I'm sorry, being a Civil Engineer myself.... that bullshit that was said during the twin towers falling.... please...
All the reporters.... "experts"... and eye witnesses are just speaking off of the moment. They don't know what happened... confusion, commotion...
Have you ever heard the sound that is made as steel busts? Or the sound as concrete crushes? It sounds like a boom.... in our Strengths of Materials lab, we took a steel rod and applied tension to it until it began to neck and bust. When it did break... the sound was similar to a gunshot (or a large firecracker). Here's the rod that I watched break:
The following semester, we did the same thing to a concrete cylinder. Now, my group had a regular strength concrete, ~2000psi. When our cylinder began to crumble, it just fell apart, but the group that got high strength concrete ~6000psi (which is all we could do at the lab at that time) EXPLODED in the upper 6000psi range. The concrete used in the towers was high strength (more like 12,000psi), much more stronger than what we had in the lab, and it was reinforced with... yep... steel rebar, or that rod you see in that picture above (only larger).
The building itself had a central core where the elevators and escape stairways were kept. Notice how there weren't any other columns in the floor plan? It was designed that way for an open floor plan. The outer shell was an inovation in the 70's. The whole exterior was a massive column that held up the building.
Now steel is burried inside concrete. Concrete is TERRIBLE in tension, but better in compression. So when a beam (horizontal) is made of concrete, it is reinforced with steel in key locations to counteract the effects of bending due to it's own weight and the load that it is carrying. But when the steel heats up, it becomes weak. It doesn't have to melt to loose it's strength. It only needs to hit a temperature which will make it behave differently that it was intended. As the temperatures rose, the steel weakened inside the concrete, which, caused more stress on the concrete to resist the tension forces. Once the steel became useless, the concrete failed, leading to a domino effect as each floor hit the next lower one. The building was never intended to experience multiple floor collapsing. The amount of weight being hit onto each level increased causing the subsequent failure in the remaining floors.
Now I could go on and on and on.. but to most of you, I would bore the shit out of... my point is that some of the stuff mentioned in Andres' link are being twisted/exaggerated. Speculation and the eagerness of some to find something so badly that they'll take apart anything they find.
Ok, so to be fair to all, I CANNOT explain the pentagon OR the crash site in PA (I think). I cannot explain why supposedly Flight 93 landed near Chicago or why someone calling thier own mother introduces themselves by thier first and last name... I'm just pointing out the reasoning (basics) of the collapse of the twin towers. Some of you may scuff at me, but this is my "expertise" if you will. Granted I could be wrong on the specifics of the twin tower's construction, I've yet to look over thier blue prints, however, trust my judgement on concrete failure.
OK, that's it for now....hope none of you have fallen asleep...