I am not an architect but I studied the design of the building which had a unique structure. If you can imagine yourself to be a six armed Hindu Goddess (I can), arms stretched out and the fingers touching the arm above and below to give support, this was the structure of the towers. Once the planes went through the building, as evidenced by the film footage, the supporting ‘fingers’ that held the steel ‘arms’ up were damaged right across the affected floors. Add the fact that some of the fire proofing to the structure was blown off and the resultant fire (again evidenced by the thick black smoke) melted the remaining steelwork. What you then had was the huge weight of the floors above the damage (hence my reference to balancing a sledgehammer), bearing down until the critical time of weakness, and the rest is history.
Make a six floored house of cards, remove the middle layer with a quick swipe, hey presto, collapse.
A fire near my house at a furniture factory left the steelwork in a state that looked like a volcano had erupted but there were no explosives; what was left was the solidified molten steelwork caused by armchairs and bar stools. Also, any engineer knows that when concrete has enormous forces applied to it, it rapidly turns to a powdery mess, also evidenced by the huge dust cloud that enveloped the streets of New York.
This is what I know, I have read all the posts by Dana but they do not convince me that it was anything other than the planes that brought the buildings down.
I saw planes hit
I saw the fire,
I saw the collapse.