In the immediate aftermath of 9/11, numerous articles were published in newspapers, with the journalists consulting experts for their opinions on what caused the towers to collapse. The experts included engineers who worked on the design and construction of the towers, and structural engineer experts. They pointed to the fact that steel is substantially weakened at high temperatures, and other factors.
Experts hypothesized that the fires "weakened the steel supports, causing the external walls to buckle and allowing the floors above to fall almost straight down. That led to catastrophic failures of the rest of the buildings."[1]
Richard M. Kielar, a spokesman for Tishman Realty and Construction Company, the construction manager for the original project said, "No structure could have sustained this kind of assault."[1] He said it was too early to piece together a precise train of events, but he agreed that weakening by fire, followed by catastrophic collapse of the floors, was the most likely possibility. "As the structure warped and weakened at the top of each tower, it -- along with concrete slabs, furniture, file cabinets and other materials -- became an enormous consolidated weight that eventually, progressively crushed each tower below," he said in a statement.[1]
According to Gregory Fenves, a professor of Civil Engineering at the University of California at Berkeley, the planes weakened the buildings' structures at key points. Fenves, working on information gleaned from preliminary TV reports, stressed that he was speculating. He said that if the planes had hit the structures higher, they could have merely damaged their tops; if they had hit lower, they would have been up against the enormous weight and resistance of the base of the buildings.[2]
"The towers took the impact," said Jack Lebduska, professor of construction and life safety at the New Jersey Institute of Technology. But the buildings ultimately failed because the extreme heat melted the floor trusses. "At 1,000 degrees, steel loses about half its strength; at 1,500 degrees it turns to taffy," said Lebduska, consultant to a Japanese firm that designed a bank on the South Tower's 80th floor. Burning jet fuel combined with plastics in computers, desks and chairs plus the extreme height presented firefighters with an impossible challenge. Ultimately, one floor fell onto the next, overloading it, and then it was like a house of cards, one falling down on top of another, Lebduska said. However, one benefit of having the load-bearing exterior was that it contained the collapse and prevented it from falling over.[3]
British engineer Professor Alastair Soane, who worked on the towers before their official opening in 1973, said "They were extremely robust buildings and built to withstand a tremendous amount. But this was of course a completely abnormal situation and one which would not have been envisaged by the people who built it. The strength of the towers was enormous but they would not have been designed for aircraft strikes."[4]
The enormous heat from the jet fuel fire probably caused the steel trusses holding up concrete-slab floors and vertical steel columns to bend like soft plastic, said Jon Magnusson, chairman and chief executive of Skilling Ward Magnusson Barkshire in Seattle, a structural engineering firm that worked out the original design.[1] "The subsequent fires caused the collapse," said Jon Magnuson, "What happens in a fire is steel at 1,500, 1,600 degrees Fahrenheit, steel loses its strength. Then it just collapses." He said nobody could design a building and account for the possibility of heat generated by burning jet fuel. The impact of the plane could have damaged the sprinkler system, allowing the fire to burn uncontrolled. The combination of the plane crashes and the heat from the fires on the steel columns probably began the collapse. A structural engineer said that after the upper floors gave way, each then overloaded the floor beneath it, continuously to the ground.[5] The skyscrapers had two means of defense against normal fire damage, Mr. Magnusson said. One, thick layers of insulation sprayed onto the steel beams, could have been breached by the initial crash, he said. Another, the building's sprinkler system, may have been disabled as well, or it may simply have been useless in the heat of the jet fuel fire.[1]
"There isn't anything particularly vulnerable about it [the design of the Twin Towers]," said Aine Brazil of Thornton-Tomasetti Engineers in New York, a structural engineering firm that worked on the Petronas Towers, the world's largest buildings, in Malaysia. Buildings are simply not designed to withstand "the extreme levels of heat that would be found in the situation with the amount of jet fuel and the explosion that occurred," Ms. Brazil said.[1]
Jack Cermak, president of Cermak Peterka Peterson in Fort Collins, Colo., the firm that did the wind-tunnel testing for the design of the towers, agreed that the impact of the crash itself probably could not have collapsed the massively reinforced building on its own. "I presume, without knowing the details, that that collapse was caused by weakening of the structure due to the heat," Dr. Cermak said.[1]
Ronald O. Hamburger, a structural engineer and member of the initial ASCE investigative team, explained "Steel is born of fire. As it's reheated, it expands and loses its rigidity. Above 1,000 degrees Fahrenheit, it loses a significant amount of its strength." He said the extreme heat from the fires might have caused the steel floors to expand and bow, which may have caused the support columns to bend inward and buckle. Heat also may have caused the steel flooring to separate from the columns, or the columns themselves may have heated up and buckled outward.[6]