Notional fire resistance

[lyledunn]

I have a 5-storey hotel in a refurbished old office building. Height to top floor is 22.4m therefore requiring 90min fire resistance for compartment floors. I have an existing floor make up which is tandg boards over a fairly robust joist structure. Below that the existing ceiling is lathe and plaster in sound condition. Is any one aware if some kind of notional fire resistance is available or acceptable? The intention is to fit a 60min ceiling below the existing. Or should the bullet be bitten and provide 90 on the new ceiling?

[Fishy]

If you needed 60 mins or less, you could potentially use BS 5268-4.2: 1990 (withdrawn, but still useful), assuming that the lath and plaster provides no contribution.  There's nothing similar for 90 mins+, though.  The other useful source is BRE's "Guidelines for the construction of fire-resisting structural elements" by Morris, Read and Cooke, but again this doesn't cover anything over 60 mins, nor does it assign any F/R to timber lath ceilings (only expamet lath).

Beware as well ceilings tested only to BS 476: Part 23 - they're not suitable for use as protection to timber floors.  You do need to remember that a floor is a load-bearing construction, so the F/R will depend upon the size of the joists, as well as the floorboard type and thickness.

[lyledunn]

Thank you for the info Fishy. BC are sympathetic but require proof of 90min. They did advise that we could explore ventilation conditions but even using that the FR must be a minimum of 75mins. The building is only a couple of metres over the magical 18m height. Had it been 18m then 60 min would suffice. It seems to me that great expense will be required to meet the book requirement. I am not sure how arbitrary the requirement or whether it would really matter one jot if the 60 min was not upgraded. I need to leave that to specialists.