Author Topic: Does 60 plus 30 equal 90  (Read 114 times)

Offline lyledunn

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Does 60 plus 30 equal 90
« on: December 04, 2018, 07:07:40 AM »
If I need 90min protection to a floor and I install a 60 min ceiling below on a metal frame grid and then another 30 min ceiling below that also on a metal frame grid, all properly supported, is it reasonable to assume that I have 90 min?

Offline SeaBass

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Re: Does 60 plus 30 equal 90
« Reply #1 on: December 04, 2018, 07:55:30 AM »
It's that principle that we use when we provide two thirty minute doors in the form of a lobby to provide sixty minute protection to stairs.  So providing each element is installed as per the test specification, then yes, it ought to.

Offline lyledunn

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Re: Does 60 plus 30 equal 90
« Reply #2 on: December 04, 2018, 03:42:48 PM »
Thanks SB. British Gypsum tech dept didn't take that view. They don't have a test to confirm the set-up so they are telling me that even though I have an existing 60, I need anther 90 below, that is if I want corroborating data.  :(

Offline Fishy

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Re: Does 60 plus 30 equal 90
« Reply #3 on: December 05, 2018, 10:21:33 AM »
Thanks SB. British Gypsum tech dept didn't take that view. They don't have a test to confirm the set-up so they are telling me that even though I have an existing 60, I need anther 90 below, that is if I want corroborating data.  :(

Two possible answers, depending upon the question...

1) Is it acceptably safe as an installation where 90 mins fire resistance is the recommended level of protection?  I'd personally not have a problem with it, but it's a qualitative argument relying upon precedents for other installations (along the lines mentioned by Seabass);

2) Could you demonstrate that it has the relevant level of fire performance recommended?  No, you couldn't, & if the configuration were actually fire tested it may or may not give 90 mins.  A particularly pertinent fact is that the furnace exposure between 60 and 90 mins (furnace temperature between 950oC and just over 1000oC) involves far higher heat fluxes and energy transfer than between 0 and 30 mins (ambient to 850oC).  The fire protection has to work much harder in the last 30 mins of a 90 min test than it does in the first 30 mins, so 30 + 60 most definitely does not reliably give 90 mins, in terms that could be demonstrated.

So... BG are absolutely right in what they say.  If this relates to building works, and is subject to Building Control inspection then I'd not take the risk, unless I'd cleared it in writing with them beforehand.  They're becoming very picky about such things these days.

Offline lyledunn

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Re: Does 60 plus 30 equal 90
« Reply #4 on: December 06, 2018, 07:11:54 AM »
Thank you Fishy, that was very well put and very interesting. You have succinctly expressed the situation we find ourselves in. I really can't see LABC accepting a determination other than from a recognised source.