The Collapse: An Engineer's Perspective
This piece by "professor" Eagar and student Musso has to be some sort of record for the greatest number of lies, and points of misdirection, ever strung together in an "engineering" article. Comment is highlighted in red.
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. I guess that Eagar realized this too, and concluded that the twin towers were also deliberately demolished. This is probably what he meant when he said that seeing Building 7 implode lead him to understand what had transpired structurally earlier that day as the Twin Towers disintegrated. However, he is not allowed to tell you this. So he produces this piece of misinformation to mislead you. Immediately above is a graphic of the initial north tower collapse. As you can see, Eagar is correct, both collapses are remarkably similar. Click the following for further information:
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".
Eagar supposes, contrary to all evidence, that the fires at the World Trade Center on September 11 were so incredibly hot that the trusses softened and failed as indicated in the animated graphic. It is of utmost importance to his theory, that the fires were considerably hotter than your average office fire, as your average office fire was (by law) considered and planned for by the building designers and, of course, they designed a structure that would not collapse in such a fire.
On February 23 1975, their design was put to the test. For on that day an intense fire broke out on the 11th floor of the north tower of the World Trade Center. The fire subsequently spread down to the 9th and up to the 19th floor, but this fire did not cause failure of the trusses (or any other major structural feature). Here is a quote from a news report:
"The fire department on arrival (at the World Trade Center) found a very intense fire. There were 125 firemen involved in fighting this fire and 28 sustained injuries from the intense heat and smoke. The cause of the fire is unknown."
A fatal flaw in Eagar's theory, is that the tops of the trusses were embedded in the concrete slab, so even if a truss was heated to the point of failure, even if it was dripping molten steel, the concrete slab would still hold the truss up and it could not possibly fall as indicated in the animation. If one truss failed, its load was redistributed to the concrete slab and all the remaining trusses associated with that slab. So the failure of one, or even many trusses, does not lead to overall failure. There is absolutely no way that the trusses could collapse one after the other, as claimed by Eagar. Here is a quote from (a section on the WTC in) Multi-Storey Buildings in Steel , by Godfrey
"Composite action between the concrete and the steelwork is ensured by extending the diagonal web members of the joists (trusses) through the steel decking and embedding them in the (concrete) slab."
Above is a photo of a number of 45 feet (13.5m) long trusses and a buckled steel column after the Broadgate Phase 8 fire (the WTC towers had 35 and 60 foot trusses). The fire occurred while the 14-story high-rise was under construction. Little of the steel was fire protected and the sprinkler system and other active measures were not yet operational. Even though a number of trusses and columns buckled, due to thermal expansion, no collapse was observed at Broadgate.
The system of design of the World Trade Center Towers is called tubular framing, since the perimeter frames of the building are designed to act as a cantilevered tube in resisting lateral forces. This design concept (the so-called tube within a tube architecture) has been employed in the construction of many of the world's tallest buildings. These include the John Hancock Center (1105 ft), the Standard Oil of Indiana Building (1125 ft) and the Sears Tower (1450 ft). In fact, it is the standard design for tall buildings. Vital to the structural integrity of these buildings are the composite floor slabs. In fact, if the floors were not composite, the buildings would eventually collapse.
Eagar totally ignores the fact that the floor slabs were composite (that is, studs or projections from the steel beams were embedded in the concrete slab) preferring to believe the fiction that the floors just rested upon the beams supporting them.
NOVA: After the planes struck and you saw those raging fires, did you think the towers would collapse?
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.
This second statement will only be true because designers of high-rise buildings would know for certain that the buildings were deliberately demolished and would consequently, "not be surprised."
NOVA: But you weren't surprised that they withstood the initial impacts, is that correct?
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.
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.
This is deliberate misdirection. It would be more accurate to say that the towers absorbed the impact of the planes as a sheet of glass absorbs the impact of a bullet. Note that a baseball does more damage to a window than a bullet (even if we arrange that both have the exactly the same momentum). As we all know, the bullet will make a neat little bullet hole while a baseball will smash most of the glass out.
