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It's my understanding that the fires from the jet fuel ignited the enormous stores of papers in the building.  

Plastics also burned.  The temperature inside the buildings rose enough to melt the slender steel support columns, and that led to the pancaking of the towers.  

The shockwave of a building about 1/4 mile high affected neighboring structures.  The pressure pulverized almost everything into a very fine powder that hung in the air for weeks.

Forensic teams studied the surviving columns and other structural materials quite extensively, and explosives experts also surveyed and analyzed the site.

In the trial of the bombers who tried to topple the WTC in 1993 by planting explosives in a basement parking garage, it came out that their intention was to knock that tower over on its side--then it could have killed tens of thousands of people.

After the 1993 attempt the authorities failed to conduct a thorough collection of data about the WTC and used that info in a probabilistic risk assessment which would have shown that the steel in the towers could not survive an intense fire.

Moi, I don't see what the difference is whether the WTC was destroyed by bombs or by planes. An attack is an attack.  And apparently it was far more successful than Osama had ever imagined.

by Plan9 on Sun Nov 13th, 2005 at 03:19:23 PM EST
It is my understanding that kerosene, paper and plastic burn at low temperatures (relatively to the melting point of steel). Also, the smoke coming from the towers was black, indicating a sooty, oxygen-starved, low-temperature fire. No flames were visible from the outside (unlike in the case of Madrid's Windsor building). It is still possible that the fires were able to weaken the steel columns, as losing structural integrity is an entirely different business from melting. But you'd have to ask a metallurgist about that, the Handbook of Chemistry and Physics will tell you the melting point of steel, but not its compression modulus as a function of temperature. Also, the steel columns were coated in concrete, which is a good heat insulator, and the floors and ceilings were also slabs of reinforced concrete.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman
by Carrie (migeru at eurotrib dot com) on Sun Nov 13th, 2005 at 03:29:03 PM EST
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Steel can survive burning jet fuel--but only for a limited time.  The buildings collapsed 110 minutes after the collisions.

There were a lot of design flaws and skirting of building codes when the World Trade Center was built.

The steel columns were supposed to be covered with asbestos, but that was only done part way up on the South Tower.

For a good description of the flaws, and why the fire (in an enclosed space and fed in part by oxygen coming up the shafts) did ultimately cause the failure of the materials, see

While the fire would not have been hot enough to melt any of the steel, the strength of the steel drops markedly with prolonged exposure to fire, while the elastic modulus of the steel reduces (stiffness drops), increasing deflections.

Modern structures are designed to resist fire for a specific length of time. Safety features such as fire retarding materials and sprinkler systems help to contain fires, help extinguish flames, or prevent steel from being exposed to excessively high temperatures. This gives occupants time to escape and allow fire fighters to extinguish blazes, before the building is catastrophically damaged.

It is possible that the blaze, started by jet fuel and then engulfing the contents of the offices, in a highly confined area, generated fire conditions significantly more severe than those anticipated in a typical office fire. These conditions may have overcome the building's fire defences considerably faster than expected. It is likely that the water pipes that supplied the fire sprinklers were severed by the plane impact, and much of the fire protective material, designed to stop the steel from being heated and losing strength, was blown off by the blast at impact.

Eventually, the loss of strength and stiffness of the materials resulting from the fire, combined with the initial impact damage, would have caused a failure of the truss system supporting a floor, or the remaining perimeter columns, or even the internal core, or some combination. Failure of the flooring system would have subsequently allowed the perimeter columns to buckle outwards. Regardless of which of these possibilities actually occurred, it would have resulted in the complete collapse of at least one complete storey at the level of impact.

Once one storey collapsed all floors above would have begun to fall.

It's difficult for me to believe in a conspiracy, given so many competing agencies and professionals studying the collapse of the towers from all kinds of different angles--materials science, fluid dynamics, physics, etc.  There really was a pretty good consensus.

It's a little like the hypothesis about global climate change.  Slowly a body of agreed-upon facts from many different sources has accumulated, peer-reviewed papers have been published, and dissenters have made their cases.  Overall at this point the conclusion of the scientific community is that the rise in global temperature is real.  People are still arguing over the details, though.

Like any scientific hypothesis, the explanation of the collapse will no doubt evolve as more information, more angles on it, and perhaps better technology arise.
Further questioning can't hurt.  We're not talking theology here.

by Plan9 on Mon Nov 14th, 2005 at 12:26:56 PM EST
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