They performed AS DESIGNED.
The hollow-tube perimeter formed by the lattices, along with the central core anchored very very VERY deep, was designed for the ultimate in vertical stability, especially considering the two towers' close proximity to other structures. Were the twin towers out in the middle of a vast unpeopled expanse, a structural failure allowing them to fall to one side or another would have been no big deal. No matter which way they fell, there wouldn't be anything to hit.
But not so with the WTC site. If you put up not one but two twin 100-story edifices on your own property, you'd better be DARN sure that should a catastrophic structural failure result from even the most remote of circumstances, that the debris would fall largely contained on your own property, minimizing the chances that a falling tower (or two) could directly wipe out nearby buildings.
So, the WTC towers were designed to drop, in case of such a catastrophic failure, right down upon themselves, the central core acting like the spindle on an old LP record changer - guiding each succeeding floor straight down and away from a perhaps even MORE catastrophic scenario where the entire building spins out of control and takes out everything adjacent to it. No. MUCH better to contain the damage to just the structures being designed.
So that's what they did, and it's been written up and posted here on FR before. The towers collapsed straight down because they were designed to do so. And, with the huge effective mechanical moment of the two planes hitting the steel members, I wouldn't look for too much insulation to stick around and join the party. That impact probably blew 95% of whatever insulation was on the beans right into dust - do not pass Go, do not collect $200.
That's it in a nutshell. The impact of the planes pulverized the insulation, the fires melted the steel, and the collapse was perfectly controlled by the buildings' design - straight down the spindle like a giant stack of old Motown records.
Case closed. Except for the omnipresent posturing of the hopelessly off-stage and desperate senators from NY.
Michael
This is required when you are building a very large structure. It appears as if this was discovered after the 1993 attack and photos exist of the condition.
Whether documentation exists of the as-built conditions on the floors most affected by the September attacks is doubtful.