It is the speed (and shape) of the projectile that determines whether the impact damage is localized or spread across a large area. The faster the projectile, the more localized the damage. Other common examples illustrating this effect are, the driving of a nail through a piece of wood, and the firing a bullet through a fencepost. Both are done at speed and thus do only local damage. In both of these examples, the wood just a centimeter or two from the impact point, is essentially undamaged. Similarly, the aircraft impacts were at great speed and severe damage localized to a few floors.
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.
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?
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. 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 http://www.pbs.org/wgbh/nova/wtc/above.html one of only four people (at least 18 survived from the impacted floors or above) 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.
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.
Caption: 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?
It's really not possible in this case.
This statement by Eagar is utterly amazing. It is a wonder MIT has not fired him. Given that the south tower did in fact tip over (and quite visibly so). This shows Eagar's desperation. One thing is certain, all buildings, even the World Trade Center towers, will tip over if enough lateral (sideways) force is applied to them.
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.
The laws of physics do not change. The same laws of physics that tell you a glass of water will tip over, also tell you that (if enough lateral force is applied) large buildings like the World Trade Center towers, will in fact, tip over. You must ask yourself why Eagar chooses to lie about this (he certainly knows the physics, but chooses to tell the world a transparent lie).
What Eagar says about the center of gravity is true, however, it does not imply that the building would come straight down, so his statement is just another piece of misdirection. His implication is clearly wrong, as shown by the fact that the south tower did in fact tip over (videos of the south tower collapse clearly show that the top 30, or so, floors tipped over, but this section was being demolished as it fell, so after a few seconds it was reduced to rubble and no longer fell as a unit).
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: I think some people were surprised when they saw this massive 110-story building collapse into a rubble pile only a few stories tall.
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?
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.
So with the World Trade Center fire, the heat was much greater than might have been expected in a typical fire?
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.
The above graphic provides us with yet another example of misinformation. The World Trade Center towers were 208 feet wide. Hence, from the graphic we can calculate that the wingspan of the pictured plane is 224 feet, however the actual wingspan of a Boeing 767 is 156 feet. Every trick in the book must be tried to convince the gullible that the aircraft strikes plus fire bought down the towers, otherwise the true culprits behind 9-11 may be discovered.
The real damage in the World Trade Center resulted from the size
of the fire. Each floor was about an acre, and the fire covered the whole floor within a few seconds. Ordinarily, it would take a lot longer. If, say, I have an acre of property, and I start a brushfire in one corner, it might take an hour, even with a good wind, to go from one corner and start burning the other corner.
That's what the designers of the World Trade Center were designing for -- a fire that starts in a wastepaper basket, for instance. By the time it gets to the far corner of the building, it has already burned up all the fuel that was back at the point of origin. So the beams where it started have already started to cool down and regain their strength before you start to weaken the ones on the other side. On September 11th, the whole floor was damaged all at once, and that's really the cause of the World Trade Center collapse. There was so much fuel spread so quickly that the entire floor got weakened all at once, whereas in a normal fire, people should not think that if there's a fire in a high-rise building that the building will come crashing down. This was a very unusual situation, in which someone dumped 10,000 gallons of jet fuel in an instant.
There are a number of major problems with Eagar's claims.
(1) One complaint is that much of the jet fuel burnt outside the buildings. This was particularly true in the case of the south tower. After the impact nearly all of the jet fuel would have been spread throughout the area as a flammable mist. When this mist ignited it would have emptied the building of almost the entire fuel load, which then "exploded" outside the building. This is exactly what was seen in the videos of the impacts.
(2) If any quantity of liquid jet fuel did manage to accumulate in the building, then its volatility would lead to large amounts of it being evaporated and not burnt (pyrolysed) in the interior of the building. This evaporated fuel would burn on exiting the building, when it finally found sufficient oxygen.
(3) The jet fuel fires were brief. Most of the jet fuel would have burnt off or evaporated within 30 seconds, and all of it within 2-3 minutes (if all 10,000 gallons of fuel were evenly spread across a single building floor as a pool, it would be consumed by fire in less than 5 minutes). The energy, from the jet fuel, not absorbed by the concrete and steel within this brief period, would have been vented to the outside world.
