Both towers were tube-like, and the majority of their support came from exterior columns. There was also a cluster of columns in the center of the building connected to the exterior columns by floor trusses.

  1. After the planes struck the towers, thousands of gallons of jet fuel burned.
  2. Thick layers of insulation on exterior and interior steel columns were breached, and sprinkler systems were disabled.
  3. Fires broke out with temperatures reaching perhaps 1,000 to 2,000 degrees Fahrenheit.
  4. The steel column systems, softened in the heat, separated from each other and buckled.
  5. Weighted down by debris, furniture, concrete and steel, floors progressively collapsed.

Sources: Urban Data Solutions; Skilling Ward Magnusson Barkshire; The Port Authority

The twin towers, framed in structural steel, had exterior moment frames with 14-in. steel box columns spaced 39 in. centers. The configuration created a complete tube around the building. The central steel core carried gravity loads only. The exterior tube provided all the lateral resistance. Horizontal steel trusses spanned 60 ft from the exterior wall to the core. Concrete on metal deck completed the floor diaphragm.

The twin towers were part of a seven-building complex designed by architect Minoru Yamasaki that covers eight city blocks. An 800 x 400-ft foundation box, 65-ft-deep and with 3-ft-thick retaining walls, is under more than half the complex, including the twin towers and the adjacent hotel. The complex was completed in phases beginning in 1970 (ENR 7/9/64 p. 36).

World Trade Center Collapsing (AP Photo)

Each tower contained about 100,000 tons of steel and 4 in. of concrete topping on the 40,000-sq-ft floors, according to Henry H. Deutch, assistant to the chief structural engineer for construction manager Tishman Realty & Construction Co. Inc., New York City, during the construction of the WTC and currently head of HHD Consultants Inc., Osceola County, Fla.

Deutch says that originally, the north tower contained asbestos in its cementitious fireproofing as did the first 30 stories of the south tower. He believes the asbestos, which had been encapsulated, was removed after the 1993 bombing. In a press conference, Mayor Rudolph W. Giuliani said the city's health department had tested the air in the area and found no undue amount of chemical agents.

Millions across the nation also "saw" the towers collapse, through live television news coverage. The south tower fell at 10 a.m. and the north tower at 10:29 a.m.

Reports indicate that the impact of each plane compromised the structural integrity of each tower, knocking out perimeter columns and the interior structure. The explosions then caused further damage, sweeping through several floors. "These were airliners scheduled for long flights, full of fuel, causing massive explosions," says Richard M. Kielar, a Tishman senior vice president. "No structure could have sustained this kind of assault," says Kielar.

As the fires burned, the structural steel on the breached floors and above would have softened and warped because of the intense heat, say sources. Fireproofed steel is only rated to resist 1,500 to 1,600 F. As the structure warped and weakened at the top of each tower, the frame, along with concrete slabs, furniture, file cabinets, and other materials, became an enormous, consolidated weight that eventually crushed the lower portions of the frame below.

Jon D. Magnusson, chairman-CEO of Skilling Ward Magnusson Barkshire Inc., Seattle, structural engineer for the original World Trade Center, agrees: "From what I observed on TV, it appeared that the floor diaphragm, necessary to brace the exterior columns, had lost connection to the exterior wall."

When the stability was lost, the exterior columns buckled outward, allowing the floors above to drop down onto floors below, overloading and failing each one as it went down, he says.

A big question for implosion expert Mark Loizeaux, president of Controlled Demolition Inc., the Phoenix, Md., is why the twin towers appeared to have collapsed in such different ways. Observing the collapses on television news, Loizeaux says the 1,362-ft-tall south tower, which was hit at about the 60th floor, failed much as one would like fell a tree. That is what was expected, says Loizeaux. But the 1,368-ft-tall north tower, similarly hit but at about the 90th floor, "telescoped," says Loizeaux. It failed vertically, he adds, rather than falling over. "I don't have a clue," says Loizeaux, regarding the cause of the telescoping.

Security measures were tightened at the 12-million-sq-ft WTC complex after a terrorist bomb on Feb. 26, 1993. That bomb blew out one section of a north tower basement X-brace between two of the perimeter columns. The blast ripped out sections of three structural slabs in the basement levels between the north tower and the hotel, threatening the structural integrity of the foundation box. It did little damage to the north tower's structural tube, other than the affected X-brace. Damage was extensive to the other building systems, however, because the bomb compromised major utility lines in the basement, and the brace compromised the central core wall, allowing soot and smoke to shoot up the building core (ENR 3/15/93 p. 12).

CLICK here for more: Structural engineers say the terrorists apparently knew they had to strike the World Trade Center as low as possible to cause the most damage & Facts about the World Trade Center