Why did the World Trade Centers collapse

health : World Trade Center: Fragile Giants

Anyone who knows New York can hardly imagine the Manhattan skyline without the typical twin towers. It seems almost as unreal that the 417-meter-high World Trade Center could be erased within a short period of time. How was it possible to bring the giant steel and concrete giant to collapse so quickly? Graphic: The World Trade Center "It was due to the combined effect of the plane impact and the terrible fire," says Klaus Rückert, professor for structural design at the Technical University of Berlin. If the terrorists had intended it that way, it would have been downright devilishly perfect. Because the towers would not have collapsed with explosives or an aircraft impact alone or with a fire. Of course, it is also possible to bring down such a massive steel-concrete colossus by means of targeted blasting. This doesn't even require a lot of explosives, but - according to Rückert - hundreds of steel girders would have to be drilled into the lower floors. Terrorists cannot do this unnoticed.

On the subject of online special: Terror against America
Photo gallery I: The attack on the WTC and the Pentagon
Photo gallery II: reactions to the attacks
Chronology: The series of attacks against the USA
Reactions: Worldwide horror
Osama bin Laden: America's public enemy number 1 is the prime suspect The plane impact could destroy some upper floors, but this would not have brought the skyscraper to its knees. Even if skyscrapers are not specially designed to withstand plane crashes, the tube construction used is very stable. The inner core, which is anchored in the ground like an oversized rod, provides the decisive support. In the hollow interior of each World Trade Center tower were 56 shafts, each with two elevators mounted one above the other, as well as supply channels with power cables, for example. According to Rückert, such hollow structures are particularly resistant to attacks from hurricanes or vibrations from earthquakes. The construction principle is, so to speak, copied from nature, which it realizes, for example, with bamboo canes.

The steel outer shell, which can be seen as a hollow square rammed into the ground, provides additional stability. Together with the core, this leads to a particularly robust "tube-in-tube construction", which is characteristic of all modern high-rise buildings. A closed facade would be the most stable, says Rückert. But it doesn't work without windows, at the World Trade Center there were even more than 40,000, but they were kept relatively small in order not to offer so much exposure to the wind. In front of the outer steel shell there was still an aluminum facade, which provided the shiny metallic exterior.

The hollow bodies placed one inside the other went 110 floors up and a few more floors down. A two-meter-thick concrete slab, which sat on the rocky rock, served as an anchor at a depth of around ten meters. The individual concrete ceiling structures were also secured by steel supports that were regularly distributed over the area at intervals of around ten meters. The great heat of the burning kerosene gradually softened these steel structures. "High-rise buildings are designed in such a way that they can withstand fires for a certain period of time," says Rückert. Ninety minutes, for example, is required by the legal regulations in Germany. The two New York towers were already so badly damaged by the planes that they only lasted half an hour. The force of the upper floors collapsing then caused the floors below to collapse after the domino effect.

The construction technology of the World Trade Center was developed by the Bangladeshi engineer Fazlar Khan based on the model of the pagodas, says the Leipzig construction technology professor Gert König. This tube construction is also used in buildings over 160 meters high in Frankfurt. The Sears Towers in Chicago are also constructed according to this principle. These buildings overtook the New York twin towers in 1974 and set the world record at 442 meters.

The entire complex of the Sears Towers consists of nine framed tubes, each of which has at least 49 floors. Some tubes have 65 or 90 floors, the two highest towers go up to the 110th floor. The principle, also conceived by Fazlar Khan, has the advantage that the attack surfaces against the strong Chicago wind are divided between the individual towers.

After 23 years, the Sears Towers lost its designation as the tallest building in the world when the 452-meter-high Petronas Twin Towers in Kuala Lumpur were completed in 1997. However, there was a dispute about it, as the pointed antennas of the Malaysian skyscrapers were included, while the Sears Tower antenna did not count. A special feature is the almost 59 meter long "skybridge" that connects the two towers at a height of around 170 meters.

The race for the tallest building in the world is certainly not over yet. The next high-altitude flight is planned for 2004 with the 677-meter-high Center of India Tower in Katangi. However, some projects have never been realized, for example the Beacon of Progress with a height of 457 meters or the Mile High Tower with an unbelievable 1609 meters. In contrast, the Chrysler Building was built in 1930 with a length of 319 meters. The record only lasted a year, then the Empire State Building took the lead at 381 meters. It was only 40 years later that the World Trade Center overtook its New York competitor.

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