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Why the Towers Fell

Animation of truss falling Animation of a floor truss in the World Trade Center giving way.
The Collapse: An Engineer's Perspective

It wasn't until Dr. Thomas Eagar saw Building 7 of the World Trade Center implode late on the afternoon of September 11th that he understood what had transpired structurally earlier that day as the Twin Towers disintegrated. A professor of materials engineering and engineering systems at the Massachusetts Institute of Technology, Eagar went on to write an influential paper in the journal of the Minerals, Metals, and Materials Society entitled "Why Did the World Trade Center Collapse? Science, Engineering, and Speculation" (JOM, December 2001). In this interview, Eagar explains the structural failure, what can be done within existing skyscrapers to improve safety, and what he believes the most likely terrorist targets of the future may be.

NOVA: After the planes struck and you saw those raging fires, did you think the towers would collapse?

Eagar: No. In fact, I was surprised. So were most structural engineers. The only people I know who weren't surprised were a few people who've designed high-rise buildings.

NOVA: But you weren't surprised that they withstood the initial impacts, is that correct?

Eagar: That's right. All buildings and most bridges have what we call redundant design. If one component breaks, the whole thing will not come crashing down. I once worked on a high-rise in New York, for example, that had a nine-foot-high beam that had a crack all the way through one of the main beams in the basement. This was along the approach to the George Washington Bridge. They shored it up and kept traffic from using that area.

Some people were concerned the building would fall down. The structural engineers knew it wouldn't, because the whole thing had an egg-crate-like construction. Or you can think of it as a net. If you lose one string on a net, yes, the net is weakened but the rest of the net still works.

Traditional design
WTC design Earlier skyscrapers (top) had columns spaced evenly across every floor. The World Trade Center (bottom) broke with tradition by having columns only in the central core and along the exterior walls.

That's essentially how the World Trade Center absorbed an airplane coming into it. It was somewhat like the way a net absorbs a baseball being thrown against it. If you lose a couple of the columns, that's not the end of the world. It will still stand up.

NOVA: The World Trade Center was also designed to take a major wind load hitting from the side.

Eagar: Yes. A skyscraper is a long, thin, vertical structure, but if you turned it sideways, it would be like a diving board, and you could bend it on the end. The wind load is trying to bend it like a diving board. It sways back and forth. If you've been on the top of the Sears Tower in Chicago or the Empire State Building on a windy day, you can actually feel it. When I was a student, I visited the observation deck of the Sears Tower, and I went into the restroom there, and I could see the water sloshing in the toilet bowl, because the wind load was causing the whole building to wave in the breeze.

NOVA: Are skyscrapers designed that way, to be a little flexible?

Eagar: Absolutely. Now, there are different ways to design things. For example, Boeing designs their aircraft wings to flap in the breeze, while McDonnell Douglas used to design a very rigid wing that would not flex as much. You can design it both ways. There are trade-offs, and there are advantages to both ways.

"Most buildings are designed to sway in the breeze."
Most buildings are designed to sway in the breeze. In fact, one of the big concerns in the early design of the World Trade Center, since it was going to be the tallest building in the world at the time, was that it not sway too much and make people sick. You can get seasick in one of these tall buildings from the wind loads. So they had to do some things to make them stiff enough that people wouldn't get sick, but not so rigid that it could snap if it got too big a load. If something's flexible, it can give; think of a willow tree. If you have a strong wind, you want the building, like the tree, to bend rather than break.

NOVA: Brian Clark, one of only four people to get out from above where United 175 hit the South Tower, says that when the plane struck, the building swayed for a full seven to 10 seconds in one direction before settling back, and he thought it was going over.

Eagar: That estimate of seven to ten seconds is probably correct, because often big buildings are designed to be stiff enough that the period to go one way and back the other way is 15 or 20 seconds, or even 30 seconds. That keeps people from getting sick.

Pancaking Upper floors pancaked down onto lower floors, causing a domino effect that left each building in ruins within ten seconds.
NOVA: The Twin Towers collapsed essentially straight down. Was there any chance they could have tipped over?

Eagar: It's really not possible in this case. In our normal experience, we deal with small things, say, a glass of water, that might tip over, and we don't realize how far something has to tip proportional to its base. The base of the World Trade Center was 208 feet on a side, and that means it would have 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. That would have been a tremendous amount of bending. In a building that is mostly air, as the World Trade Center was, there would have been buckling columns, and it would have come straight down before it ever tipped over.

Have you ever seen the demolition of buildings? They blow them up, and they implode. Well, I once asked demolition experts, "How do you get it to implode and not fall outward?" They said, "Oh, it's really how you time and place the explosives." I always accepted that answer, until the World Trade Center, when I thought about it myself. And that's not the correct answer. The correct answer is, there's no other way for them to go but down. They're too big. With anything that massive -- each of the World Trade Center towers weighed half a million tons -- there's nothing that can exert a big enough force to push it sideways.

Impact Even traveling at hundreds of miles an hour, the planes that struck the World Trade Center did not have enough force to knock the towers over.
NOVA: I think some people were surprised when they saw this massive 110-story building collapse into a rubble pile only a few stories tall.

Eagar: Well, like most buildings, the World Trade Center was mostly air. It looked like a huge building if you walked inside, but it was just like this room we're in. The walls are a very small fraction of the total room. The World Trade Center collapse proved that with a 110-story building, if 95 percent of it's air, as was the case here, you're only going to have about five stories of rubble at the bottom after it falls.

NOVA: You've said that the fire is the most misunderstood part of the World Trade Center collapse. Why?

Eagar: The problem is that most people, even some engineers, talk about temperature and heat as if they're identical. In fact, scientifically, they're only related to each other. Temperature tells me the intensity of the heat -- is it 100 degrees, 200 degrees, 300 degrees? The heat tells me how big the thing is that gets hot. I mean, I could boil a cup of water to make a cup of tea, or I could boil ten gallons of water to cook a bunch of lobsters. So it takes a lot more energy to cook the lobsters -- heat is related to energy. That's the difference: We call the intensity of heat the temperature, and the amount of heat the energy.

Continue: The heat was much greater than might have been expected in a typical fire?

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