Structural steel beams, columns etc. have to be protected by a fire retarding casing, or by 'intumescent' paint, which foams into a carbon-rich protective shell on burning.
This is because at around 660F the iron molecules re-arrange into a "different metal", similar to the structure of aluminium or copper. (BCC molecular lattice changes to an FCC lattice). At around this temperature, though steel actually increases in strength, it becomes very brittle and shock sensitive- known as 'blue-brittleness' due to that being the color of the metal (often seen in tempered tools). Higher than 660F, the strength falls away fast, and by 1550F will rupture at less than 2 tons/sq in. Being a good heat conductor, it will rapidly attain the above temperatures in a fire if not sheathed. If you look at photos of burned buildings, you often see the steelwork has sagged or collapsed. The point of sheathing is to retain some structural integrity long enough to give the occupants of a burning building time to get out. Eventually, practically all common building materials fail in a fire, but wood in thick sections chars slowly, even if directly exposed. I'm not quite so sure about those thin webs, they sure won't give 30 minutes grace once alight, and are a critical structural element of the beams. But, they are usually sheathed themselves by the flooring and ceiling plaster-board (sheet-rock?)- which by the way is a good fire protecting material.
Alan
