On a clear, beautiful day a few years ago, Masoud Sanayei, professor of civil and environmental engineering, decided to visit the World Trade Center and see for himself the two towers that dominated the New York City skyline. He took the nearly one-minute elevator ride up 110 stories, enjoyed the breathtaking view and had nothing but admiration for the architects, civil engineers and construction workers who had made the buildings possible.
"As a structural engineer, I wanted to see these buildings," he remembers. "I was so impressed with the people who designed, financed and constructed the towers."
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Sanayei took a stroll on the rooftop. He felt so relaxed that he sat down on a bench in the viewing area and took a nap.
On September 11, another clear and beautiful day, Sanayei, along with the rest of the world, watched in horror as the towers collapsed.
Sanayei and Daniel Abramson, associate professor of art history at Tufts, co-teach a course called "Skyscrapers" that is offered to both engineering and liberal arts students. They recently assessed the World Trade Center, both aesthetically and in terms of its engineering and design.
In his new book, Skyscraper Rivals (Princeton Architectural Press), Abramson explains that the Singer Tower, built in 1908, was demolished to make way for the World Trade Center, which was completed in 1974. The Singer Tower was 47 stories high and remains the tallest building ever deliberately brought down.
"Aesthetically, the World Trade Center was never thought of highly," said Abramson. "It was considered fairly bland, fairly blank and was not particularly interestingly articulated. The columns were very close together, and the windows about 30 inches wide. Because the windows were so narrow, we saw a blank faade."
Even the plaza at the base of the towers was dull, he said. Although architecturally uninteresting, the towers' fame rested on their height.
Briefly the tallest buildings in the world until the construction of the Sears Tower in Chicago and the Petronas Towers in Kuala Lumpur, the World Trade Center dwarfed everything else in lower Manhattan. Since their completion, nothing else built in New York City has come close to their height.
"New York is a great skyscraper city, and skyscrapers are the archetypal building type of the 20th century," said Abramson. "I'm dumbfounded to think that something so big could come down so quickly. One presumed they would never be torn down and that they would always be there. This would be as if the Pyramids were destroyed."
Heat brought them down
Sanayei said the buildings were strong enough to survive the impact of the airplanes but collapsed as a result of the ensuing fire, which may have burned with temperatures greater than 1000 degrees Fahrenheit.
The buildings were designed, he said, using a tube-in-tube structural arrangement, "creating a system that is very stiff and strong and able to resist lateral loads such as wind or an earthquake. The towers survived the initial impact. Each building rocked back and forth, and people would have felt the buildings move."
But, Sanayei said, the large amount of jet fuel delivered to the towers followed by explosions and subsequent fire weakened the floor systems and the columns. "I suspect the fire created temperatures higher than what is normally experienced in office building fires caused by burning furniture or rugs or paper," he said.
Structural steel melts at around 2000 degrees Fahrenheit but starts to soften at around 800 degrees. Somewhere between 1200 and 1500 degrees, it loses two-thirds of its strength, Sanayei said.
"The building still had to carry the massive loads of higher floors above the plane crash location. In the first tower, one third of the building was above the plane, and in the second, about half the floors were above the impact. The structural columns could no longer carry the gravity loads of the floors above the fire.
"In addition," Sanayei said, "the heat could have weakened the floor systems above, as well as the floor-to-column connections. This combination caused the top-down collapse, producing a domino effect. The exterior columns that formed the outside tube of the World Trade Center buildings guided the self-contained collapse within these buildings. It looked like a designed and time-delayed implosion that took only a few seconds to bring each tower down. It should have felt like an earthquake to the surrounding buildings. In such catastrophic events, it is hard for anyone to survive."
If there had been no fire, he said, potentially the buildings could have survived, although at a later date, they might have had to be brought down. Without the collapse, he noted, not as many lives would have been lost.
Lessons for the future
"There are lessons to be learned from this tragedy," Sanayei said. "Our structural and geotechnical engineers and architects who design these buildings are doing a fine job of designing safe, high-rise buildings, as is shown by the fact that both towers survived the initial impact.
But the challenge ahead, Sanayei said, is to make these buildings more fire-resistant, to develop better systems to extinguish fires, even one like this. "It is going to happen again someday, if not by aircraft crash than by something else," he said.
"We need to develop more reliable methods of evacuating tall buildings. We need redundant systems such that if one evacuation system fails, there are other effective ways of getting out. The airplane crash, the subsequent explosion and fire and the catastrophic collapse of both towers will drastically change the way investors, architects and engineers will construct our future skyscrapers. We need to rethink our cities."