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.

Above, is an animated graphic of the collapse of World Trade Center Building 7. This is clearly a controlled demolition.

Caption: Animation of a floor truss in the World Trade Center giving way.

Eagar claims that the collapse of the twin towers was primarily due to failure of certain structural components (pictured in the animated graphic above) used to support sections of the concrete floor slabs. He refers to them as trusses. However, the word truss generally refers to the diagonal reinforcement of a rectangular frame, and so can be applied to a wide variety of structures. The items in question, are more correctly called "open web joists" or "bar joists".

Part of the problem is that people (including engineers) often confuse temperature and heat. While they are related, they are not the same. Thermodynamically, the heat contained in a material is related to the temperature through the heat capacity and the density (or mass). Temperature is defined as an intensive property, meaning that it does not vary with the quantity of material, while the heat is an extensive property, which does vary with the amount of material. One way to distinguish the two is to note that if a second log is added to the fireplace, the temperature does not double; it stays roughly the same, but the size of the fire or the length of time the fire burns, or a combination of the two, doubles. Thus, the fact that there were 90,000 L of jet fuel on a few floors of the WTC does not mean that this was an unusually hot fire. The temperature of the fire at the WTC was not unusual, and it was most definitely not capable of melting steel.

It is known that structural steel begins to soften around 425°C and loses about half of its strength at 650°C.4 This is why steel is stress relieved in this temperature range. But even a 50% loss of strength is still insufficient, by itself, to explain the WTC collapse. It was noted above that the wind load controlled the design allowables. The WTC, on this low-wind day, was likely not stressed more than a third of the design allowable, which is roughly one-fifth of the yield strength of the steel. Even with its strength halved, the steel could still support two to three times the stresses imposed by a 650°C fire.

The additional problem was distortion of the steel in the fire. The temperature of the fire was not uniform everywhere, and the temperature on the outside of the box columns was clearly lower than on the side facing the fire. The temperature along the 18 m long joists was certainly not uniform. Given the thermal expansion of steel, a 150°C temperature difference from one location to another will produce yield-level residual stresses. This produced distortions in the slender structural steel, which resulted in buckling failures. Thus, the failure of the steel was due to two factors: loss of strength due to the temperature of the fire, and loss of structural integrity due to distortion of the steel from the non-uniform temperatures in the fire.

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.

Caption: 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.

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.

Picture of the World Trade Center south tower tipping over.
NOVA: Was there any chance they could have tipped over?
Eagar: It's really not possible in this case.

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.

Eagar makes a real ass of himself in this article. To see how much of a fool Eagar is making of himself and his profession, think through the following thought experiment. Take the WTC and remove ten floors, but only say, the eastern half of each floor (so you have a situation analogous to a lumberjack cutting a slot half way through a tree), and imagine how the WTC (tree) would fall. One things for sure, it would not fall straight down. By Eagar's "logic" a tree that is extremely massive must fall down through itself, rather than tip over (because a tree is made of atoms and atoms are mostly empty space). What a dope.

Caption: 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: 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.

Caption: Watch an animation of the Boeing 767 aircraft hitting the North Tower and the rapid spread of the resulting fireball through the building.

NOVA: So with the World Trade Center fire, the heat was much greater than might have been expected in a typical fire?

Eagar: Right. We had all this extra fuel from the aircraft. Now, there have been fires in skyscrapers before. The Hotel Meridien in Philadelphia had a fire, but it didn't do this kind of damage.

Eagar is referring to the One Meridian Plaza fire of February 23-24, 1991, which burnt for 19 hours. Strange how Eagar manages to "forget" the 1975 World Trade Center north tower fire. When he says "it (the One Meridian Plaza fire) didn't do this kind of damage" he means that One Meridian Plaza did not collapse. So here is another example (the first being the World Trade Center North Tower in 1975) of a skyscraper that endured much more serious fires than those of September 11, without collapsing. In fact, before September 11, no steel framed skyscraper had ever collapsed due to fire. However, on September 11, it is claimed that three steel framed skyscrapers (both towers and World Trade Center Seven) collapsed mainly, or totally, due to fire.

But the steel still had plenty of strength, until it reached temperatures of 1,100 degrees F to 1,300 degrees F. In this range, the steel started losing a lot of strength, and the bending became greater. Eventually the steel lost 80 percent of its strength, because of this fire that consumed the whole floor. If it had only occurred in one little corner, such as a trashcan caught on fire, you might have had to repair that corner, but the whole building wouldn't have come crashing down. The problem was, it was such a widely distributed fire, and then you got this domino effect. Once you started to get angle clips to fail in one area, it put extra load on other angle clips, and then it unzipped around the building on that floor in a matter of seconds.

NOVA: Many other engineers also feel the weak link was these angle clips, which held the floor trusses between the inner core of columns and the exterior columns. Is that simply because they were much smaller pieces of steel?

Eagar: Exactly. That's the easiest way to look at it. If you look at the whole structure, they are the smallest piece of steel. As everything begins to distort, the smallest piece is going to become the weak link in the chain. They were plenty strong for holding up one truss, but when you lost several trusses, the trusses adjacent to those had to hold two or three times what they were expected to hold.

More crap from Eagar. Does he really believe that the towers were only held together with a couple of rivets and duct tape. Here is a quote from the FEMA report into the WTC collapse (Chapter 2).

Pairs of flat bars extended diagonally from the exterior wall to the top chord of adjacent trusses. These diagonal flat bars, which were typically provided with shear studs, provided horizontal shear transfer between the floor slab and exterior wall, as well as out-of-plane bracing for perimeter columns not directly supporting floor trusses.

Eagar claims that the trusses were connected to the perimeter wall only by what he calls, "angle clips". The truth is that every 160 inches, the perimeter wall was solidly attached to a 24 x 18 inch metal plate that was covered with shear studs and set in the concrete slab. In addition a pair of 6 foot long, flat, steel bars lined with shear studs were welded to the plate and to the top chord of the adjacent trusses. These bars were also set in the concrete slab. Between these plates similar pairs of 6 foot long, flat, steel bars connected directly to tabs on the perimeter columns. So these features, as well as the angle clips, connected the perimeter wall to the concrete slab and hence to the rest of the building. Below, is a picture of these plates and steel bars before the concrete slab was poured. The plates are the dark rectangular objects along the perimeter wall. The steel bars are the V-like features