by a computer-programed control system, and eight “kangaroo” cranes built in Australia were used to hoist the elements into place.
The towers’ interiors were column-free, providing 4.48 million square feet (418,600 square meters) of net rentable space—about 75 percent of the total area (the average for contemporary high-rise buildings in the U.S. was only two-thirds of that). The vertical transportation system, which required fewer elevator shafts, also contributed to that efficiency. Each tower had ninety-nine elevators grouped in three vertical “zones”. Express elevators served skylobbies at the forty-first and seventy-fourth floors; from the skylobbies and the plaza, four banks of local elevators carried passengers to each zone.
The World Trade Center complex opened in December 1970 and was dedicated in April 1973. During the course of construction the cost rose from the estimated $350 million to $800 million.
In the September 2001 attack, more than 5,000 people were killed and thousands more were injured. In late September, the last standing piece of the center, a seven-story wall from the south tower, was torn down and saved for possible use in a future memorial. It is unlikely that the elegant towers will rise again.

On the morning of 11 September 2001, terrorists targeted the World Trade Center in Manhattan, first crashing a hijacked commercial jetliner into the upper levels of One World Trade Center, one of its twin 110-story iconic skyscrapers. A few minutes later a second hijacked aircraft sliced through the middle levels of Two World Trade Center, the other tower. (A third airliner crashed into the Pentagon outside Washington, D.C., while a fourth crash landed in a field in Pennsylvania, its intended target undetermined.) The effects were predictably devastating: both buildings burned fiercely before totally collapsing in clouds of dust and rubble that darkened the sky above Manhattan. A third building in the complex, a forty-seven-story office block (Seven World Trade Center), damaged by flying debris, followed soon after.
In an earlier raid in February 1993 Arab terrorists exploded a 1,200-pound (550-kilogram) truck bomb in the Center’s parking garage, creating a 150-foot diameter (46-meter) crater. Six people died, and over 1,000 were injured. Floors were destroyed for three levels below the point of detonation, but because of the load-bearing exterior walls, the structural stability of the building was largely unaffected. Tenants returned to their offices by the end of March. The cost of repairs was $250 million. The World Trade Center occupied a 16-acre (6.5 hectare) site a few blocks from Wall Street at the southwestern tip of Manhattan Island, near the bank of the Hudson River. Designed by Minoru Yamasaki and Associates and supervised by Emery Roth and Sons, it was the core of an urban renewal scheme sponsored by the Port Authority of New York and New Jersey to attract international firms to downtown Manhattan.
The surviving parts of the complex are a twenty-two story, 818-room hotel (Three World Trade Center); two nine-story office buildings (Four and Five World Trade Center); and an eight-story Customs House (Six World Trade Center). With the destroyed buildings, they were grouped around the 5-acre (2 hectare) landscaped Austin J. Tobin Plaza. Beneath it is The Mall, with about sixty specialty shops, banks, restaurants, and function spaces. Before the tragedy, about 500 international companies were located in the center, employing 50,000 people. It had its own subway stations and its own zip code. In March 1999 U.S. construction executives named the World Trade Center among the top ten construction achievements of the twentieth century.
For a short while the One and Two World Trade Center towers, at around 1,353 feet (411 meters), were the world’s tallest buildings, but they were superseded in 1974 by the 1,442-foot (450-meter) Sears Tower in Chicago. In 1998 the Petronas Twin Towers in Kuala Lumpur, Malaysia, reached 1,483 feet (452 meters). Even higher buildings have been projected: for example, the Taipei Financial Center, to be completed in August 2002, will stand 1,660 feet (508 meters) tall and Hong Kong’s Kowloon MTR Tower will be 1,903 feet (580 meters). As technically demanding as it is, great height does not qualify a building as an architectural feat. It was their structural system and the consequent creation of usable space that made the New York World Trade Center’s towers remarkable. Ironically, it also was a contributor to their collapse.
Yamasaki’s team was selected over a dozen other American architects. During the preliminary design phase, more than 100 proposals were reviewed, ranging from a single 150-story tower (its scale was far too large) to a series of lower towers (which “looked too much like a housing project”). Following Yamasaki’s recommendation, the Port Authority decided on the twin towers as the focus for the complex. The final plan was produced, subject to some minor changes, in 1964 and construction began in 1966. The project, administered by the Tishman Realty and Construction Co., entailed over 700 contracts.
The site was first isolated by building a 3-foot (0.9-meter) thick, 70-foot-high (21.5-meter) wall, keyed into the bedrock. A six-level basement was built in the foundation excavation, and the spoil was used to create over 23 acres (9.3 hectares) of new land in adjacent Battery Park. The towers, each 208 feet (63.6 meters) square in plan, started to rise in March 1969.
The structural engineering firm of Worthington, Skilling, Helle and Jackson designed the world’s highest load-bearing walls as vertical cantilevered steel tubes. Fourteen-inch square (0.36-meter), aluminum-clad box columns at 39-inch (1-meter) centers around the perimeter of the towers had spandrels welded to them at each floor, effectively making them into huge trusses. The facades were in effect steel lattices, a light, economical structure that provided efficient wind bracing at the outside surface of the building. Less than a third of the towers’ surface area was glass; the slot-like windows were narrower than the columns between them. Thirty-three-inch- deep (0.84-meter) prefabricated steel trusses, spanning 60 feet (18.4 meters) to the towers’ central service cores, carried the floors. Installation of 200,000 structural steel components—they were prefabricated in Seattle, St. Louis, and Los Angeles—presented a major logistical problem, because of the tight urban building site. Delivery and fixing was managed by a computer-programed control system, and eight “kangaroo” cranes built in Australia were used to hoist the elements into place.
The towers’ interiors were column-free, providing 4.48 million square feet (418,600 square meters) of net rentable space—about 75 percent of the total area (the average for contemporary high-rise buildings in the U.S. was only two-thirds of that). The vertical transportation system, which required fewer elevator shafts, also contributed to that efficiency. Each tower had ninety-nine elevators grouped in three vertical “zones”. Express elevators served skylobbies at the forty-first and seventy-fourth floors; from the skylobbies and the plaza, four banks of local elevators carried passengers to each zone.
The World Trade Center complex opened in December 1970 and was dedicated in April 1973. During the course of construction the cost rose from the estimated $350 million to $800 million.
In the September 2001 attack, more than 5,000 people were killed and thousands more were injured. In late September, the last standing piece of the center, a seven-story wall from the south tower, was torn down and saved for possible use in a future memorial. It is unlikely that the elegant towers will rise again.