After the collapse of the World Trade Center towers in New York City last week, a team of structural engineers formed by the American Society of Civil Engineers has been actively studying the structural failures of the buildings, hoping to gather information that may benefit the design of future skyscrapers.
Heat was the key factor in the building’s collapse, said Max Porter, professor of civil and construction engineering.
Porter said he believes that, in addition to the obvious gap in the structure left by the crash of the plane, the collapse may have been be due to a decrease in the strength of the steel frame of the building caused by the massive amount of heat generated by burning jet fuel.
“The WTC towers’ structures were based upon a tube-within-a-tube design,” said Porter, president of the Structural Engineering Institute, a semi-autonomous branch of the ASCE.
“At the center was a steel and concrete core, while the outside was comprised of steel columns spaced very close together running the entire vertical height of the building,” he said. “Steel, like all building materials, is only able to handle a certain amount of heat before reaching a failure point in terms of strength.”
Porter said the towers’ design, which set the standard for skyscraper architecture in the 1960s, was ahead of its time for more than just its extraordinary height.
“The building relied extensively on the outer steel frame which, when connected to the concrete core, formed a diaphragm,” he said. “This system was very effective in dealing with the stresses caused by high velocity wind at that height.”
Porter said many of his colleagues believe the connections between the trusses running underneath the floors and the steel columns may have failed as a result of the heat. This failure would have caused the structure to become unstable, and the floor resting on the trusses to fall.
“Once one floor fell and landed on the next floor, the building was bound to collapse,” Porter said. “This, along with the overheated steel, allowed for the structures to implode, or collapse inward, as many people witnessed on TV.”
Mardith Baenziger, a former structural engineering consultant with more than 30 years of industry experience, said she did not have enough information to say for sure what caused the collapse.
Based upon information she heard in news reports and read on the Internet, however, Baenziger also believed overheated steel may have been the cause for the collapse.
“Buildings, skyscrapers or not, are generally only designed against regular office fires,” said Baenziger, associate professor of civil and construction engineering. “These fires were fueled by thousands and thousands of pounds of jet fuel. Imagine the heat.”
Both structural engineers emphasized that buildings only can be fire-proofed effectively to a certain point. After that point, it becomes both structurally and economically unsound.
“Steel is often fire-proofed with concrete,” Baenziger said. “That concrete, despite its thin layering, is still very heavy, and applying too much of it would put too much weight at the top, causing the building to be unstable.”
Porter said the real problem is not whether a fire-proofing system could be designed to withstand such fires, but whether it was realistic from a money standpoint.
“The cost would just be unfeasible,” he said.
Both structural engineers said they believe, despite the eventual collapse of the towers, that the frames performed remarkably well.
“The fact that both of them held up for more than an hour after a giant jet had crashed into them, which allowed hundreds to be saved, testifies to their sound design,” Baenziger said. “They were designed to withstand natural phenomena, such as hurricane winds, but not commercial jets.”
Porter said the buildings were designed to withstand the crash of a 707 commercial airliner.
“After a plane crashed into the Empire State Building, the designers worried about planes crashing into the buildings just peaking above the city’s skyline,” Porter said. “However, commercial jets larger than the 707 were not a factor.”