Why did the World Trade Center towers collapse? Weren't they designed to be earthquake-proof and impact-proof?
The 110-story twin towers of the World Trade Center, which stood at a height of 1,353 feet, were designed to withstand earthquakes as well as impacts like that of a plane. This was evidenced by the fact that the buildings did not fall over on impact, says Brian Markham, a structural engineer with the Ove Arup & Partners engineering firm. The more likely cause of the buildings' collapse was the resulting fire, exacerbated by the huge quantities of jet fuel present.
When a fire ignites in a large building, its steel core does not melt, but over time it weakens. As the steel supporting the floors collapses, a "pancaking" effect will result, with each of the upper floors collapsing onto the floor below. This is why the disintegration of the towers was not limited to the top floors. With the accumulated weight of each collapsed floor, the stacked floors continued to fall. This explains why the building collapsed vertically, rather than tipping over.
Big buildings are structurally designed to withstand a blaze for between 2 to 3 hours--time enough to put out the fire and evacuate occupants. Both World Trade Center towers collapsed in about an hour, probably because the jet fuel caused the fire to burn hotter and faster than any would have anticipated.
Markham, while unaware of the fire rating for the World Trade Center buildings, suggests that even with a standard 2- to 3-hour burn time, it would have been extremely difficult to evacuate the 50,000 people estimated possibly to have been inside the buildings. He adds that the tragedy would have been compounded because, while many of the buildings' occupants could not have been evacuated via the exits, they would have been moved to ostensibly "safer floors." As it turned out, there were no safer floors.
Explainer thanks Brian Markham.