Twin Towers’ steel and concrete construction couldn’t have sustained the hit
NEW YORK (AP) – The image of the World Trade Center’s 110-story twin towers crumbling seemed a scene of impossible destruction.
But the miraculous steel and concrete architecture that made them could not withstand the power of Tuesday’s attack and ensuing fire. No building designed today could, said Masoud Sanayei, a civil engineering professor at Tufts University.
Experts in skyscraper construction said video of the collapse led them to believe the towers were perhaps weakened by the initial impact of the airplanes that hit them Tuesday, but that heat from the resulting fire was likely the most punishing blow.
Hyman Brown, a University of Colorado civil engineering professor and the Trade Center’s construction manager, speculated that flames fueled by thousands of gallons of aviation fuel melted steel supports.
”This building would have stood had a plane or a force caused by a plane smashed into it,” he said. ”But steel melts, and 24,000 gallons of aviation fluid melted the steel. Nothing is designed or will be designed to withstand that fire.”
Sanayei said the heat may have disconnected one of the towers’ concrete floors from the tubular steel columns that ringed the buildings. If one or two floors collapsed, it would have created a pancake effect of one massive floor caving into the next.
”In my opinion, the fire weakened the connection between the floor system and the columns on the higher floors and caused a couple of the floors to collapse,” Sanayei said. ”The floors are very heavy, made of reinforced concrete, so when one hits the next, they cause a domino effect … and it can go all the way down to the first floor.”
Architect Minoru Yamasaki, who died in 1986, worked with engineers John Skilling and Leslie E. Robertson to design the fabled twin towers, once the world’s tallest buildings.
In his 2000 book ”Building Big,” architect David MaCaulay described the towers’ engineering as ”a series of load bearing exterior columns spaced 3 feet apart and tied together at every floor by a deep horizontal beam, creating a strong lattice of square tubing around each tower.”
The core surrounding the elevators inside was much the same, with a giant lattice work of steel covered by poured concrete connecting the interior columns to the exterior ones. The design was free enough for each of the towers to hold 4 million square feet of space unencumbered by columns or load bearing walls.
Sections of exterior wall were wrapped around the outside in 24- and 36-foot high sections, creating a sort of patchwork so that not all the floor joints would meet walls at the same height, according to MaCaulay.
Both Brown and Saw-teen See, a managing partner in Robertson’s engineering firm, said the twin towers were originally designed to sustain a direct hit by a large jetliner, but that such construction couldn’t make them fire- or bombproof.
Brown said it appeared the attack was meticulously planned.
”If they did it lower in the building the fire department could have gotten to it sooner. In its simplicity, it was brilliant.”
He said that the two towers have staircases in all four corners of the buildings and were designed to be evacuated in an hour, but it appeared that since the planes crashed into the corners, escape was cut off for those on the floors above.
”I could never conceive of anybody being able to bring down those two buildings,” Brown added.
Minoru Yamasaki Associates issued a statement Tuesday saying the firm was in contact with authorities and had offered assistance.
”We believe that any speculation regarding the specifics of these tragic events would be irresponsible,” the statement said. ”For obvious reasons, MYA has no further comment at this time.”
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