Fire resistant periods

[GB]

I am trying to determine the FR period for a ceiling of 15mm plasterboard with 4mm skim coat fixed to timber joists with 450mm mineral wool above.
I have spoken with the board suppliers technical department who haven't conducted a test on this configuration, however suspect it will provide 30 minutes, but I am looking for any evidence?
Does anyone know where I can find any test or similar information?

[Fishy]

Firstly - not sure why you would need a F/R ceiling?  If it's a floor that is being discussed then you also need to consider load-bearing capacity & I'd always recommend that the Eurocode approach is used (Eurocode 5: BS EN 1995-1-2).  The upside would be that you might get a contribution from any floorboards too.

If it is just a ceiling, and protection is required with fire from the underside (not from top down), then if we refer to BS 5268-4, Section 4.2 - "Recommendations for calculating fire resistance of timber stud walls and joisted floor constructions" (withdrawn unfortunately - 'cos it was a really useful standard) then it has no data for 15mm plasterboard, but it does tell you that 19mm (2 layers of 9.5mm) plasterboard with a 5mm plaster skim doesn't give you 30 minutes integrity or insulation on its own (you need 2 x 12.5mm or more).  You'd only get a contribution from the insulation material if a) it's definitely mineral fibre, not glass fibre (which melts) and b) it's fixed to the joists.  Also need to remember that this is BS 476 performance & the new Euroclassification uses a slightly tougher test regime - but this might only be relevant to a new construction against the current spec.

Joist spacing is also important, by the way (BS 5268 states <457mm centres).

If the suppliers are confident that it would give you EI30, then they could always have it assessed, but that costs money & they probably wouldn't want to do it for a small job (or - in this case - a job where it's an existing construction so zero sales potential)!  I have to say, though, that the likes of British Gypsum have probably tested most permutations and combinations of their products, so if they don't have the test evidence to hand then it probably indicates it wouldn't give 30 mins reliably (it might be close, though).  I would trawl through the fire resistance data from Knauf & British Gypsum  that you can find on their websites, though - there might just be something lurking there.

[GB]

Thanks Fishy, the BSI still have the standard but at ?186 for a withdrawn standard is more than this poor Scotsman can bare!

In terms of why the FR ceiling - the compartment walls between the apartments do not extend to the roof space - instead we have 30 minute barriers every 20m in the roof space which covers approx. 2 apartments.

I have tried British Gypsum and Knauf technical departments and whilst they state their opinion that the 4mm plaster skim on 15mm plasterboard nailed directly to the roof timbers with 450mm mineral wool should achieve 30 minutes, they have no test data to back that up and you are correct that with no sales potential - wouldn't conduct a test.

I have blown the dust of my SFPE Handbook and turned the dusty pages and looked at the make up.

Okay my rationale is that plasterboard consists of approximately 21% water by weight which contributes to the plasters effectiveness as a fire barrier.

We know that the water acts as a thermal barrier by turning to steam within the board which is the calcination of the board. This calcination starts at a temperature that is slightly higher than 100 degrees C.

We have sprinklers in the apartments (but not the roof space) therefore would not expect the calcination process to commence within the board at all or if it does, only in very limited places for a very short period of time.

Some simple CFAST Zone modelling conducted for the apartment show the temperature below 100 degrees C which provides some indication that the calcination will not take place within the plasterboard or be very limited.

The SFPE Handbook of Fire Protection Engineering 4th Edition Section 4 Chapter 13 Table 4-13.1 provides times for the fire resistance of Gypsum Boards which is 20 minutes for a 16mm board and 25 minutes for effectively 19mm of gypsum board.

The detail analysed within the apartment is effectively 19mm of plaster made up of a 15mm board and 4mm skim which as Table 4-13.1 would be expected to provide 25 minutes FR.

The Fire Resistance Design Manual, Gypsum Association, Washington DC, 2000 that if we were to use 2 layers of 12.5mm plasterboard nailed to the timbers with 450mm mineral wool, we would achieve 60 minutes FR.

Table 4-13.1 states that two layers of 13mm wall board provides 40 minutes FR without the mineral wool.

Mineral wool has very good fire retardant properties and are generally resistant to temperatures in excess of 500 degrees which again due to the sprinklers is not anticipated within this scenario.

It could be deduced that the presence of the mineral wool would have an anticipated effect of up to 20 minutes which is beyond that which we require.

With the plaster providing 25 minutes of FR, the mineral wool needing only provide 5 minutes FR then I would expect the detail to achieve 30 minutes ignoring the presence of the sprinklers completely.
Thoughts

[Fishy]

I follow the sprinklers/temperature issues - slightly concerned that the rationale mixes fire test performance & modelling a 'real' fire in the building (they're not the same thing at all, and they're not supposed to be) but that's something for discussion with client & AHJ, as the sprinklers will undoubtedly have a significant fire risk reduction benefit, however you justify it. It does, though make the question to be asked rather different, I would suggest, as you're no longer trying to justify that the ceiling provides 30 minutes fire test performance (you can't do this using CFAST) - you're trying to demonstrate that a lesser period of F/R performance is sufficient in these particular circumstances?  Nothing necessarily wrong with that, but it's not the same thing.

On the mineral fibre - as I said you have to be sure that it's rock fibre, not glass fibre (the latter makes little contribution to F/R - in fact it can make things worse in certain circumstances) and you do have to make sure it's fixed to the joists - 'cos it'll just fall out as soon as the plasterboard falls away if it isn't.

[GB]

The CFAST model is not supposed to be the main thread of the methodology - I am using the SFPE Handbook - the modelling is nothing more than anectodal evidence which suggests that the sprinklers would stop calcination. That is why I am taking the duration of 25 minutes as SFPE handbook and not extending it.

Take the point about the mineral wool which is rock fibre.
Thanks Fishy

[wee brian]

Sorry GB that all sounds very convincing, but its not.

Plasterboard usually fails at the fixings and if you are using nails rather than screws then you wont get any data to support all this guff.

Have a look at the Biritsh Gypsum white book to see what constructions have been tested and check the fising details carefully.

[GB]

Thanks Wee Brian - I have never before sounded convincing therefore I will take that as a positive!
Screws were used at 230mm centres - very precise thanks to an eagle eyed CoW!