A hot enough fire weakens metal.  Jet fuel, diesel or heating oil burn very hot.  If there is enough fuel to burn for an extended period of time things happen. 

What caused the twin towers to collapse?  It wasn't the direct hit from the airplanes or the weight of the airplanes on the structure.  Without the extended fire from the jet fuel the buildings wouldn't have collapsed.

If you have an extended hot fire at the base of a building it will also cause a structural failure and then there is nothing to hold the building up.

The buildings that collapsed experienced very unique and unusual circumstances.