Reply #14: Yes, apparently so.

[maha]

http://www.steel.org/news/innews/pr_amm_01_0912.htm

Structural engineers said the design integrity of the twin towers of New York's World Trade Center was such that the buildings could withstand the impact of two large passenger planes crashing into them. The buildings collapsed about two hours later after fire caused the steel supporting the buildings to lose its elasticity.

"Steel has a fire rating," said a structural engineer based in Pittsburgh. "A steel building like that will have about one hour (to stand) but when it gets hot enough, the building will come down."

Intense heat causes what is called the modulous elasticity of steel to be reduced. In the case of the World Trade Center crashes, thousands of gallons of aviation fuel were spilled into the building and ignited fires that burned at temperatures estimated between 800 and 1,000-degrees Fahrenheit.

"And then what you have is sort of a zipper effect," the engineer said. "When the steel began to weaken, the weight of the floors collapsed and it went down from there. When the planes went in, they probably took out support columns (at their points of impact). Then you have the weight of each floor falling down upon the next one and that brought the buildings down."

The buildings remained intact for more than an hour after impact, giving thousands a chance to escape before the buildings fell.

"I believe tens of thousands of lives have been saved by the structural integrity of the building," John Knapton, a professor of structural engineering at Newcastle University in England, said in an interview with BBC News Online. "It had a lot of its structure taken out, yet remained intact for more than an hour, allowing thousands to escape."

Ray Steeb, vice president and general manager with the Pittsburgh office of Turner Construction Co., said, "With such intense heat, steel loses its strength. It becomes like a noodle after it's cooked. The construction industry puts up fireproofing around the steel columns that give people one hour to two hours to get out of buildings in the case of fires. The columns (with proper fireproofing) should withstand 1,600-degree heat for one to two hours, and in most buildings that would give people time to get out."

Steeb called the collapse of the two towers "predictable," given their structure. "They came down just like they were supposed to," he said. "Once you get gravity involved, that's what happens. The buildings simply implode on themselves."