So... if we all agree that the lower structure (below impact floors) could have held the weight of the upper mass in a hypothetical situation where the impact floors were removed, and the upper mass was slowly lowered onto the lower floors/structure... than what we are saying is that the force from this weight (of the upper mass) falling 50-70 feet is what caused all of the lower floors to fail.
No. In a structure, you have various types of forces that are skillfully directed so as to work toward the strengths of the materials used in that particular arrangement. Dead (meaning constant) loads are carefully transferred to the ground in a certain way. When you change that static loading to be dynamic, live loads, such as a weight that is falling, they don't behave in the same way. Specifically, in the case of the towers, shear forces are at work that exceed design capacity, leading to global collapse. A shear force is one that cuts across a material, rather than being transfered length-wise.
I'll give you an example. Imagine a large, heavy truck that is resting on a small bridge. It's not much below the posted weight limit but the bridge supports it while at rest, or while driving across the span. However, if you lift the large truck with a crane and drop it on the bridge from a height, your results will not be as favorable.
Here's another. Imagine an obese woman standing on the end of a diving board. (I have had the unfortunate experience of witnessing this particular example in person, I'm afraid) Several other people of normal size have previously used this springboard to dive into the water below. It even supports her weight as she contemplates her dive. Hovever, as she bounces, and then lands to spring . . . it shatters. Why?
Doesn't NIST say... the collapse increased in speed as it progressed downwards? Meaning the resistance from the lower floors didn't slow the momentum of the upper mass, but actually increased it.
Also, your understanding of how forces, and resistance to them (other forces) work is below elementary. There are a couple of things, at a bare minimum, that you need to understand.
First off, if you look at something as simple as a
coefficient of friction for a static body as opposed to a moving body, you will see that they are noticeably different. There's a simple test you can make of this. Take a small, cubed block of wood and place it at one end of a longer plank of the same variety. Now, slowly raise the end of the plank that the block is on to incline it. As you increase the angle, the vertical component of the force acting on the block, acting upon it to fall, increases. At a certain angle, that component will surpass the static coefficient of friction between the block and the plank, and the block will begin to
accellerate toward the lower end of the plank. It won't slow down, even if you begin to lower the plank, until such a point that the kinetic friction is more than the vertical component of the force acting on the body. At that point it will begin to slow, until it stops.
Kinetic friction is almost always less than the static friction, and that's why ABS that keep your tires from skidding will stop your car faster than if your tires skid.
You also need to undertand why the forces are acting the way they are, and effecting the objects with predictable, and calculable, effects. I won't go into the calculus of an accelleration or anything like that, but perhaps I can explain some really simple
vector analysis when it comes to forces. Vectors are typically described as having a direction and a magnitude.
Picture it as a rope, that is being pulled in a certain direction at a certain rate. If you have a large force pulling on one end of the rope, it will move in the direction of that force. If you put a smaller force on the other end, pulling in the opposite direction, it will still accellerate in the direction of the larger force, but not as fast if there were no resistance. The only way that it will slow, once moving, is if the resistance force becomes larger than the pulling force. If the forces are equal, the rope will continue moving, if it was moving, or remain stopped, if it was stopped.
For instance, the force of gravity on an object is its weight, directed toward the center of the Earth, which, we will call "down" for simplicity. In a static environ, such as a built structure that is at rest, this force of gravity is resisted by the strength of materials in that building, in the exact opposite direction, "up", and actually in the exact same amount that the gravity force is pushing down.
In the case of that structure moving downward, that basically means that the downward force has exceeded the upward component capacity of that structure, just like your wood block on the wood plank. The structure won't decelerate because of the resistant force it encounters, but it will accelerate slower than it would have if the only force acting on it was downward. That's why you observed a collapse time of about 14 - 15 seconds for the structure instead of under 9, which would have been the free fall time without resistance.
