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Why the Towers Fell
Did You Know?
  1. Most structural engineers were surprised when the World Trade Center towers collapsed.

  2. Engineers believe that part of the reason why the towers remained standing as long as they did after impact was because of redundancy in their design: The weight of upper floors pushing down on columns lost in the impact was transferred to other columns nearby that were left intact.

  3. Only four people escaped either tower from above the floors where the planes struck, using what appears to have been the only stairwell not destroyed or blocked by the impacts: Stairway A in the South Tower.

  4. One of those survivors recalled that when struck by United 175, the South Tower swayed in one direction for seven to ten seconds before swinging back and stabilizing.

  5. The World Trade Center was designed to withstand hurricane-force winds.

  6. It was also designed to withstand the impact of a Boeing 707, though engineers apparently did not take into consideration the plane's fuel load.

  7. Each plane that hit the Twin Towers released an estimated 10,000 gallons of flaming jet fuel into the buildings.

  8. Temperatures of the fuel fire may have reached 2,000°F.

  9. Though no evidence has turned up that the fires burned hot enough to melt any of the steel, eventually the steel lost 80 percent of its strength because of the intensity of the fire.

  10. While there are signs that the fire melted aluminum from the fuselage or wings of at least one of the planes, there is no evidence that the aluminum burned.

  11. Many structural engineers feel the weak link in the chain within the towers was the angle clips that held the floor trusses between the interior and exterior steel columns.

  12. The angle clips were smaller pieces of steel than the columns and therefore gave out first.

  13. Each floor was designed to support approximately 1,300 tons beyond its own weight, but when one or more gave way in the intense fire of the impact zone, the combined weight of higher floors crashing down reached into the tens of thousands of tons.

  14. Each tower weighed about 500,000 tons.

  15. There was no chance of either tower tipping over, for a 500,000-ton building has too much inertia to fall any way except virtually straight down.

  16. Each 208-foot-wide building would had to have tipped at least 100 feet to one side in order to move its center of gravity from the center of the building out beyond its base.

  17. Each building collapsed in about ten seconds, hitting the ground with an estimated speed of about 125 miles per hour.

  18. The collapse was a near free-fall. With no restraint, the collapse would have taken eight seconds and would have impacted at about 185 miles per hour.

  19. The reason the 110-story towers collapsed into a rubble pile only a few stories high was that they were about 95 percent air.

  20. The roughly 300,000 tons of steel from the World Trade Center is fully recyclable and represents just a single day's production by the U.S. steel industry.


Note: Unless otherwise specified, all sources are NOVA/WGBH.

7. "Towers Fell as Intense Fire Beat Defenses, Report Says," by James Glanz and Eric Lipton, The New York Times, 3/29/02, p. A14.
8. Ibid, p. A1.
13. "Why Did the World Trade Center Collapse? Science, Engineering, and Speculation," by Thomas W. Eagar and Christopher Musso, JOM: The Journal of the Minerals, Metals, and Materials Society, December 2001, available at
15. Ibid.
17. Ibid.
18. Ibid.
20. Ibid.

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