This means that the jet fuel fire did not heat the concrete slabs or fire protected steel appreciably. Large columns such as the core columns would also not heat appreciably, even if they had lost all their fire-protection. Unprotected trusses may have experienced a more sizeable temperature increase. The jet fuel fire was so brief that the concrete and steel simply could not absorb the heat fast enough, and consequently, most of the heat was lost to the atmosphere through the smoke plume.
(4) Even if the fire-rated suspended ceiling and the spray on fire-protection of the trusses was removed by the impacts and the trusses were heated till they had lost most of their room temperature strength, we know from the Cardington tests and real fires like Broadgate, that the relatively cold concrete slab will supply strength to the structural system, and collapse will not occur. Remember, that at Broadgate and Cardington, the beams/trusses were not fire-protected.
(5) Since the jet fuel fire was brief, and the building still stood, we know that the composite floor slab survived and continued to function as designed (until the buildings were demolished one or two hours later). After the jet fuel fire was over, burning desks, books, plastic, carpets, etc, contributed to the fire. So now we have a typical office fire. The fact that the trusses received some advanced heating will be of little consequence. After some minutes the fires would have been indistinguishable from a typical office fire, and we know that the truss-slab combination will survive such fires, because they did so in the 1975 fire.
(6) Of course, most of the weight of the building was supported by the central core columns. Eagar doesn't bother to tell us how these 47 massive columns might have failed (at least in the case of the north tower, some of these columns, perhaps two or three, would have been displaced by the impacts). We know that the jet fuel fire was too brief to heat them appreciably. Since the central core area contained only lift shafts and stairwells, it contained very little flammable material. This meant that the core columns could only have been heated by the office fire burning in the adjacent region. Consequently, the core columns would have never got hot enough to fail. But we already know this because they did not fail in the 1975 WTC office fire.
(7) You should consider that it has been calculated that if the entire 10,000 gallons of jet fuel from the aircraft was injected into just one floor of the World Trade Center, that the jet fuel burnt with the perfect efficency, that no hot gases left this floor and that no heat escaped this floor by conduction, then the jet fuel could have only raised the temperature of this floor to, at the very most, 536 degrees F (280 degrees C). You can find the calculation here http://members.fortunecity.com/911/wtc/how-hot.htm
(8) Another reason that we know the fires were not serious enough to cause structural failure, is that witnesses tell us this. The impact floors of the south tower were 78-84. Here are a few words from some of the witnesses:
Stanley Praimnath was on the 81st floor of the south tower: The plane impacts. I try to get up and then I realize that I'm covered up to my shoulder in debris. And when I'm digging through under all this rubble, I can see the bottom wing starting to burn, and that wing is wedged 20 feet in my office doorway.
Donovan Cowan was in an open elevator at the 78th floor sky-lobby: We went into the elevator. As soon as I hit the button, that's when there was a big boom. We both got knocked down. I remember feeling this intense heat. The doors were still open. The heat lasted for maybe 15 to 20 seconds I guess. Then it stopped.
Ling Young was in her 78th floor office: Only in my area were people alive, and the people alive were from my office. I figured that out later because I sat around in there for 10 or 15 minutes. That's how I got so burned.
Eagar claims temperatures were hot enough to cause the trusses of the south tower to fail, but here we have eye-witnesses stating that temperatures were cool enough for them to walk away.
Interestingly, a tape of radio conversations between firefighters exists (but only relatives of the dead men have been allowed to hear it). Kevin Flynn, of the New York Times, reported:
Chief Orio Palmer says from an upper floor of the badly damaged south tower at the World Trade Center. Just two hose lines to attack two isolated pockets of fire. "We should be able to knock it down with two lines," he tells the firefighters of Ladder Co. 15 who were following him up the stairs of the doomed tower. Lt. Joseph G. Leavey is heard responding: "Orio, we're on 78 but we're in the B stairway. Trapped in here. We got to put some fire out to get to you." The time was 9:56 a.m.
