A note from someone who went to architecture school...
As R.L. points out, you don't need to melt steel into liquid to cause structural failure. The strength of structural steel goes to hell at even moderate fire temperatures. At 550 C it's down to about 60 percent of strength, which uses up most of the safety factor. It drops rapidly from there.
Steel buildings insulate their structural elements by coating them with heat-resistant foams or other materials. You will see this in some buildings under construction.