BUILDINGS

Behavior of Twin Towers In Collapse Mystifies Experts (9/11/01)

By Nadine M. Post



MINUTES BEFORE COLLAPSE Manhattan's twin 110-story towers burned for more than an hour before collapsing. (Photo by Tom Sawyer for ENR)

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n what appear to be coordinated and parallel terrorist attacks in New York City and Washington, D.C., airplanes crashed into the World Trade Center towers and the Pentagon on Tuesday. At least two planes, believed to be hijacked, crashed into the towers in lower Manhattan, which later collapsed. Another airliner crashed into a section of the Pentagon not long after the attack in New York. Later in the afternoon, 7 World Trade Center across the street from the twin towers, was on the verge of collapse as a result of the earlier failures in lower Manhattan.

As the nation and the world absorb the events of Sept. 11, there remain more questions than answers. One big one for Mark Loizeaux, president of Controlled Demolition Inc., the Phoenix, Md.-based implosion consultant, is why the twin towers of the World Trade Center collapsed in such different ways, after both were compromised by airplanes and fire.

The south tower, which was hit second but collapsed first, failed much like a felled tree, as would have been expected after the plane compromised its exterior columns. But the north tower, similarly hit, "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. He wonders whether there was an explosion at the base of the building. "I don't understand," he adds, unless there was another "agency," at work, he says.

The twin towers were both 110 stories tall. The 1,368-ft-tall north tower, with an antenna on top, was hit by an airplane at 8:45 a.m., above the 70th floor. The second 1,362-ft-tall tower was hit at 9:03 a.m., at a lower elevation. The south tower collapsed at 10 a.m.; the north tower at 10:29 a.m.

After its upper floors burned for about an hour, the north tower came down in a progressive collapse. "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," according to Jon D. Magnusson, chairman-CEO of Skilling Ward Magnusson Barkshire Inc., Seattle, one of the successor firms of Skilling Ward Christiansen Robertson, structural engineer of record for the World Trade Center.

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.

The twin towers, framed in structural steel, had exterior moment frames with 14-in. steel box columns spaced 39 in. on center. The configuration created a complete tube around the building. The central steel core carried gravity loads only. The exterior turbe 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 planes compromised the structural tube and the fires that followed, likely fed by jet fuel, probably burned to temperatures beyond the integrity of the fireproofed steel, which is designed to withstand 1,500 to 1,600° F heat.

They 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). Major subways, including the PATH trains to New Jersey, pass under the complex.

Security measures had been tightened at the 12-million-sq-ft World Trade Center complex, which housed approximately 50,000 workers and received thousands of visitors daily, 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).

In the aftermath of the bomb, WTC structural consultant Leslie E. Robertson, who was on Skilling's original team for the WTC, was convinced that the terrorists had meant to take down the twin towers in 1993 but had failed. After the events of Sept. 11, it seems clear Robertson was correct.