So now we know that, just a few minutes before the collapse of the south tower, firefighters did not consider the fires to be that serious, and were in fact able to get right into the impact region without being killed by the heat that was (according to Eagar) so intense that the trusses glowed red-hot and failed.
How high did the temperatures get, and what did that do to the steel columns?
The maximum temperature would have been 1,600 degrees F or 1,700 degrees F.
It's impossible to generate temperatures much above that in most cases with just normal fuel, in pure air ("pure" air is only 21% oxygen).
In fact, I think the World Trade Center fire was probably only 1,200 degrees F or 1,300 degrees F.
Eagar randomly settles on a temperature between 1,200 degrees F to 1,300 degrees F. He does this so that his "estimate" would be higher than the first "critical" temperature for open web steel joists of 1,100 degrees F. He does not differentiate between atmospheric temperatures and steel temperatures. The critical temperature is defined as approximately the temperature where the steel has lost approximately 50 percent of its yield strength from that at room temperature. It turns out, that for composite steel structures, the first "critical" temperature, is not really that critical. Here are the critical temperatures adopted by the North American Test Standard (the ASTM E119 standard)
Type of Steel Critical Temperature
Columns 1,000 degrees F (538 degrees C)
Beams 1,100 degrees F (593 degrees C)
Open Web Steel Joists 1,100 degrees F (593 degrees C)
Reinforcing Steel 1,100 degrees F (593 degrees C)
Prestressed Steel 800 degrees F (426 degrees C)
These critical temperatures are only part of the picture. If individual components are exposed to temperatures in excess of those quoted, then they may fail. However, these same components when incorporated in larger structures can be heated to much greater temperatures before failure occurs. The June 1990 Broadgate fire occurred in a high-rise while under construction. Consequently, little of the steel was fire protected. Even though the fire blazed for 4.5 hours, the building did not collapse and runaway type failures did not occur. To investigate the implications of the Broadgate fire on fire standards, the British Steel and the Building Research Establishment performed a series of six experiments at Cardington on a simulated, eight-story building. Here is a quote from one of the research reports from these experiments.
Steel beams in standard fire tests reach a state of deflections and runaway well below temperatures achieved in real fires. In a composite steel frame structure these beams are designed to support the composite deck slab. It is therefore quite understandable that they are fire protected to avoid runaway failures. The fire at Broadgate showed that this (runaway failure) didn't actually happen in a real structure. Subsequently, six full-scale fire tests on a real composite frame structure at Cardington showed that despite large deflections of structural members affected by fire, runaway type failures did not occur in real frame structures when subjected to realistic fires in a variety of compartments. 
And here is a quote from the FEMA report into the WTC collapse (Appendix A).
In the mid-1990s British Steel and the Building Research Establishment performed a series of six experiments at Cardington to investigate the behavior of steel frame buildings. These experiments were conducted in a simulated, eight-story building. Secondary steel beams were not (fire) protected. Despite the temperature of the steel beams reaching 1,500-1,700 degrees F (800-900 degrees C) in three of the tests (well above the traditionally assumed critical temperature of 1,100 degrees F (600 degrees C), no collapse was observed in any of the six experiments.
To get a feeling for how amazingly fire-resistant composite steel structures really are, consider this:
Test 6: The office demonstration test fire at Cardington:
A compartment 18m wide and up to 10m deep with a floor area of 135m^2, was constructed on the second floor, using concrete blockwork. The compartment represented an open plan office and contained a series of work-stations consisting of modern day furnishings, computers and filing systems. The test conditions were set to create a very severe fire by incorporating additional wood/plastic cribs to create a total fire load of 9.4 pounds per square foot (46kg per square meter). Less than 5% of offices would exceed this level (mainly office libraries). The fire load was made up of 69% wood, 20% plastic and 11% paper.
