Different failure modes.

Mild steel becomes ductile at relatively low temperatures, way before it actually melts, hence why you sometimes see steel beams deform and drape over timber beams in a fire. The ductile transformation happens relatively quickly once you get to the critical temperature on the iron carbon phase diagram for typical construction grade steels. Hence why Steel is usually protected with fire rated drywall like boards, intumescent paint or weird spray on insulation that looks like porridge.

Whereas with timber the rate at which it will burn is reliably predicted, so exposed timber structures are designed to be oversized so that the reduced beam sizes will still be effective. This is done using a charring calculation to give the required amount of structural integrity duration which varies depending on how tall the building is and what it is used for.

All types of structure will fail eventually in a fire, the assumption is that the fire brigade will tackle it before it goes too far.

In the UK, the main fire related problem with CLT is that the exposed surface doesn't meet the requirements for the speed of surface spread of flames so it has to be treated with a fire retardant or you need sprinklers if you want to expose the material. Both of which are expensive.

Today I learned why the bottoms of the staircases in the building where I work look like they've been sprayed with porridge. Thank you!!! (I had assumed it was insulation of some kind but could not figure out why they'd put it there)

Steel performs more poorly than you would expect in a fire

I actually wonder if CLT/wood would perform better than steel during an all-out fire in terms of the structural integrity, at least for an initial period of time.

The steel structure would more likely survive in-tact, but the wood structure could guarantee a certain minimum bound in terms of evacuation time available to occupants (which might be superior to steel).

From my very novice perspective, I feel like wood does not attenuate in structural integrity as its temperature rises (at least not in the same way as steel), and it seems it would also absorb and conduct heat much more slowly than steel.

On the other hand, I feel that once a wood-based structure gets hit with a big enough fire, the entire building is guaranteed to be lost due to how hard it could be to fully extinguish the fire (e.g. if the steel in a skyscraper could burn openly, what would that be like?).

I did find this related paper which may be of interest to HN: https://www.fpl.fs.fed.us/documnts/pdf2012/fpl_2012_kukay001...

Not bad if we believe this video:

https://www.youtube.com/watch?v=YuAya0hRjwU

It burns slowly enough that it doesn't conflagrate like unprotected hardwood or plywood might, so there's much more time to evacuate. If the fire isn't put out quick enough, however, the building may lose its structural integrity

Have you ever tried to set fire to a single log?