Notional Boundaries

Two sleeping buildings on the same property. C/AS2 stipulates a 6.0 metre distance from the unprotected external wall to the notional boundary. Also states that the same notional boundary be used for both buildings. So a 12 metre distance between the two buildings results. Have I got this right? In the definition section it is suggested that the notional boundary can be moved for each building. The sums suggest a 6.0 metre distance between the two is OK, so as usual I ma rather confused. Any help appreciated.

I take it from your discourse that you are dealing with a unsprinklered firecell with a width greater than 5 m. What this means in effect is that if you set your notional boundary 6 m away then the radiation at the notional boundary will be less than 30 kW/m2 and less than 16 kW/m2 1 m beyond the notional boundary.
If you then take your 2nd unsprinklered firecell, presumably also greater than 5 m width, then you must place that firecell 6 m away from the notional boundary giving a 12 m fire separation according to C/AS2 5.2.9.
This arrangement probably produces a highly conservative result as a 7 m building to building separation should comply with the building code (6 m + 1 m) as would be the separation distance from a “relevant boundary” calculation.

Thanks Robert, yes it does seem very conservative, hence my query. So C/AS2 is more conservative than the building code per se? If you know what I mean.

I’ve submitted a number of fire reports recently with just this situation and I took the Comment point 2 under the definition of notional boundary to mean you could shift it back and forward as it says no closer than 1.0m to the receiving building, so as Robert says - 7m separation. I’ve never had it queried.

Thank you Rob. I was unsure which clause took precedence. much appreciated.

Your post raises several interesting points…
“I’ve never had it queried” - I would be careful to provide statements on the method of compliance that allude to departure from strict compliance with C/ASx . I certainly would not rely on a query from council to pick up any such depatures.
Unfortunately, we have been told by the purveyors of the MBIE Acceptable solutions that they are rigid documents and have little or no space for intelligent manoeuvre (Unlike the old pre 2012 C/AS1).
My approach is to either use the C/VM2 commentary method for boundary separation or, now that the C/ASx docs have removed any engineering capability (apart from reading some tables in the C/VM2 commentary), just lodge an alternative solution with full compliance with C/AS2 (except for boundary separation) and boundary separation complying with C/VM2 (radiation calculations).
On the matter of the definition of “notional boundary” I think that C/AS2 5.2.9 is quite explicit and is a requirement of that acceptable solution and appears to override comment 2 to the definition.

I think as Robert has noted that it is important to read all clauses. 5.2.9 is quite clear when it says that both firecells must be analysed against the same notional boundary. How can it be the same notional boundary if it moves?

I’m curious as to why the comment point 2 was therefore included? Is 5.2.9 an extra safety factor for when both firecells are sleeping perhaps?

The relevance of item © in the definition of relevant boundary seems to have escaped scrutiny in these discussions. The circumstances described are reasonably common, so this might provide an answer to the first question.

I’m not sure from Tim’s post that there are any relevant boundaries present for his problem. My understanding of a relevant boundary is that it is a hard line on a title plan.
Tim, it would appear that the authors of the C/ASx docs consider that 16 kw/m2 radiation at the face of sleeping occupancy building or an exitway to be more than they would like for their acceptable solutions and thus have elected to take a more conservative approach.

After this conversation, I agree Robert, thanks for your input.

Gentlemen, I thank you. Much much appreciated.