The time taken for an uncollapsed floor to become a collapsed floor in the face of even a fraction of a tower falling onto it is unimaginably brief. In short,

The time taken for an uncollapsed floor to become a collapsed floor in the face of even a fraction of a tower falling onto it is unimaginably brief. In short, the reason that the towers' collapses looked like controlled demolitions is because controlled demolitions don't rely on the explosives to destroy the building, they rely on destabilising it enough that it falls in on itself. The "controlled" part is just making sure it collapses inwards.