Steel loses the majority of its tensile strength when it's heated to the same temperature as burning jetfuel. How do you think blacksmithing works exactly? They don't melt swords.
I appoligise, it's just that i've constantly heard the meme that JetFuel can't melt steel beam's that I disregarded the law's of physic's and basic fact's in favor of internet logic. Sorry guy's.
The Steel beam's would indeed be weakened by the amount of heat being transfered onto it and vibrate immensely at intense temperature's and the molocule's inside the beam's would be seperated aswell. The Mass of the Plane + The weakened beam's and also the state of the building due to the fire spreading inside etc... would indeed cause the structure to collapse.
Forgive me for not listening to my brain Maxwell.