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FIRE SERVICE AND GENERAL FIRE SAFETY TOPICS => Fire Safety => Topic started by: Tall Paul on July 02, 2013, 12:18:04 PM
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I have been exploring AD B and the requirement for fire compartmentation in buildings of three storeys converted to flats.
In general, purpose built blocks of flats are constructed with 60 minutes fire resisting compartmentation to facilitate the stay put approach in the event of a fire. Table A2 points back to para 7.9 and 7.10 which allow for 30 minutes fire resistance for converted buildings having no more than three storeys. There is no indication in the paragraph of a need for a fire alarm system to compensate for the reduced level of fire resistance, and discussions with a local building control officer have confirmed that no further compensation would be required.
The difficulty that this raises in my mind is in relation to the principle of the stay put procedure, which appears to be founded in the 60 minute separation, and the suggestion that those on the second floor of a converted building will be ok to stay put with only 30 minutes separation.
I note that this option for reducing FR in converted flats is omitted in BS 9991.
I would tend towards seeking 60 minutes FR even in flat conversions where possible, using BRE Digest 208 to upgrade the floors where necessary. I would welcome your thoughts on this one - particularly those who are aware of the founding principles or rational that led to the writing of these two paragraphs - so that I can better understand why this reduction is acceptable in conversions when it is clearly not acceptable in new-build.
Thank you.
Paul
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The 30 minutes is a concession. When they produce a new copy of ADB they have to think about what the consequences of their recommendations will be. If they asked for 60 minutes in all converted flats there would be tens, maybe hundreds, of thousands of existing properties that do not meet that standards. Of course, existing properties do not have to meet the recommendations of ADB but, in practice, many fire risk assessors use the current recommendations as guidance and many might feel obliged to recommmend that the fire resistance is increased to 60 minutes (which would not be simple job).
The more judicious fire risk assessors might find that the particular circumstances of a case would justify only 30 minutes separation but those who have less experience and less confidence in their own judgement might feel obliged to fall back to the ADB recommendations. The authors of ADB are aware of this and will make recommendations that best suit a situation.
Stu
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Thank you Stu, I thought that the rational might have been something like that. I am interested in your second paragraph. There are instances in which a fire alarm system has been installed as a compensatory feature - but this is over and above the requirements of the AD B section discussed. The building regulations approach to flats is reliant on stay-put in the event of fire. This in itself is based on the fact that a fire will generally be controlled either by fire service intervention or by fuel limitation within 60 minutes. 30 minutes reduces that time frame significantly which would seem to suggest that stay-put is a bit more precarious.
I appreciate that people will leave their own flat if they start to feel threatened by a fire in a neighbouring flat, but if they are asleep and without the benefit of an alarm system (either part 1 in the common area or part 6 in the dwelling) then they are unlikely to be aware of the fire starting to spread to their own property. Do you have a particular set of circumstances that would lead you to accept a stay-put policy with 30 minutes FR, or would you be leaning towards evacuation triggered by an alarm system (with all the inherent limitations of alarm systems in communal areas of course)?
I have my own views, based on fire investigation and fire risk assessment experience, but am interested in exploring this this in a wider context and beyond the written word.
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See the LGA, which accepts 30 minutes in many existing blocks of flats. After that, see every old tenement in Scotland. Wonder at the ingenuity and education of the indigenous population. Then wonder at the fact that, in Scotland, those who die in flats are invariably those who have a fire in their own flat, not those affected by other people's fires (though I would have to admit that sometimes the turntable ladder comes in handy).
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An interesting comment, and one which would surely suggest that the requirement for 60 minutes FR in flats is excessive overkill? Or are those north of the border simply made of sterner stuff?
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I think Colin's last sentence says it all.
"though I would have to admit that sometimes the turntable ladder comes in handy". Hardly the essence of the stay put policy!
And the 30 minute concession in ADB does not apply to the protection of the means of escape and only applies to 3 storey conversions so all in all there aint many folk at risk and they don't have to travel far to reach a protected route.
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I understand the sentiment fully Kurnal, but I think that this could be explored further. AD B does not make concession for all three storey blocks of flats - only converted properties. People don't have far to travel to reach a protected route in any block of flats, regardless of height. The stay-put principle is based not so much on the travel distance to a protected escape route but on the ability of a fire to be held in check in the flat of origin sufficiently to reduce the need for escape for residents elsewhere. If it is sufficient to accept 30 minutes for three storey converted flats then the same should be true of three storey new-build flats if the argument is sound. The fire will not discriminate between the two, it will either be held in check or it won't.
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Well said little Pauley, exactly the point.
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The stay-put principle is based not so much on the travel distance to a protected escape route but on the ability of a fire to be held in check in the flat of origin sufficiently to reduce the need for escape for residents elsewhere. If it is sufficient to accept 30 minutes for three storey converted flats then the same should be true of three storey new-build flats if the argument is sound. The fire will not discriminate between the two, it will either be held in check or it won't.
Someone once told me that the entire contents of the flat would burn out in much less than an hour and so therefore a one hour standard of resistance will suffice to support the stay put policy. Of course that idea does not address arguments such as the realism of BS476 tests, whether the fire is ventilated or unventilated etc. On the other hand the strategy may also rely on an element of intervention by the fire service (which even by todays standards we would hope should arrive sometime on the same day as the fire.:D)
So do we have an opinion? Is a half hour standard sufficient to support a stay put policy or not? My opinion is that in general it is not and that the ADB position represents a pragmatic solution to support the conversion of a large number of small existing buildings where otherwise it may not be reasonably practicable and the politics of needing to meet the demand for affordable housing. To suggest it may be adopted in a new building is probably an example of having been given an inch then asking for a mile.
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I would agree Kurnal. The issue is a pragmatic one, the conversion of an existing building is different from the construction of a new one. With the constuction of a new build the issue of providing half hour or one hour seperation is a matter of the materials used and the design the cost of the materials will not be substancially different. However the conversion of an existing building to bring it up to one hour standard is likely to incur much greater labour and material costs as well as greater difficulty.
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Thank you Kurnal. That was the sort of engagement that I was seeking. I think that hoping for a fire to be held in check based on 30 minutes FR, ventilation effects not withstanding, is stretching it. I would tend, as stated earlier, to recommend a fire alarm system (with all of its inherent inadequacies in the case of residential accommodation) as a compensatory feature where upgrading of the structure is impractical. This approach is not implied within the AD B para 7.9-7.10 and has met some resistance on that point.
This is all a matter of risk assessment, but there are occasions when interpretations on various paragraphs are worth exploring more deeply.
Paul
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I would agree with you Paul.
A fire consultant I dealt with years back told me that the theory in small rise flats was that thirty minutes f/r was allowable because a fire would need to compromise two sets of thirty minutes f/r doors to jeopardise a neighbouring flat, thus giving you a 'notional sixty mins f/r'.He went on to say that even then the fire would need to jump across the communal area.
He didn't inspire me with confidence. As you say each situation needs to be assesed on its own merits but i dont like the idea of a stay put policy without 60 mins f/r although there are always some exceptions.