SFPE Discussions

Notional Boundary Analysis

In a situation where a notional boundary analysis is required, C/AS2 paragraph 5.2.9 states that analysis should be conducted separately for each firecell using the same notional boundary. However, in a case where only one of the two firecells contains a sleeping risk or exitway, we are only required to consider radiation in one direction right? In other words, in this scenario is it acceptable to set a notional boundary at 1 metre from the sleeping firecell and use the relevant factors for the non-sleeping firecell to determine the remaining distance separation? Just checking because I have had this queried recently.

I suggest having a look at C/AS2:2017 Comment on Notional Boundary definition, then you can see the concept behind Notional Boundary analysis and how to use it.

Happy to help.

By using C/AS2 5.2.9 for notional boundaries your calculations will probably yield a far more conservative result than those results provided for in title boundary calculations.
The relevant building regulation here is NZBC C3.6. C/VM2 Appendix C tables are supposed to address compliance with NZBC C3.6. The tables in C/AS2 section 5, in some cases, do not meet the requirements of NZBC C3.6.
That aside, one approach you could take that would comply with the building code is to use two notional boundaries:
When looking at two buildings on the same title you would place a notional boundary 1.0 m away from each target building. That way your design should result in no more than 30 kW/m² at each notional boundary line and no more than 16 kW/m² at each target building.
This method would be an alternative solution to C/AS2 5.2.9 that complies with the NZ Building code.
I don’t recommend using the C/AS2 unprotected area tables in section 5 unless you have done backup calculations to check the veracity of those tables for your specific situation.

might be semantics but C3.6 is specific to relevant boundaries only. I realise that C/AS1 and 2 use the terms relevant and notional interchangeably and this works in the context of s22 of the building act. essentially the chief executive deems this appropriate regardless of whether code clause compliance is demonstrated, or not. However it all falls down at building code level as the only code clause you can fall back on is C4.3. note also the test of when notional boundaries need to be imposed, exitways, sleeping, etc, both of which lend themselves to means of escape from fire, not protection of other property, (via the definition of relevant boundary), in a s112 context. This reinforces the premise of C4.3 being the relevant code clause.
therefore when imposing notional boundaries you need to keep within the scope of the acceptable solution, not venture to C3.6 and also consider them in the context of s112

NZBC C3.6 is all about preventing fire spread to buildings on other property WITH a limitation that the prevention of fire spread is only addressed up to 1.0 m over the title boundary.
For buildings and firecells on the same property that same limitation is provided by the use of a notional boundary and the same unprotected area tables supposedly based on C3.6.
So, C/AS2 provides for placing a notional boundary 1.0 m from a building/firecell. The problem with this approach is that it unnecessarily constrains the allowable unprotected openings in the building/firecell closest to the notional boundary. If building/firecell A can roast building/firecell B with 16 kW/m² then why can’t building/firecell B roast building/firecell A with 16 kW/m²?
Notional boundaries are generally about preventing fire spread between buildings/firecells. That is also my understanding of the intent of C/AS2 5.2.9 - to prevent fire spread either between two firecells on the same property, or two separate buildings on the same property.
Which ever way you cut it you don’t want to radiate more than 16 kW/m² on the target firecell/building for sleeping risk groups or exitways.
If you choose to evaluate C/AS2 5.2.9 in terms of C4.3 FED Thermal compliance, you would be in serious strife proving compliance using the C/AS2 Tables 5.1, 5.2, and 5.3 (C/AS2 5.2.8).
I suggest that compliance with NZBC C4.3 is not considered in C/AS2 Part 5.
I don’t understand the relevance of s112 of the Building Act 2004 in the context of Control of External Fire Spread. Control of External Fire Spread is normally addressed in s116 Building Act 2004 incursions.

I don’t disagree, but it still doesn’t make C3.6 applicable as its obligation is to relevant boundaries and 5.2.9 identifies the intent of notional boundaries in a life safety context. All I am saying is that we need to understand the difference between building act compliance and conformance to an acceptable solution as conflating them can lead to issues. There are countless examples in the acceptable solutions of requirements more onerous than, or that demonstrate non compliance to the building code and in the context of s22 that’s fine. If we elect to use an acceptable solution s22 simply requires that we be treated as if we have complied with the building code which is very different from actually complying with the building code.