Task force to determine chain of events in collapse of twin towers

Maas, Angela

Jon Magnusson had the television on the. morning of Sept. 11 when the program was suddenly interrupted. The sight of the burning North Tower of the World Trade Center at first stunned Magnusson, chairman/CEO of Skilling Ward Magnusson Barkshire, one of two successor firms to the original firm that designed the building.

“My next emotion was great relief that the building had taken such a huge hit and was still standing,” he says. “Ninety-nine percent of buildings will collapse upon impact of a 767. Most buildings wouldn’t get to the fire stage.”

It was that fire, experts agree, that caused the collapse of the buildings. But beyond that point, experts differ on their opinions of the nature of the collapse. A structural engineering coalition, headed up by the American Society of Civil Engineers/Structural Engineering Institute, has been formed to investigate. Members include the American Institute of Steel Construction, the American Concrete Institute, the National Fire Protection Association, the Society of Fire Protection Engineers and the National Council of Structural Engineers Association/Structural Engineering Association of New York.

The towers were constructed with 61 steel supporting columns, each 14 inches in diameter and separated from the next by 39 inches, center to center, resulting in a steel tube frame. The advantage of such densely placed elements is that the structure can take a great deal of mechanical damage. Most experts seem to agree with Scott Melnick, editor of Modern Steel Construction, who says that the impact weakened the steel supporting columns, and then the conflagration, fed by enormous amounts of jet fuel, further weakened the building.

The fire proofing protection was “undoubtedly damaged,” which resulted in local collapses that started a chain of events, says Ronald Hamburger, chief structural engineer for ABS Consulting, who first thought that the collapse was due to charges going off in the building. “The failure of the structure in the area of the fire was not a huge surprise to any design professional,” he says, remembering that he anticipated simply a local collapse within three hours.

Hamburger, however, contends that “the global nature of the collapse was surprising and very distressing.” Hamburger, whose specialty is earthquake-zone construction, states that partial collapse – where the building pancakes at one story and a 20-story building ends up 19 stories – is common with earthquake destruction. “I would have hoped that the buildings would have performed in this manner,” he says.

“The building shrugged with the initial impact,” says Hamburger, noting that there was a great deal of redundancy built into the structure. “The force of the plane was a fraction of the force of the wind loading that the buildings were exposed to daily.”

Magnusson admits he is not sure if we’ll ever really know why the towers collapsed. He offers that a collapse of the interior columns, a collapse of the trusses supporting the floors, a separation between the floors and the exterior walls, or a combination of any of these could have led to the collapse.

Magnusson also says that the mechanism for collapse may have been different in each tower. Hamburger agrees, noting that the South Tower, the first to collapse, started to veer to the side that was impacted before coming straight down. With the North Tower, however, the outside walls looked like they were almost sucked in; it appeared almost implosive,” Hamburger says, leading him to wonder if failures in the elevator core may have initiated the collapse.

What people seem to forget, says Dr. W. Gene Corley, structural engineer with Construction Technology Laboratories, is that the buildings stood long enough for most of the tenants to make it out. “if there were 50,000 people in the buildings, about 45,000 must have gotten out,” he says.

Will the incident prompt standards changes? “Current design practices and procedures are adequate for fires in normal buildings,” says Hamburger, who believes modifications to high-risk high rises and government buildings are possible, as well as changes to emergency response procedures. But Magnusson questions whether such improvements are possible. “No technology would allow a building to take that kind of hit,” he says.

“It appears to me that the terrorists had very clear advice on what to do to cause the most damage possible,” says Dr. Corley, who led the investigation into the Murrah Federal Building that Timothy McVeigh blew up in 1995. “The person behind it had a great deal of engineering knowledge. They knew where and how to hit it to cause the most damage.”

Most experts, though, think that the real answer isn’t making buildings strong enough to survive a 767 crashing into them. “The question should focus on how to keep airplanes from hitting buildings,” Magnusson maintains. “We have to take away the weapon.”

– Angela Maas

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