I also found that damaging the outer structure was critical to obtaining a collapse. However, in my experiment, only the impacted wall and opposite wall were damaged initially. This eventually led to one corner succumbing to stress and collapsing. At first, this only caused the upper portion of the building to drop down about 1/2 floor and rest on the section below, where it remained until subsequent fires that spread to the lower floors eventually weakened the structure. There was eventually enough of a shift downward that the top section began falling. In my experiment, it also remained intact until it reached the ground. Which is not entirely dissimilar to the WTC, as the upper section of one of the towers
appeared to remain intact at least halfway down, IIRC (the white antenna still visible until obscured by dust).
In the end, not an exact recreation of the collapse, but visually very similar.
---------- Post added at 11:26 AM ---------- Previous post was at 11:21 AM ----------
I also made the central charges a bit stronger than the outer ones, to simulate the ignition of the jet fuel having more of an overall effect on the structure than the initial impact.
I used 2kg charges on the impact side, 5kg charges on the central columns, and 1 kg charges on the opposite side to account for the building's partial absorbing of the impact force.
---------- Post added at 11:28 AM ---------- Previous post was at 11:26 AM ----------
That happens if you destroy a building, quit to main menu. Then load another building. The result is this building collapsing on it's own.
The solution is to quit to desktop and start the game again.
It happens to me on some levels regardless of whether I have previously loaded a building or not. For example, I have yet to be able to load the bridge without beginning with an initial damage of 10% already destroyed.
I did find a workaround though, at least on some structures. For me, if I remove all explosives, then reset the building, I get 0% damage.