>From what I remember of my BSc education stuff on materials, you don't
need to melt steel, or anything like it, to cause a loaded structure to
fail. Heating for a time (dunno what time but not as long as one would
think) will lead to a loss of strength and increased elasticity. Once
structural members in one place have distorted sufficiently for, say, a
floor section to drop out onto the one below - also weakened and even less
likely to take the increased weight, much less the impact force of an
accelerating load - the whole pack of cards comes down. As we saw :-(
I suspect there will be a lot of work done to assess the structural capabilities of large buildings to sustained heating. And also impacts of large aircraft. It seems people thought about the impact of a large aircraft - the building withstood this - but perhaps didn't think enough about the impact of very large quantities of fuel burning (and potentially detonating from vapour traps in the structure?). As always with engineering disasters, the thing that gets you is the thing that one hadn't thought of before...