Structural engineer can discuss vulnerability of World Trade Center, similar buildings, to terrorist attack

Thomas Harmon, Ph.D.

Clifford W. Murphy Professor of Civil Engineering;
Director of the Construction Materials and Management Center

Harmon, a structural engineer, has been involved with the design of buildings similar to the World Trade Center. He is available to discuss design and engineering issues that may have contributed to the building's collapse. Harmon says that the World Trade Center building design is known as a tube structure. It consists of lots of small, closely spaced columns that resist two sets of loads, vertical and lateral. Harmon worked on the analysis of the Standard Oil Building in the early 1970s, an 87-story structure that is in some ways similar to the World Trade Center. The Sears Tower in Chicago also is similar in design and was built in the same era. All three buildings are designed as structural tubes.

"The idea behind the World Trade Center design is to make the exterior wall as efficient as possible," Harmon says. "One way to do that is to make the space between the columns on the perimeter wall as short as possible so that the horizontal members, the beams, are very stiff to resist the lateral load."

Harmon speculates that the short distance between exterior columns in the World Trade Center buildings — about four to five feet apart, compared with 15 feet apart in the Sears Tower — might have made the buildings more vulnerable to the impact of the planes and less resistant to fire. The columns of the world trade towers are smaller because they are closer together.

"As the fire went up and destroyed the upper floors, the trusses probably collapsed under their own dead weight because the steel was heated and maybe the fire proofing was removed by the impact," he says. "What then probably happened was the floors collapsed on top of each other and you got a domino effect. All these floors holding the walls up, and they all just cave in."

He said that he believed the structure of the World Trade Center buildings is an interior core with long trusses going from that core to the perimeter walls.

"As the upper building collapses and hits those floors, they collapsed," he said. "When there's nothing to support the exterior walls, the walls probably just crumpled in on the building."

Harmon expressed incredulity that the buildings collapsed, speculating that it was due to the melting of the fire.

"I was utterly shocked that the buildings actually collapsed," he says. "I can't imagine that whoever did this thought the buildings were going to totally collapse."

For interview
Contact Harmon directly at his office: (314) 935-4536; or home: (314) 993-6220; or via e-mail:
Or contact Tony Fitzpatrick in the Office of Public Affairs: (314) 935-5272 or 935-5230;
or e-mail:


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