Sounds like my model test at
http://heiwaco.tripod.com/nist1.htm#6 . Very popular. I use a weight of 1 750 kgs or almost 3 900 lb. The force on the lower structure is about 17 185 Newton, but the stresses in the structure below before heating up is just 0.3 of yield. Exactly as in WTC1/2.
But nothing is pushed to an edge and nothing is free falling and and nothing is impacting something below. The support structure of the upper block is heated, so it slowly loses strength and then the support structure starts to deform, bend, buckle and fail and the 3900 lbs upper block moves down ... but there is no free fall and no impact.
Reason is that there is always an opposing force of exactly 17 185 Newton in the failing structure (all the time) ... that prevents free fall. The local failures will just extend over the heated area and then stop. The upper block will remain on top of the lower, unheated structure! No collapse initiation will take place. The lower structure arrests any collapse. Happens all the time.
Only way to produce free fall/impact is by LCD (Local Controlled Demolition) that instantaneously removes the support structure below (as you suggest - push it to the edge). Then there is no opposing force.
And this is what we see on all videos of the WTC1/2 destructions. No opposing force that should arrest the local failures but sudden destructions BANG, BANG, BANG ... from top down of complete floors structures. It seems LCD is applied at every 5th or 10th floors with 0.5 to 1 second intervals. The upper block disappears first ... as
tanabear points out. The massive amounts of dust, débrise and rubble is evidence of LCD.
Quite obvious actually. The NIST suggestion (no evidence of course) that potential energy released by the upper block exceeded the strain energy that could be absorbed by the structure below is sectarian nonsense.
It seems that the sect has infiltrated NIST and is influencing many people at JREF Forum.