|Dean said that if I'd look at his video link, he'd look at the links I sent. He certainly didn't look very much, because explanations, like Fred's excellent explanation above, were in several of those links I supplied. It's like I said, when someone gets too deep into a conspiracy theory, the theory takes over and if someone shoots down one piece of false evidence the 'truther' comes up with another. Discussions with them become like whack-a-mole but not as much fun.|
Imagine even just one floor collapsing; one second, there is a 10 ft space of air. When the floor above collapses, at some point, that 10 ft of air is now compressed to 1 inch. What compression ratio is that? Where does that air go? At what speed does that overpressure propagate inside the building to the floors below? As a non-engineer, I'd add this... When a floor collapses, the weight of that floor, and the weight of all of the floors above it, land on the floor just below. This is a conversion of what had been a static load, an amount the support columns were designed to handle, into a dynamic load. Those support columns, instead of holding x number of tons which are not in motion, are now trying to hold up against the shock of x number of tons multiplied by y feet per second of motion. Velocity is the great multiplier of force. (Imagine holding a 40 caliber bullet in the palm of your hand, versus getting shot in the hand with that bullet.)
What you are seeing is the internal overpressure in the building exceeding the ability of the windows to stay intact in the floors below; that overpressure is experienced over the entire window and resisted only by the frame. It doesn't take much overpressure to create a huge force.
What 'precedes' the falling of the floors above looks entirely consistent with overpressure wave propagating ahead of the giant 'piston' falling from above. The pressure wave from that piston travels through the building at the speed of sound, not the speed of 'free fall.'
And, if there is any longitudinal elastiscity in the support columns, that is, any ability to compress some small amount without loss in support integrity, that capacity will be spread out over the columns below during the passage of a small unit of time. As each floor below starts to compress, there is a touch less force to be converted to those still further down. When there are 100 floors below, that's a lot of floors to spread out the force of the 102nd floor driving into the 101st floor. But each time this happens there are fewer flights below, and that capacity to accept some degree of compression is disappearing - so the force generated by each floor collapsing is going to increase - because of the greater weight now in motion, but also in the loss of ability to absorb that lies below. Each floor that is currently receiving the collapse is subjected to the full force of the weight above times the speed of collapse. Thus, with each floor's collapse, there is more weight added to that which is in motion and fewer floors below to absorb a fraction of the force with non-destructive compression. Faster collapses as this happens will increase the forces Fred described (the same amount of air being compressed the same amount - but in slightly less time). The other thing I remember from one of my classes, eons ago, is the gas law. To decease the volume of air that much in that short a time is not only going to generate massive pressure, but also a huge increase in temperature. I wonder if anyone has computed the pressure related temperatures?