Chit Chat 51

An Engineering Assessment of the World Trade Center Twin Towels Attack

Bob Chen

The editorial forwarded by Bill Kao's better half, Karen, and mentioned in Gina and Steve Cheng's e-mail to us all is greatly appreciated by this writer. The changing of the Guards in Buckingham Palace on 9/12/01 with the playing of the American national anthem, and some of the Cold Stream Guards in black uniforms was the only show of sympathy from one of the Marshall plan benefactors. The Canadians later had a ceremony in Ottawa was quite neighborly too. The obvious missing French and Japanese expressions were unforgivable in my mind. May be they are true realists, and not wanting to jeopardize their oil supplies.

Aside from the human tragedies that we all saw on TV over and over, I would like to give an engineering assessment of the events leading to the collapses of the twin towels. The information was given by Peter Jennings of ABC right after the incidents. He interviewed a civil/structural engineer who gave a good account of the collapse of the second towel. He said that the impolsion was due to the intense heat ( 1600 deg. F to 2000 deg. F) melted the steel columns, and the weight of the top 50 to 60 floors that buckled the outer walls. This showed the terrorists were very knowledgeable. The first one goofed in hitting too high. He hit the 101st. floor out of a 110th floor building. the delta weight of the towel was no more than 10% of the total, while the second one hit the 40th. plus floor, the delta weight was at least 50%. The shock of the collapse of the second towel weakened the first towel to collapse it later. If the second hijacker did not know enough engineering he wouldn't hit so low. They were well informed of the fuel capacities of the 767, and 757 airliners too. They picked the longest routes, Boston to LA, Newark to San Francisco, and Dulles to LA. All these long cross country flights carrying lots of fuel. After the initial 40 to 45 minutes flight, there were ample fuel left to burn. Also, they must know
the structure of the twin towels well. According to the civil/structural engineer Jennings talked to, the center line to center line spacing of the twin towels were 39 inches. The building code requirement is 10 feet, or 120 inches. This three times denser spacing acted as a net to keep the crashed aircraft in place to burn. What an irony that the redundancy of the steel beams contributed to the downfall of the twin towels! I believe the terrorists were led by people that know more than killing themselves will lead them directly to heaven. The second hijacker had in him above average intelligence and self-motivation to finish the job, or get the job done. The latter quality is rare these days. It is a pity that he went astray.