The diagrams I linked to in this post are part of this archived page from 911research.wtc7.net: https://archive.is/GIb3g It has info about the construction of the towers and is well worth a look before you read the rest of this post.

When I used to work in construction we installed floor systems similar to that used in the WTC so I am more than familiar with it. The system uses bar joists onto which steel decking is laid and then concrete is poured onto that steel decking to create the floor slab but while looking at this floor diagram from the WTC something jumped out at me that I had not noticed before and have never heard mentioned.

Look carefully at the floor support beams in the diagram, titled

Constructional features of a prefabricated floor unit

The floor support trusses (24) are called bar joists. They use diagonal cross members in place of the solid steel you would see in an i-beam. If you look carefully you can see those bar joists run both ways, there are the main ones running left to right in the diagram but look carefully, there also secondary ones that run laterally. They are slightly shallower than the main ones so they can sit between the top and bottom members of the main joists so they are actually inside the frame of the main bar joists.

You can see how all those intersected steel bar joists (14 and 15) create a grid pattern when looking at it from above in the diagram titled: Structural system for typical floor

Now the thing is, looking back at the the construction features diagram again, you see how the steel decking (23) is laid between the top members of the main bar joists so it actually sits on top of those secondary, lateral bar joists. This ensures the steel decking sits level beside the top members of the main bar joists. If you look carefully at those top members, you can see how the ends of the diagonals extend through them so you can see them sticking up? That means when the concrete is poured, not only is it bonded to the steel decking and to the top members of the main bar joists beside it, but those ends of the diagonal members that stick up actually end up embedded in the concrete floor.

This means the entire steel floor structure becomes bonded by the concrete to form a single composite and rigid floor connected all the way around, every 6 feet, to the support columns both at the center and at the perimeter of the building so even if any of those bar joists was heated enough to become malleable, the composite rigidity of that concrete and steel floor system as a single entity would not allow the ends of the bar joist to detatch from the vertical column that supports it so therefore the NIST explanation could not happen.