The steel columns were fire protected but the primary and secondary beams (and their connections) were not. The maximum atmosphere temperature was 2215 degrees F (1213 degrees C) and the maximum average temperature was approximately 1650 degrees F (900 degrees C). The maximum temperature of the unprotected steel was 2100 degrees F (1150 degrees C) with a maximum average temperature of about 1750 degrees F (950 degrees C). The steel beams would have only have had 3% of their strength at 2000 degrees F (1100 degrees C), with such little remaining strength left in the steel, the beams could only contribute as catenary tension members. It is also clear that the concrete floors were supplying strength to the structural system by membrane action.
The structure showed no signs of collapse.
One of the conclusions derived from the Cardington tests, was that fire protection for the beams (trusses) was not necessary (in a composite steel structure).
Investigations of fires in other buildings with steel have shown that fires don't usually even melt the aluminum, which melts around 1,200 degrees F. Most fires don't get above 900 degrees F to 1,100 degrees F. The World Trade Center fire did melt some of the aluminum in the aircraft and hence it probably got to 1,300 degrees F or 1,400 degrees F.
This is almost certainly a lie. It is no surprise Eagar does not give a source for this information.
But that's all it would have taken to trigger the collapse, according to my "back of the envelope"
You've pointed out that structural steel loses about half its strength at 1,200 degrees F, yet even a 50 percent loss of strength is insufficient, by itself, to explain the collapse.
Well, normally the biggest load on this building was the wind load (actually the biggest load was the gravity load),
trying to push it sideways and make it vibrate like a flag in the breeze. The World Trade Center building was designed to withstand a hurricane of about 140 miles an hour, but September 11th wasn't a windy day, so the major loads it was designed for were not on it at the time.
As a result, the World Trade Center, at the time each airplane hit it, was only loaded to about 20 percent of its capacity. That means it had to lose five times its capacity either due to temperature or buckling -- the temperature weakening the steel, the buckling changing the strength of a member because it's bent rather than straight. You can't explain the collapse just in terms of temperature, and you can't explain it just in terms of buckling. It was a combination.
Eagar claims that the exterior columns buckled. The exterior columns were visible from outside the building. There was no visible evidence that these columns buckled before the collapse. There is also no visible evidence that these columns were very hot. Photographs of these columns in the debris heap, showed no indications of thermal buckling (I guess the conspirators will claim that the reason no photographs showed thermal buckling of the exterior columns, was that they made sure that such columns were the first to hauled away and melted down). Eagar jumps from buckled columns to buckled beams in a few more lines, mixing up the two as if they are essentially the same.
So can you give a sequence of events that likely took place in the structural failure?
Well, first you had the impact of the plane, of course, and then this spreading of the fireball all the way across within seconds. Then you had a hot fire, but it wasn't an absolutely uniform fire everywhere. You had a wind blowing, so the smoke was going one way more than another way, (by the way, cross ventilation is known to cool a fire)
which means the heat was going one way more than another way. That caused some of the beams to distort, even at fairly low temperatures. You can permanently distort the beams with a temperature difference of only about 300 degrees F.
You mean one part of a beam is 300 degrees F hotter than another part of the same beam?
Exactly. If there was one part of the building in which a beam had a temperature difference of 300 degrees F, then that beam would have become permanently distorted at relatively low temperatures. So instead of being nice and straight, it had a gentle curve. If you press down on a soda straw, you know that if it's perfectly straight, it will support a lot more load than if you start to put a little sideways bend in it. That's what happened in terms of the beams. They were weakened because they were bent by the fire.
Eagar is, as usual, incorrect here. Buckling of beams does not necessarily lead to failure, in fact, in fires it is beneficial. For example, a laterally restrained beam (that will buckle at relatively low temperatures due to the lateral restraint) will not suffer runaway till around 900 degrees C, whereas, a simply supported beam carrying the same loads (that will not buckle) will suffer runaway at around 450 degrees C. So the beam that undergoes buckling is much preferred in a fire situation. Here are two more quotes from research papers examining the Cardington experiments.
