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The Durande
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Let me start by emphasising that I am not, I repeat, not, a conspiacy theorist.

But I have some questions about the world trade center terrorist attacks.

Hopefully, someone here, with adequate knowledge about the physics and facts involved can answer some questions that I am trying to debate ith an acquantance of mine who is a conspiracy theorist.

1. Could wtc #7 have toppled neatly (in about 8 seconds) and looked as if it was a professional demolition job, simply due to fires smoldering all day in the lower floors? And even if diesel tanks blew up in the basement, would it have toppled like that?

2. Could wtcs 1&2 have toppled neatly to the ground the way they did, simply from the fires in the buildings ( supposedly melting and/or weakening the steel structure?)

3. Let me put extra EMPHASIS on the fact that I am arguing AGAINST this conspiracy theorist, not trying to play devil's advocate like they always do. (but, in conversations with some firemen and engineers that I know, I have run into some skepticism about the way that things have been reported as happened)

So, in this case, i do feel that I am not aruing against the arbitrary.

Personally, I feel that the reporting of the events on 9/11 was accurate, not because I inherently trust the media and government (who could in this day and age?), but because I fail to see the motive for a cover-up. And I seriously object to anyone (my acquantance included) who thinks it was all arranged by some members of either our gov. and/or our intelligence community to put the Patriot Act in place. Also, I don't see any major problems with the Patriot Act, because, even before it, if the gov. wanted tp invade anyone's privacy - it could. I am well-schooled enough to know that any time a gov official with the ability wants to invade my privacy it can do so quite easily, Patriot Act or no Patriot Act.

Anyway, I would appreciate any answers dealing with the physics and chemistry involved in the wtc bombings. ***Please dont give replies that deal with me debating with a conspiracy theorist. I am quite aware of the fact that one does not argue with the arbitrary. Like I have said, my discussions with engineers have led me to believe that, at least in the case of wtc #7, arguments for a cover-up are not completely arbitrary. Thanks in advance for any replies

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1.  Could wtc #7 have toppled neatly (in about 8 seconds) and looked as if it was a professional demolition job, simply due to fires smoldering all day in the lower floors?  And even if diesel tanks blew up in the basement, would it have toppled like that? 

2.  Could wtcs 1&2 have toppled neatly to the ground the way they did, simply from the fires  in the buildings ( supposedly melting and/or weakening the steel structure?)

First, I'm not a demolition expert, but I do have a degree in structural engineering.

Intense heat does weaken structural steel. The members (columns, girders, etc.) are insulated to protect against this. In the WTC main towers, sprayed asbestos was used up to the 63rd floor, if I remember correctly. This was about the time during its construction that problems with asbestos became known and a different insulating sustance was used above this level.

The collapse of WTC 1 and 2 both began at the point of impact. They collapsed from the top down, unlike a controlled demolition where the collapse begins at the bottom. They fell straight down because that's the direction gravity works. Once the top section of the buildings (above the point of impact/failure) begins moving and gathering momentum, the floors below and the structures supporting them would collapse like toothpicks. No commercial structural system in the world is designed to withstand that kind of impact loading.

As for WTC 7, I have only spent some brief amount of time looking at a couple of conspiracy sites. They point to "smoke plumes" from the upper floors at the moment of collapse as evidence of demolition charges. But demolition charges are never placed in upper floors. They're at the bottom, where the structrual members are severed. When the building began to collapse, the air and smoke inside had to go somewhere. My guess would be that these plumes are just smoke escaping from a window, freshly shatted by the building's movement. Also, they claim that the fires in the building were minor, yet from the pictures, the fire appears to be anything but minor; practically the whole building is engulfed in smoke.

So why did the whole building move at once? Wouldn't this have required the instantaneous failure of every column at the bottom of the structure? It is important to remember that failure by heating is not an instantaneous failure, but a gradual weakening of the structural members. As a particular column weakens, the load on it is partially taken up by those around it, so that the weakened member remains on the verge of failure without having actually collapsed. (I won't go in to the exact dynamics of how this occurs.) Of course, if the basement of the building is engulfed in a diesel fire, this gradual weakening is going on throughout the structual system, with loads dynamically shifting back and forth between members until every member is on the verge of total failure. At this point, as soon as one member fails, the load it bore will immediately be transfered to the members around it, which in turn triggers their failure and so forth. In this way the whole system collapses, virtually instantaneously. It's neither miracle, nor conspiracy; it's just physics.

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The fire in the two World Trade Center towers was much more than hot enough to very substantially weaken the structural steel.

Steel loses its strength rapidly as temperature rises, and remember, those planes were fully of kerosene (i.e., jet fuel) when the terrorists crashed them. How hot was the fire? I've heard numbers well in excess of 2000F.

A few data on the high-temperature strength of structural steel:

- At about 1300F, it has lost over 75% of its room-temperature strength, and its strength drops very rapidly after that. (This document has a graph that shows this: document) So 1300F would be enough to seriously weaken the buildings' structure, and the fire was much hotter than that.

- Steel is commonly hot-rolled when it's made in the steel mill at temperatures as low as about 2100F. That means that it has almost no strength at that temperature; that's the temperature at which it is soft enough so that huge slabs can be squashed into shape in a steel mill. So structural steel is just about absolutely worthless once it reaches temperatures above 2000F. (See The Making, Shaping and Treating of Steel for an in-depth discussion of both the behavior of steel at high temperatures, and the use of high temperatures in processes to form steel.)

The reason the buildings stood for as long as they did was that whatever insulation there was on the steel structural members slowed down the temperature rise somewhat.

If you want to do some further research, you could search the internet for information on:

- High temperature strength of ordinary structural steels.

- The combustion temperature of kerosene.

- Specific information on the temperature that the WTC was subjected to in its destruction by the terrorists.

That should be enough to put these conspiracy theories to rest.

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