The Pile Driver/Smashdown Theory
NIST avoids explicitly endorsing the piledriver theory,
but its theory depends on it.
It gets closer to explicitly endorsing it in the few additions
of the Final Report to the Draft Report
Failure of the south wall in WTC 1 and east wall in WTC2
caused the portion of the building above to tilt in the direction
of the failed wall.
The tilting was accompanied by a downward movement.
The story immediately below the stories in which the columns failed
was not able to arrest this initial movement
as evidenced by videos from several vantage points.
The structure below the level of the collapse initiation offered
minimal resistance to the falling building mass at and above the impact zone.
The potential energy released by the downward movement
of the large building mass far exceeded the capacity of the intact structures
below to absorb that through energy of deformation.
Since the stories below the level of collapse initiation
provided little resistance to the tremendous energy
released by the falling building mass,
the building section above came down essentially in free fall,
as seen in videos.
As the stories below sequentially failed, the falling mass increases,
further increasing the demand of the floors below,
which were unable to arrest the moving mass.