In structures such as the composite steel frame at Cardington, the slab strongly restrains the thermal expansion strains and consequently develops large membrane compression and tension forces in the composite steel-concrete floor system. The membrane compressions can be limited by the large downward deflections which occur through thermo-mechanical post-buckling effects and thermal bowing (these are nonlinearly additive). The resulting behaviour is then a combination of displacement and force responses. The heated steel part of this composite system, if unprotected, rapidly reaches its axial capacity (through local buckling and strength degradation), and produces a beneficial effect by limiting and then reducing the total membrane compression, so allowing increased expansion of the steel through softening and ductility. This is clearly a desirable behaviour here, as it reduces the force imposed on the structure by the expansion forces and allows the damage to be localized. 
In composite floor slabs, buckling of the steel beams as a result of large compressions induced by restrained thermal expansions, is a positive event. The buckle allows the increase in length, as a result of thermal expansion, to be accommodated in downward deflections relieving axial compressions. 
So, in buildings comparable to the World Trade Center, buckling, paradoxically, has a beneficial effect.
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.
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?
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
Those angle clips probably had two or three or four times the strength that they originally needed. They didn't have the same factor-of-five safety as the columns did, but they still had plenty of safety factor to have people and equipment on those floors. It was not that the angle clips were inadequately designed; it was just that there were so many of them that the engineers were able to design them with less safety factor. In a very unusual loading situation like this, they became the weak link.
I've read that the collapse was a near free-fall.
Yes. That's because the forces, it's been estimated, were anywhere from 10 to 100 times greater than an individual floor could support. First of all, you had 10 or 20 floors above that came crashing down. That's about 10 or 20 times the weight you'd ever expect on one angle clip. There's also the impact force, that is, if something hits very hard, there's a bigger force than if you lower it down very gently.
Here is an article that has been posted at various sites on the internet.
All that one needs to know, to be able to conclusively prove that the Twin Towers were demolished, is that the towers fell in roughly 10 seconds, that is, that they fell at about the same rate that an object falls through air.
Anyone with a little common sense will realize that the top of a building does not pass through the concrete and steel that comprises the lower portion of the building at the same rate that it falls through air. This just doesn't happen, unless, of course, the lower part of the building has lost its structural integrity (and this is usually due to the detonation of a multitude of small explosive charges as seen in controlled demolitions).
The fact that the towers collapsed in about 10 seconds is a statement that the upper portion of each of the towers passed through the lower portion at about the same rate that it would have fallen through air. The fact that the towers fell this quickly (essentially at the rate of free-fall) is conclusive evidence that they were deliberately demolished.
Believing that there is nothing wrong with the towers collapsing so quickly, is roughly analogous to believing that people pass through closed doors as quickly as they pass through open doors.
The fact that they fell at such a rate means that they encountered essentially no resistance from the supposedly undamaged parts of the structure. That is, no resistance was encountered from any of the immensely strong parts of the structure that had held the building up for the last 30 years. From this, one can conclude that the lower undamaged parts were actually very damaged (probably by the detonation of a multitude of small explosive charges as is usual in a controlled demolition).
Miraculously, a number of firefighters survived inside Tower One. They were on the third or fourth floor in a stairwell, and immediately after the collapse they looked up and saw blue sky above their heads -- their part of the stairwell survived. How is that possible, with all the force of that 500,000-ton building coming down?
Well, you have to understand the stairwells were reinforced areas of the building. The stairwells were in the central core, which had more steel than the outer areas, which were big open floors. So that extra steel formed a little cage to protect them. It's still amazing, though. Now, there could have been someone two floors below who could have been completely crushed. It just depends on how the steel buckled. If you take that soda straw again, and you push it sideways, it will develop a buckle at some location, probably somewhere in the middle third. Well, if you happen to be where the buckling occurs, that area is going to get smashed, but if you're, say, below where the buckling occurred, basically the whole thing can push sideways. They were very, very fortunate that they happened to be in an area that was somewhat shielded and protected by all the extra steel in the central core.
