¶ … structural engineers were stunned when the twin towers of the World Trade Center crashed to the ground on Sept. 11. How could these buildings have collapsed? Numerous causes have been looked at including fireproofing, height, impact of the planes, heat from the fire from spilled fuel, and the shell and core of the building itself.
The twin towers were part of a seven-building complex designed by architect Minoru Yamasaki that covered eight city blocks. An 800 x 400-ft foundation box, 65-ft-deep and with 3-ft-thick retaining walls, was under more than half the complex, including the twin towers and the adjacent hotel. The complex was completed in phases beginning in 1970 . The 1.8-million-sq-ft Seven World Trade Center, constructed in the mid-80s, also had a steel moment frame from the seventh story up. (ENR) Each of the twin towers stood more than 415 meters tall, and the buildings' floors enclosed more than one acre of office space. Seen from above, the 110-story twin towers were approximately squares, 209 feet on a side, with 59 columns on each face. The core, containing the elevators, stairwells and mechanical equipment, consisted of a rectangular arrangement of 47 heavier columns. The core columns carried about 60% and the exterior columns 40% of the towers' weight, which totaled 276,000 tons each above the plaza level.
Steel-framed buildings, such as the World Trade Center towers, are encased in cementitious fireproofing to protect framing members -- typically to provide structural integrity for up to three hours. But the fire loads that govern the amount of applied fireproofing are based on requirements for fires fed by materials found in offices such as paper, furniture and drapes, and not by volatile jet fuel.
The buildings, which for a while in the 1970s were the world's tallest, didn't have the traditional skyscraper's skeleton. They were designed to withstand the impact of a Boeing 707, which was the largest commercial aircraft at the time the buildings were designed. An exterior skeleton of closely spaced steel columns along with steel columns ringing a core of elevator shafts, air ducts and stair enclosures held up the towers. "No building before or since has put so much rentable space so high up in the air," says Leslie Robertson, the structural engineer for the towers. (Insight on the News -- Dec. 2001)
On September 11th, 2 Boeing 767s crashed into the World Trade Center, one into the North tower at about the 90th floor and penetrated approximately 150 feet into the building, while the other slammed into the South tower at about the 60th floor. The second jet proceeded to blast a hole through the adjacent side of the building. The impact shattered each of the aircraft, spraying jet fuel throughout the building. It is estimated that each of the 200 ton jets were traveling at least 300 miles per hour and carrying 9,000 gallons of jet fuel. Each impact severed many floors of the towers vertical support columns, presumably disabling the sprinkler system. An explosion of jet fuel created a fire with flames as hot as 2000 degrees Fahrenheit. Abolhassan Astaneh-Asl, a professor at the University of California at Berkeley, said preliminary evidence indicates the structures withstood the impacts of the planes. (CNN -- Oct. 2001) Astaneh-Asl, 53, led a team that studied damage to the San Francisco-Oakland Bay Bridge after the 1989 Bay Area earthquake, and has investigated methods to make buildings bomb-resistant.
Many have questioned why the 1,362-foot tall South tower, failed much as one would expect a felled tree, but the 1,368-foot tall North tower, similarly hit but at a higher floor, telescoped, failing vertically, rather than falling over. A question remaining is why the twin towers appeared to have collapsed in such different ways.
The American Institute of Steel Construction has said that there is much speculation regarding what caused the twin towers to collapse after the large, fuel-laden passenger jets flew into the buildings. (Robertson-2001)...
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