The two towers of the World Trade Center may have collapsed because the planes which crashed into them this morning scraped away protective coatings on steel beams allowing incredibly intense fires to soften supporting beams, according to a San Jose State University professor who has worked on buildings of similar size.
While not drawing a conclusion as to the cause of the collapse, a Stanford University professor has estimated the intensity of the fires, likening them to the explosion of an atomic bomb.
"The planes hitting the sides of the buildings probably did not do that much structural damage," says Kurt McMullin, professor of civil engineering at San Jose State.
"The impact of the jets probably knocked a lot of fireproofing off [the steel girders]," he says. "Losing the glass windows allowed the fire to travel to several floors. It just led to a complete collapse of the steel frame which then dropped all the floors above.
"We design buildings to withstand high intensity fires for a limited time," he says. "[But] once steel becomes hot it becomes soft and it loses its strength."
The buildings, engineered to withstand the force of a hurricane, should have been able to withstand the impact of the planes, he says.
But the impact was hardly inconsequential, according to a Stanford University professor.
According to applied physics professor Steve Block's calculations, the amount of energy released by each plane that hit the World Trade Center was equivalent to about 1,000 tons of TNT-- or 1/20th of the energy released by the Hiroshima atomic bomb in 1945. Both planes combined released one-tenth of the energy produced at Hiroshima, according to Mr. Block's calculations.
A typical commercial Boeing 767 jet weighs about 412,000 pounds on takeoff -- including people, luggage and 24,000 gallons of fuel, he says. If you combine the energy released by the burning fuel [equivalent to about 1,000 tons of TNT] with the kinetic (motion) energy caused by the collision [equivalent to about one ton of TNT], you get the equivalent of roughly 1,001 tons (1.001 kilotons) of TNT -- or one-twentieth of the Hiroshima bomb, Mr. Block says.
Mr. McMullin noted that the Empire State Building withstood the impact of a World War II bomber which plowed into it in a dense fog in 1945. The difference this time, he believes, may have been the size of the fires generated by the impacts.