I read one of those people's statements in the paper the other day, and he said that if they'd been in the lobby, they'd be gone. I was in the lobby of the World Trade Center years ago, and it was some three or four stories tall. What was going to buckle? Well, the lobby had the longest columns, so they were going to buckle. Those firefighters were just above that, so they were protected by the buckling underneath, within this sort of steel cage.
In fact, that's how they design automobiles for crashworthiness. They try to design the passenger compartment to be a cage, and the hood and trunk are supposed to deform and absorb the energy so that you're protected by this little cage of steel that hopefully won't deform.
Caption: Engineers have found evidence that the aluminum of the planes' fuselages and wings may have melted, but there is no evidence that it burned.
There's a theory that the aluminum of the planes caught fire.
Yes, a number of people have tried to reinforce that theory. Now, the aluminum of the planes would have burned just like a flare. Flares are made out of aluminum and magnesium, so are fireworks, and they burn hot enough to melt steel in certain cases. However, they have had people sorting through the steel from the World Trade Center, and no one has reported finding melted steel, which means that we didn't have that aluminum flare. In any case, burning aluminum would have been white-hot, about 4,000 degrees F, and someone would have seen it even through that dense black smoke. Of course, aluminum can burn. That's what demolished the [British destroyer] Sheffield
in the Falklands War [when it was struck by an Argentinian missile]. It wasn't the Exocet missile that destroyed the superstructure of the Sheffield
. The missile wasn't big enough, just like the plane wasn't big enough to bring down the World Trade Center. That Exocet missile did damage the Sheffield,
but what doomed the Sheffield
was the aluminum superstructure caught fire and burned. So you suddenly had something like 1,000 or 10,000 times as much fuel as you had in that Exocet missile. Now, this is not
a type of fire we have to worry about in buildings. We don't have anywhere close to those types of conditions. And we didn't have those in the World Trade Center, in my opinion.
How soon will a definitive report of the causes of the collapse be released?
Well, there's some very sophisticated analysis that various people in the government, at universities, and at structural engineering firms are doing to understand it. Most of those people have not yet published any conclusions. To do a good job of research on something like this can typically take one to two years. I don't expect to see any conclusive reports probably until about the first anniversary of the attack.
There are different levels of analysis. You can do the back-of-the-envelope, which was what I and other people did early on. But to do the full analysis will take much longer. I suspect there will still be people worrying about this ten years from now.
In your back-of-the-envelope analysis, you concluded the World Trade Center was not defectively designed, but not everyone apparently accepts that conclusion.
A lot of people said, Well, the building failed. That's true, but nothing is indestructible. The question is, why did it fail? In this case, as I've explained, it was the fire covering the whole floor in a few seconds that made this different from any other fire that anyone had ever designed for.
If people say, Well, couldn't we have designed it for this, I say, Yes, we could have. We could build buildings that could survive a jet running into them with a full fuel load. In fact, the military does. But they're bunkers. We build these things for the President and the rest of the 150 leaders of the country to go to as a secure area. You can do that, but your building costs go up by a factor of about 100. Well, do we want to have 100 times fewer homes for people to live in? Do we want to have 100 times fewer roads? If we were to harden everything against a terrorist attack, we'd push ourselves back into the first half of the 19th century in terms of living style. Now, some people might consider that an improvement, but not everybody, so society has some important tradeoffs here. There's got to be some middle ground where we can make things more secure but not destroy our standard of living.
Anything we should do now to retrofit existing skyscrapers like the Sears Tower?
Well, one of the things that's really important and is relatively inexpensive is a public communication system. I've been in high-rises when the fire alarm goes off, and everyone looks around the room and decides, Should we just continue meeting and ignore the fire alarm, or should we evacuate? Fortunately, in most cases -- and I've had to be the person in a few of those cases -- people say, Look, it's a fire alarm. We don't know if it's real. Evacuate. So you need better public-address systems to inform people that this is not a test, this is not a false alarm, you'd better get out of the building.
Caption: Better communications systems may have allowed more people to escape the towers before they collapsed, Eagar believes. For instance, if more people had known that Stairway A in the South Tower, shown here in green, had survived the impact, more people may have gotten out before the building collapsed.
