After about 14 second from "collapse" initiation both twin towers were reduced to rubble. The exact time each took to come down is in dispute, but it was less than 9.1 seconds which would have been free fall If the towers had been instantly exploded all through its height whatever was left would have descended at close to free fall. That didn't happen.
Once there was a global failure up top at around the crash zones the collapses began or what appeared to be top down collapses.
NIST seemed to take the approach that they would model a series of events to led to the release of the mass which would facilitate a top down collapse. They did not demonstrate how the bottom of the structure would succumb once the collapse was initiated. Why?
We don't know, but I suspect that the mechanism of collapse was linked to the collapse initiation and if without having to deal with how the structure was destroyed "sequentially" or progressively, all they could side step the issue and come up with a narrative which allowed them to conclude that fires from the plane strikes were the cause. They tried to turn local failures into global collapse and destruction.
Many countered that it defied the laws of physics to have a big blog destroyed by a small block. This is completely factual but really not related to what was going on. Is a table a rectangular block or 4 legs joined to a table top which defines a cubic solid? The answer is: it's not a block. Tables can behave as cubic solids because their leg to top joints are rigid. But if a table can support a concrete block if you turn it on its side and put the concrete block at the end of a leg it will likely snap it off. Here the table is behaving like an assembly of discrete elements which individual structural properties.
The columns each was designed to carry a specific load and carried a safety factor. If columns were compromised the remaining columns would then carry additional loads. If the table weighs 40 pounds each of the 4 legs will carry 10 pounds. If one leg is removed the other three share the 10 pounds and each leg now supports 13 1/2 pounds. If their safe working load is 15 pounds the table will stand on 3 legs. If you add 10 pounds of weight on the table top each leg will not have to support an additional 3 1/3 pounds and this will mean16 2/3 pounds each and this will exceed the safety factor and the leg would buckle the top would drop to the floor.
When the planes stuck columns were destroyed but the loads were redistributed and the towers stood.
What could make them collapse?
In the south tower it appeared that enough columns on on the SE side were destroyed and the loads were transferred to the NW side. This would require that the top acted like a rigid block and the unsupported side was cantilevered off to the SE. The top was like a 4 legged table with on removed but the weight over the area with the lost columns in SE side began to pull down. If the top was a block... ie if its structural connections could hold the tens of thousands of structural elements together it would act as block and tilt down. Would it rotate about an axis inside the footprint and lift the side opposite the the falling one? or would the pivot point be the opposite corner?
The top acted like a block through about 20° of tilt. As the unsupported side dropped down all the columns of the top section moved off the columns they had been resting in. The top section no longer had support and was beginning free fall down into the bottom structure. The floors on the dropping side collided, destroying the falling corner of the top block and the top corner of the bottom block. As the force of gravity began to acceleration the top block down and the floors interactions reached the opposite side. Now all the floor area were being crushed and the core columns were acting like the bristles of two brushes jamming together. One brush fixed on the table, the other being pushed down into it. The meeting columns offset each other destroying the cross beams in the core of the bottom and the top. The top inertia continued moving the mass to the SE as it was engaged in mutual destruction with the bottom block.
This might have destroyed enough floors, released them from their supports broken them apart and had them drop to the undamaged floors below. The undamaged floors below soon became overloaded with the floors which crashed down on them. They failed in one of two ways: broke apart cracking mid span breaking the trusses... or acting like a membrane and remaining rigid and overwhelming their connections to the columns which supported them at the core side and the facade side. If the over loading of an undamaged floor was uniform throughout the floor the weakest links would fail at the same time. Either the floors would shatter at once or the connections would shatter at once. In the latter case the entire mass would drop like a record on a changer. No likely, but possible.
We didn't get to see this unfold. Massive explosions went off at the top as the two masses were destroying each other by breaking apart the floors which held the tube to the core.
What this likely did is cause a series of undamaged floors to be freed from the core side supports - the beam stubs - at once. These floors caved in like water pulled into a drain. The facade side now was carrying the entire floor loads and this time the connections were likely overloaded and the truss to facade connections failed at the perimeter by shearing the top chord or the bolts that held them. The facade panels lost their lateral support.
Several corners connecting the facades together were also attacked with incendiaries or explosives in the first wave of engineered attacks. With the corners no longer holding the facade side together, the top of the tube design had turned into 4 walls of columns. The floors were collapsing down and the four walls were losing more and more lateral bracing. They eventually were unstable enough to fall away outside.
Could this mass of falling floor debris be stopped by any floor below. No. Each has the same strength connections to the columns which held it up. The columns were stronger the lower the collapse front descended, but the floor connections to it were not. The mass of falling debris was increasing but the connections and their strength remained the same... except at the mechanical floors.
These had no floor trusses and had heavy steel beam framing and thicker slabs, but still had beam stubs to frame the perimeter floor area slabs at the mechanical floors to the perimeter core columns. The mechanical floors were also the easiest to access for mischief and the columns and beam seats likely exposed and not hidden in dropped ceilings and framed out architectural features. It is likely in the mechanical floors that explosive were placed between at the beam stubs, blasting them free and forcing the perimeter core columns out of alignment and into the core enabling overloading of the unattacked core columns.. This also free the perimeter floors at the core side and overloaded the facade side connections. The heavy mechanical floors collapsed inward as well as the avalanche from above came crashing through.
If the mass of falling debris became organized it would push at attached floor below and act like a piston compressing the air explosively out the windows and into the shaft walls of the elevators. This would happen in rapid succession quickly overcoming each undamaged floor on the way down. As the debris pile of mass in the avalanche built up it pushed the facade panels out and they fell away. The collapsing floors were no longer pistons completely confined by the facade which was peeling away in sequence 1, 2, 3, 1, 2 , 3 as they were staggered in placement vertically.
The floors were not pancaking they were in a vertical avalanche a pyroclastic like flow of debris grinding itself to bits and generating heat in the process.
The top block had long ago dissociated itself and was partly fallen over the side, partly exploded to bits and mostly part of the avalanche of debris.
The bottom "block" was being attacked by growing inertia of the avalanche overwhelming the floor connections to the perimeter core columns and the facade columns.
The core columns, a lattice with increasingly stronger 3 story tall columns at the bottom has the lateral members mangled by falling steel leaving the columns like tall spires supporting nothing and with less and less lateral support. Their splices failed, were pushed out of line and they plunged like falling icicles.
This process would have to break every floor connection and this was resistance slowing the collapse. But the yield strength point was reached very quickly with the enormous dynamic loading. The process accelerated at less than 1 G and in 15 seconds it was all over.
The enormous quantity of pulverized dust and debris hit bottom at speeds of over 100 mph and dust quickly spread in all directions in rapidly propagating billowing clouds of hot air from the friction of the collapse.
Few if any columns were crushed. No small block crushing a stronger big block. No piston pushing out squibs 20 floors below the collapse front. No pancakes to stack up.
Simply destroy and dissociate enough floor material, explode some perimeter core columns to the side at enough locations and the collapse will progress from the top down and appear as we see in the videos.