Alexis Nihon Plaza Montreal, Canada
Steel frame with composite steel beam and deck floors; fire resistive without sprinklers
15 floors, Office
Oct. 26, 1986, after 5 hour fire, which then continued for 13 hours
Partial 11th floor collapse
One New York Plaza New York, NY, USA:
Steel framing with reinforced concrete core, fire resistive with no sprinklers.
50 floors, Office
August 5, 1970
Connection bolts sheared during fire, causing several steel filler beams on the 33-34th floors to fall and rest on the bottom flanges of their supporting girders.
So, shit happens. If you heat steel in a fire, it changes size, it changes strength, those little structural quirks become huge structural weaknesses. It does not have to melt. It just has to change strength.