I haven't read it all, but it should be a must read for those nongs who don't believe that the fire could have collapsed the building...
The University of Sydney Civil Engineering, Centre for Advanced Structural Engineering.
It describes the structure of the buildings, the trusses which were the only support, and how the impact and fire damaged the trusses...
After the initial plane impacts, it appeared to most observers that the structures had been severely damaged, but not necessarily fatally.At the end of the paper it mentions theories and questions about demolition, and even mentions breifly WTC7, which seems to have the troofers convinced of the conspiracy.
It appears likely that the impact of the plane crash destroyed a significant number of perimeter columns on several floors of the building, severely weakening the entire system. Initially this was not enough to cause collapse.
However, as fire raged in the upper floors, the heat would have been gradually affecting the behaviour of the remaining material. As the planes had only recently taken off, the fire would have been initially fuelled by large volumes of jet fuel, which then ignited any combustible material in the building. 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.
And here are most of the questions and answers at the end of the article:
The fire wasn't hot enough to melt the steel
There has never been a claim that the steel melted in the fire before the buildings collapsed, however the fire would have been very hot. Even though the steel didnt melt, the type of temperatures in the fire would have roughly halved its strength.
There would have been variations in the distribution of the temperature both in place in time. There are photos that show people in the areas opened up by the impact, so it obviously wasnt too hot when those photos were taken, but this is not to say that other parts of the building, further inside were not hotter. In addition, to make a reasonable conclusion from these photos, it would be important to know when they were taken. It might be possible that just after the impact the area wasnt very hot, but as the fire took hold the area got hotter.
The way the building collapsed must have been caused by explosions
One demolition expert on the day of the collapse said it looked like implosion but this is not very strong evidence. Implosion firstly requires a lot of explosives placed in strategic areas all around the building. When and how was this explosive placed in the building without anyone knowing about it. Second, implosion required more than just explosives. Demolition experts spend weeks inside a derelict building planning an event. Many of the beams are cut through by about 90% so that the explosion only has to break a small bit of steel. In this state the building is highly dangerous, and there is no way such a prepared building could still be running day to day like WTC was.
Why did the building fall so quickly?
The buildings did fall quickly - almost (but not exactly) at the same speed as if there was no resistance. Shouldn't the floors below have slowed it down? The huge dynamic loads due to the very large momentum of the upper floors falling were so great that they smashed through the lower floors very quickly. The columns were not designed to carry these huge loads and they provided little resistance.
What about World Trade Center 7?
I have not studied WTC in any great detail and cannot offer any theories on its collapse mechanism. In the chaos of the day, little attention was paid to WTC7, so there is less evidence available on the damage it sustained before it collapsed. However, some questions that you may want to ponder ...
* While it did not receive any direct impact form the planes, how much debris hit at as the main towers collapsed and what damage did it cause?
* To what extent (if any) did the shock or vibrations caused by the collapse of WTC1 & 2 affect the integrity of WTC7?
* Did any unseen damage to the WTC7 foundations occur in the collapse of WTC 1 & 2?
* Did any of the fire suppression systems in WTC7 function?
Stacks has it in a nutshell:
To me the old saying sums it all up. "For those who believe, no explanation is necessary. For those who do not believe, no explanation is possible."