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1. How much strength would the steel have to lose for the WTC to collapse?
2. What temperature would the steel have to reach to occasion this loss of strength?
3. What was the temperature of the fire in the WTC; i.e., did it reach the critically weakening temperature?
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http://www.serendipity.li/wot/mslp_ii.htm
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Question 1:
In the original article, I cited my own experience that a support device must be capable of bearing three times the maximum load that would ever be applied.
It turns out that this rule-of-thumb is applicable only to dynamic loads, not static (structural) loads of commercial buildings. Since then, I have been informed by a commercial structural engineer that the standard ratio for static loads is five, not three. That is, if a bridge is rated to carry 1 ton, it should be capable of bearing 5 tons without collapsing at the time the bridge is built.
Going back to the fire at the WTC, we can see that reducing the steel structure to 60% its rated strength should NOT have weakened it to catastrophic collapse, because at 60% it would still support three times the rated load. The steel structure would have to be reduced to 20% of its rated strength to collapse.
Thus, even if the fire had heated the steel to 550 degrees C (1022 F), that would not have been sufficient to cause the towers to collapse.
Question 2:
The Corus page on fire vs. steel supports (
http://www.corusconstruction.com/fire/fr006.htm) shows that the steel would have to be heated to about 720 degrees C (1320 F) to weaken the steel to 20% of its cool strength.
The text on that page discusses another change in the steel above 550 degrees C (1022 F): It looses elasticity and becomes plastic. Elasticity means that when the steel is bent, it returns to its original shape; it springs back. Plasticity means that the steel is permanently deformed and does not spring back to the original shape.
Springing back or not, our only concern with this page is to determine the point on the graph where the steel would be weakened to 20% its original strength, and that point is 720 degrees C (1320 F).
For steel, 550 degrees C (1022 F) is an important threshold, however, and we should not be glib with it. If a steel tower were heated to 550 C, loss of elasticity could mean that the tower would not spring back to the original shape after a gust of wind, and a series of buffets might cause the tower to fail -- if the strain exceeded the reduced strength of the hot steel.
Question 3:
Now let us make a guess on the actual heat of the fire.
Fortunately, a number of studies have been done under very similar conditions. In Europe, multi-storied "car parks" are often built of steel, and the possibility of vehicle fire is a distinct possibility. A parked vehicle, loaded with gasoline, diesel, tires, engine oil, engine tar, upholstery, hydraulic fluid, etc. can cause a fire that seems very hot. A number of other vehicles could be parked close to the burning one, and they too could catch fire, with a general conflagration. Any number of cars could contain almost any household items from shopping, etc.
These materials are similar to the materials we would expect in the burning offices of the WTC: jet fuel (which is a refined kerosene, very similar to the diesel used in some European cars), oil, upholstery, etc.
A summary of the results of these studies is published on the Corus page. Go to
http://www.corusconstruction.com/ and click on "Fire". Individual articles are listed across the top of the window. The fourth article, "Fire in Car Parks," discusses the temperatures of "any fires that are likely to occur" in a car park this web page is now at
http://www.corusconstruction.com/page_137.htm.
Presumably, one car could catch fire and inflame other cars parked closely nearby. As explained below, "The maximum temperatures reached [in actual test fires] in open sided car parks in four countries" was 360 degrees C (680 F), and structural steel has "sufficient inherent resistance to withstand the effects of any fires that are likely to occur."
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Steel is an excellent conductor of heat, so when you apply heat to a steel structure the heat spreads quickly. So the heat from the fires would have spread through the entire steel structure of each tower. The Twin Towers contained 200,000 tons of steel.
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I'm really looking for all the scientific data I can to prove this thing one way or another .. but for now I think the US govt played a passive role in 9/11 happening.