The clear problem with this jeffrey is that if you watch the video or see images of the tower all the material is being ejected out the side of the building and not stacking up the towers
Judging on that picture at least 80 percent of the material is ejected laterally and not causing any force on the still intact floors below.
The buildings do not slow down or de accelerate but maintain a constant speed. However its not free fall speed its rather close.
I am pretty certain that what is ejected outward... breaking the windows in the process is the CONTENTS between the floor slabs.. ceiling tiles, GWB, furniture, window shades, paper, carpet and so forth NOT concrete from the slabs.
The reason I say this is because what was taking place was a vertical avalanche of sorts of building parts dropping down smashing into one floor after another. The building parts I refer to were the steel from the hat truss, mechanical equipment from the mech floors on 108 and 109, the massive sub station transformers up there, tanks for water, the antenna parts, restaurant equipment, elevator machinery and of course shatter floor slabs. Each floor added something like 1600 tons of mass for the slabs and loads outside the core... not counting the facade the core columns or the inside the core slabs and live loads. So the top 12 floors were dropping something in the order of 30,000 tons of material on the 97th floor of one acre. The vertical avalanche had to also push the air out of its path... much like a car does. The air was 18,000 cu yards per floor and it was moved in something like 0.1 seconds. Where did it go? Out the windows! And it took with it everything on the floor. The wind pressure created winds pulse or gusts of up to 400 mph which will pretty much destroy anything on a twin tower floor. Think of a sheet of plywood falling over on dusty floor times 100,000 or something... I don't know the scale.
The floor concrete became pulverized as it was ground to bits by millions of collisions on the way down and especially when it hit bottom. The concrete was not very strong to begin with with no stone aggregate and something like volcanic slag for aggregate.
If you knew the mass distribution you would know that your 80% figure is way off.