According to explosions (=dust ejections) seen in the available videos, there are some explosions made every floor (1F), some others made every three floors (3F). The visible characteristics are:
- Very few 3F explosions could be found in the videos.
- 3F explosions are mostly localized in the upper floors and in the initiation time. After that time, it’s very difficult to identify 3F explosions, but that does not mean they do not exist.
- There are too many 1F explosions in videos.
- In all videos we can find 1F explosions.
We can tell that 3F explosions are used to cut the core columns. 3 Floor explosives should become bigger and bigger during the collapse progression. That's for cutting bigger core columns.
But why and how did they use 1F explosives?
1F explosives are intended to destroy the floor trusses and make them fall by their own gravity. 1F explosives should be placed on the connection between perimeter core columns and floor trusses seats. 1F explosives should remain in the same size all over the collapse; they destroy same or similar floor trusses. Using only this kind of explosives, it's possible to demolish all floors and all outer perimeter columns (= Open Office Space Demolition).
But making that will leave the core erected after the collapse without any one of the floor trusses seats (in all pictures and videos I saw NO one floor trusses seat. Did anybody see?). In that case, after the collapse, it will be easily seen that the floor trusses seats are attacked.
To avoid that situation, it's necessary to destroy the core columns also. This could be done by exploding these columns at their junctions. But this destruction may not be complete in all core columns junctions.
Exploding the impact level's core columns is required to make the upper part collapse. As the aircraft impacted only H type small core columns floors, that destruction of core columns is also easy. But lower floors core columns may not be exploded on all their junctions.
For the lower floors, some weakening of the core columns is enough to make them collapse by falling debris. To weaken them, just exploding their horizontal and diagonal connections is enough. All these connections are too much smaller than core columns. In that way, the explosives remain small and less audible.
For final collapse, to be sure that the core columns will collapse, it’s necessary to destroy all core columns junctions over some floors, let’s say 1, 3, 5, 7, 9, 12, and 15. But at that time, every people near the towers should be running, the noise very big, … and they can use bigger explosives for that purpose. From 15 to 77 the core columns may not be destroyed. That’s why some core columns remained erected during the collapse.
That explanation is consistent with all visible strong evidence.