My understanding is that timber, while it burns and will
eventually be destroyed by fire, will retain a sufficient percentage of design strength longer than a similarly strong steel element. That's because a) It burns mostly only on the outside, with a limited burn rate and b) conducts heat much slower than steel, so it stays cooler inside for longer (also, a wooden beam of same strengh is thicker.
Put conversely: The heat of fire is conducted to the core of a steel element pretty fast - and steel is thus susceptible to yielding within a short amount of time. My structural engineering friend has also explained to me that when a steel structure fails from fire, it is liable to do so with little to no warning, whereas wooden structures somehow broadcast coming failure a bit more ahead of time.
I like to show the effects of a fire in a workshop of a local cooky factury that burned several years ago at a time when they were producing a christmas variety of chocolate cake - photo is my own:
View attachment 63236
True, this a simple one-story workshop, the steel structure had only to hold up the roof, and obviously no fire proofing.
One could say that "
chocolate cake fires melt steel!"
Point is: JayUtah had an anecdote about timbers holding up such a roof even after a fire - steel does not.