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9/11 Twin Towers

In this Moment of Science, we explain how and why the collapse of the Twin Towers occurred.

The Twin Towers memorial at night.

Photo: dliberty3 (Flickr)

Built in the 1960s, each Twin Tower had a tube structure, meaning that the concrete-encased steel columns that supported the building and bore its weight were on the building's outside.

Amid the shocking events that occurred on September 11th, one of the most startling was the sight of the collapse of the Twin Towers soon after hijacked planes hit them.

Structure And Design

Designed and built in the 1960s, each tower had a tube structure, meaning that the concrete-encased steel columns that held the building up and bore its weight were on the building’s exterior. Inside, each Tower had a vertical steel core at its center that housed stairwells and elevators.

Steel beams radiated outward from the core, supporting the floors and attaching to the exterior columns. Part of the reason that the buildings collapsed as they did is because aside from the floor reinforcements, there were no columns holding the building up from the inside.

Cause Of Collapse

When the planes hit they severely damaged the exterior columns. But the impact alone was not what caused the collapse; it was the intense heat generated by exploding airplane fuel. Each plane carried 24,000 gallons of fuel, enough to create fires hotter than 1472 degrees Fahrenheit.

Exposed to such intense heat, both the exterior columns and floor beams began to weaken. Consequently, the upper floors caved in, collapsing the floors below. The accumulated weight was enough to destroy the entire building.

What Could’ve Been

Even though they eventually gave way, the Towers’ structure saved thousands of lives by withstanding the heat for as long as they did, allowing many of the buildings’ occupants to reach the ground.

Had they been designed differently, the Towers might have fallen over like giant trees, creating a disaster of even larger proportions.

  • Anonymous

    Sorry! I don’t speak english.  It’s wonderful… United States, our big brother is stand up again. Before, the Twin Towers were material US biulding icon … today its bigger and greater than before because the 3D Twin celestial lights Towers it’s a heavenly US icon, non destructible never.  Thanks for the American Great Spirit and Hearts.  Love,  isolda

  • Anonymous

    Sept. 12, 2011Sorry! I don’t speak english.  It’s wonderful… United States, our big brother is stand up again. Before, the Twin Towers were material US biulding icon … today its bigger and greater than before because the 3D Twin celestial lights Towers it’s a heavenly US icon, non destructible never.  Thanks for the American Great Spirit and Hearts.  Love,  isolda

  • Keith Miller

    This article is an awfully brief description that may be enough to satisfy a non-discerning mind that doesn’t want to take the time to really consider what is being said. So, steel beams radiated out from the core. Well, the beams had to be attached to the core which surely would provide added support for the floor beams that were attached to the exterior columns. But forget the floor reinforcement which was surely both at the inner core and the exterior columns. The article seems to be getting us to think just in terms of the horizontal floors being able to collapse just as the Commission Report description did. What they both ignore is that continuous VERTICAL columns don’t and can’t just collapse in on themselves, especially not 287 all at once with no resistance in a matter of seconds. Some claim that the vertical columns above the fire/heat failed because the fire insulation was torn or blown off (but this of course would ONLY have occurred to the small percentage of columns in the immediate vicinity of the plane impact but as for all of the columns below the heat and fire there was nothing to remove the insulation and not much heat to weaken the 287 VERTICAL columns. Some also say that fuel ran down the elevator shafts heating and weakening the lower columns. But ALL of the fuel was on fire all at once so how far is it going to flow. Most agree that ALL the fuel burnt up in the first 10 minutes or so. Even if it did flow down the shafts this would only effect the few columns in the immediate vicinity of the shafts but we are talking about 47 inner core and 240 exterior columns These columns were riveted and welded and were continuously equally strong from the bottom up. The joints we can be sure were not weak joints by any means. They are not going to just fly apart without resistance which is in fact what we see, unless some unusual force is applied. If you think about it all lower portions of buildings are build to resist the force of gravity of the portions of the building above. The lower portion is not going to just give up all resistance in a flash. You can see for yourself in short videos another important point. One video is “Firefighters Architects Engineers Expose 9/11 Myths”. Just see from 1:09:00 to 1:16:30 (or prefferably start from1:00:00) and the same point can be seen in a video “WTC Tower 1 Collapse NBC Slow Motion” and you can start and stop to see the full evidence that the upper portion of the building shown is being blown up and is no longer even a solid object with the ability to force the lower building to be crushed. Another video explains the same thing “9/11 North Tower and The Pile Driver That Wasn’t”. Another fascinating article to read is “Where Are The 9/11 Whistleblowers?”. Another video also of great import. “9/11 Commission Report Bars 503 1st Responder Eyewitnesses”.

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