Survivors from the World Trade Center have said that some people took four or five minutes to figure out there was something more than just some false alarm. Other people started moving immediately. Obviously, the quicker people started to move, the better chance they had of reaching safety.
How about improving the fire safety of the building or putting in extra stairwells?
These are very difficult things to redesign into current buildings. They can and will be added to future buildings. The simplest thing is the communication system. And better training of firefighters. Those things will definitely be done. If you look at the World Trade Center disaster, it would have been greatly minimized if the safety personnel had been aware of the danger they were in. They didn't realize it was going to collapse. As I said earlier, there are only a few engineers in the country who had ever designed skyscrapers like this who would have realized, but they couldn't communicate within that first hour with the people at ground zero. Nobody could call to New York City at that time.
So better communication. The military's known that for years. They've invested tremendous amounts of money in better communications. That's been one of the differences in having fewer lives lost on the American side in recent wars. We've got much better C3I -- Command, Control, Communications, and Intelligence. They've spent billions of dollars, and it's saved thousands and thousands of lives in the military. We can do that on the civilian side as well for these big structures, though, in my opinion, skyscrapers are not the problem anymore.
I think the terrorist danger will be other things. A terrorist is not going to attack the things you expect him to attack. The real problem is pipelines, electrical transmission, dams, nuclear plants, railroads. A terrorist's job is to scare people. He or she doesn't have to harm very many people. Anthrax is a perfect example. If someone could wipe out one electrical transmission line and cause a brownout in all of New York City or Los Angeles, there would be hysteria, if people realized it was a terrorist that did it.
Fortunately, we have enough redundancy -- the same type of redundancy we talk about structurally in the World Trade Center -- in our electrical distribution. We have that redundancy built in. I shouldn't say this, but this was how Enron was able to build up a business, because they could transfer their energy from wherever they were producing it into California, which was having problems, and make a fortune -- for a short period of time.
Gas pipelines don't have redundancy built in, though.
No, but one advantage of a gas pipeline is the damage you can do to it is relatively limited. You might be able to destroy several hundred yards of it, but that's not wiping out a whole city. The bigger problem with taking out a gas pipeline is if you do it in the middle of winter, and that gas pipeline is heating 20 percent of the homes in the Northeast. Then all of a sudden you have 20 percent less fuel, and everybody's going to have to turn the thermostat down, and you're going to terrorize 30 million people. The lesson we have to learn about this kind of terrorism is we have to design flexible and redundant systems, so that we're not completely dependent on any one thing, whether it's a single gas pipeline bringing heat to a particular area or whatever.
Remember the energy crisis in 1973? That terrorized people. People were sitting in long lines at gas pumps. It takes five or 10 years for society to readjust to a problem like that. What happened in the energy crisis in 1973 was we had essentially all our eggs in one basket -- the oil basket. But by 1983, electric generating plants could flip a switch and change from oil to coal or gas, so no one could hold a gun to our head like they did before.
Thomas Eagar is Thomas Lord Professor of Materials Engineering and Engineering
Systems at MIT. He was recently nominated to serve on a National Research
Council committee on homeland security. To see Eagar's article, "Why Did the
World Trade Center Collapse? Science, Engineering, and Speculation," which was
coauthored by MIT graduate student Christopher Musso, go to
Thomas Eagar is employed by MIT. He blackens the good name of MIT as well as his own.
Interview conducted by Peter Tyson, editor in chief of NOVA Online
 Multi-Storey Building in Steel, GB Godfrey (Editor), Collins, London, England, 1985.
 Behaviour of Steel Framed Structures under Fire Conditions; School of Civil & Environmental Engineering; The University of Edinburgh.
 Structural Performance of Redundant Structures under Local Fires; J.M.Rotter, A.M.Sanad, A.S.Usmani and M.Gillie; Proceedings of Interflam99 - Edinburgh.
 The Behaviour of Multi-storey Composite Steel Framed Structures in Response to Compartment Fires; Susan Lamont. PhD Thesis, University of Edinburgh, 2001.