SFPE Discussions

C/AS1 Multi-unit Dwellings

I have recently undertaken a couple of fire reports for two or three level residential dwellings where a lower floor or part of a lower floor is to be a separate minor dwelling firecell (granny flat). C/AS1 quite correctly addresses the firecell separation and egress requirements (the egress requirements could do with some work).
The part of C/AS1 I cannot see the rationale for is 5.1.1 b). If we have say a single two or three storey residence the only external wall fire separation requirements are for external walls within 1.0 m from the relevant boundary.
If we provide a separate firecell on a lower level (reducing the fire exposure to 1 or more boundaries) then C/AS1 requires all external walls (excluding windows) within 5.0 m from the relevant boundaries to be fire rated. For most modern properties this means that 2 or 3 of the external walls require fire rating. When you take into consideration the stability of the external walls then this generally means that floors affecting the stability of the fire rated external walls need to be fire rated as well.
In fact is some situations most of the dwelling structure is fire rated as it affects the stability of the fire rated external walls.
My question to the fire engineering community is: Does anyone know what problem is the C/AS1 5.0 m external wall fire rating requirement trying to solve?

Regards
Robert Peart

This one has always got me. You could have the entire wall as glass, and no fire rating is needed, yet add a tiny bit of wall, then this bit has to be fire rated, with all the knock on effects.

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if you run the numbers the 1m rule of C/AS1 will not ensure compliance with C3.6 and so adequacy is based on 2 assumptions. One that the FENZ response time will be short to allow them to prevent/ limit the spread of fire to other property and also that the fire load will be relevant to a single HHU. The rationale behind the 5m rule was to recognise this non compliance and the effects of the increased fire load. You also have to remember that the comment regarding granny flats is just a comment and so does not form a part of the acceptable solution. The correct test being whether you have 1 or 2 household units as defined, basically social cohesion is the correct test.

The adequacy of the 1 m rule is also based on real residential fire events.
If you take a two or three level house as a single firecell (single household unit) you require a 1 m boundary separation. If you decide to take that same house and fire separate off part of, or all of a level or even multiple levels, you have reduced the fire load for a fire within that building, not increased it. You almost certainly will have reduced the boundary radiation exposure by changing the house from one firecell to two or more firecells.
It seems to me that the C/AS1 5.1.1b 5 m rule is:
a) dissuading people from subdividing houses to create a flat, and
b) creating incorrectly constructed fire separations because fire engineers/architects/builders/BCA’s do not take account of the stability requirements for the (in my opinion) unnecessarily fire rated external walls.
There is a reduction in fire load by taking a single large firecell and dividing it into two smaller firecells.
So, I don’t see how two fire separated residences, one above the other, creates a more hazardous situation to the neighbours than a single residence of the same size.
The radiation to the boundary is not a fire load issue; it is a radiator area/radiator temperature issue.
Chopping your house into multiple firecells does amazing things to radiator areas.
It leaves me with the thought of having to play code lawyers by recommending to our clients to build the entire external wall from glass and construct privacy/thermal partitions behind the glass.
The way C/AS1 5.1.1b is currently written, I believe, is having unintended consequences. This paragraph needs to be re-written with some clear and understood objectives.
Mike, would the clause have been written with the expectation that a fire separation between household units located one above the other will be compromised or ineffective?

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I agree that the 1m rule stems from historical data but as we are moving away from an assumed one dwelling per quarter acre i assume someone asked whether this initial assumption was still valid and subsequently adjusted the rules when the opportunity arose in 2012. I also assume that the 5m rule was intended as a stepping stone to align with other jurisdictions where it is normal to assess similar buildings in a medium density situations up to 6m where generally speaking FRR of 60minutes is the norm. Based on what empirical or quantitative evidence is the real question and it would be good to see the associated cost benefit analysis?