good GRIEF! Hasn't anyone paid attention to the fact that THIS WAS NOT A STANDARD OLD-STYLE STEEL STRUCTURE, likesay, The Empire State Building..
There were NOT huge, thick steel beams in the towers, there were grids of smaller pieces, carefully interlocked, and coated with fireproofing material, **BECAUSE** they were not as fire resistant as old style girder! This was partially to SAVE WEIGHT in a tall building. This fireproofing was to deal with possible fires generated INSIDE the building.

When the impact shattered much of the structure, it also dislodged the fireproofing! Then, heat softened the remaining pieces...