I'm not a structural engineer. Nor am I an architect or a builder.
But, these words "reinforced steel" are repeating and some information has come to light.
There was speculation of which I have not researched that the Twin Towers had some kind of structural deformity.
After the planes hit the towers and the investigation ensued, this aspect was "talked about" as to why the buildings crumbled as they did.
How could a jet hitting a particular area of a building cause what appeared to be a "controlled detonation" unless their were some deformity present?
Now, if we look at it this way, we'll have to say that the TWIN TOWERS WERE BUILT WITH THIS DEFORMITY so that when the "terrorist attack" occured the structural deformity would be a "weak" explanation as to the way they "inverted" into themselves.
Drone aircraft come to mind. However, looking at pictures available, these aircraft do not represent what hit the towers.