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FIRE SERVICE AND GENERAL FIRE SAFETY TOPICS => Technical Advice => Topic started by: Revol on June 22, 2016, 05:57:29 PM
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Is anyone aware of a minimum factor of safety (%) given within the BS 7974 suite for ASET/RSET assessments?
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I wish.
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Would a percentage be relevant? The ASET / RSET comprise a range of factors and a straight percentage may be too simplistic if applied across the board?
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Never seen one applied. Generally it's simply ASET>=RSET = :) ASET<RSET = :(
There should be a level of conservatism built into the models & input data, but I'm not sure it's ever explicitly stated. It's only an approximation to reality, in any case!
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I agree. The assumptions within PD6 are mostly ridiculously over the top. Particularly in regards to pre-movement times (in most situations), and the method of applying the lower percentile, in addition to the upper percentile.
Anything over the top, in terms of additional percentage, is a bonus in my view.
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A lot of the ASET calculations, in 7974 for example, have no safety factor built into them, though some do. To be conservative, the fire engineer using these formulae should use conservative inputs.
RSET can be based on some calculations but a significant element, as we all know, is pre-movement time which cannot be calculated. It is best if the pre-movement time is engineered out of the equation by the use of rigorous, reliable and robust fire safety management procedures such as training, well practised evacuation procedures and the like.
If suitably, but not overly, pessimistic figures are derived for both ASET and RSET then there is little need for a large safety margin. But you would need to be confident about your results.
If you were to use average inputs into the various calculations instead of pessimistic ones then you would get average outputs and you would not know the full range of possible results and so you would have no idea how big your safety margin should be and no one would know if your building was really safe until it had a significant fire. [In general, this is a poor way of testing your hypotheses].
The more sensitivity analysis that is satisfactorily undertaken ['what if?' analysis], the more likely the building is to be safe.
I would rather see realistically pessimistic inputs and a small but adequate safety margin than average inputs and an arbitrarily large safety margin.