You take the whole top 20 floors of a 100 story build and smash them up into rubble the lower 80 are still holding the same amount of weight, of course if you dump all that rubble on one spot of floor on floor 79 it's likely to fall through to 78 and carry that weight plus the floor down to 77 and so on, smashing and damaging as it goes, but that's another story. Whacking at the top of a tree with an ax generally will not knock it over. That's not to say the strike and accompanying damage weren't a factor, or that supports which might otherwise be fairly insulated from melting might not get exposed more directly through cracks from the strike, but I couldn't see it making much difference. I think people over complicate this, grab an old roof antenna or something else metal with lots of bits or spokes or whatever, and hang weight off it, non flammable weight, it sags a bit and stabilizes, now pour rubbing alcohol of kerosene on it and light it up, it will sag more and at a certain point bits are likely to snap off, then smack other bits on the way down and snap them off. Wrapping it up stone or whatever helps keeps the burning fuel off the metal but a good smack might open cracks for it to get in, and even without that, insulation generally works both ways, so to speak.

It's entirely possible that without all the broken windows, since those would have been sturdy material themselves, a fire couldn't have gotten air fast enough to heat the building up, I mean you need a god awful lot of oxygen to run a fire big and hot enough to do that, honestly off the top of my head I'd guess that breaking all those windows - along with delivery the fuel - were the only two real significant 'impacts' of the planes impact... however I am not an engineer or architect, so grain of salt and all. You figure as maybe previous fires in skyscrapers, Towering Inferno and all, probably contained enough flammable material in terms of raw heat production to parallel several thousand pounds of jet fuel but just couldn't get enough air to burn it fast enough to melt supports, couldn't really say.