Reply #29: Consider this

[LARED]

The strength of the floor support was most likely fairly constant through the building, so the amount of energy to break a floor free will be about the same.

The strength of the columns increased approaching grade to handle the increasing weight of the building and column itself. What many CT'er fail to grasp is that without the floors providing lateral support, the column lose stability and will fail. In fact if you removed the practical need for floors as a work area, the WTC would still have required many dozen lateral supports structures to keep the columns stable. Also the perimeter columns failed largely at the bolted connection, at each column section as it was assembled, the weakest element of the columns. This would have been a weak point along the entire column assembly from top to bottom.

You also need to grasp a concept about the strength of the building. It was designed to handle gravity loads and wind load as a structural system. Once a collapse mechanism started this system was degraded and the load were highly dynamic; very different from design criteria.