Fire rated boundary wall design actions to prevent consequential collapse  

How is everyone approaching the stability and after-fire stability of fire rated boundary walls for single storey buildings designed in line with the acceptable solution C/AS2 (not SH risk group), particularly in relation to preventing consequential collapse?

There seems to be the assertion that designing a fire rated boundary walls as free-standing to resist a 0.5kPa face loading (B1/VM1 Clause 2.2.4) is acceptable to demonstrate compliance with NZBC Clause C6—Structural stability, no matter the situation.

Fire engineers and Architects often cite the Fire Rated Systems Manual 2018, page 104, which notes “When a building is located sufficiently close to a property boundary, an FRR is often required to manage the risk of fire spread to adjacent property. NZBC Clause B1 — Verification
Method B1/VM1 provides guidelines for checking the lateral stability of fire rated boundary walls. The simplest and most commonly applied criterion is the ability to resist ‘a uniformly distributed face load of 0.5kPa in any direction’.“

This is also supported by an 2015 opinion published in BRANZ Build 146, which I agree with is a sensible approach for residential buildings with 30min FRR, as the risk is low, there also been research in to similar scenarios, which is discussed in BRANZ Build 160. 

The problem comes when similar thinking is extrapolated out to 60min FRR boundary walls, for C/AS2 designs, and there is an expectation from Architects, Fire Consultants and Council Inspectors, that a Structural Engineer is able to design and detail the wall as freestanding to comply with B1/VM1 and C6.   

C/AS2 Providing horizontal stability, clause 4.3.5 states as one way of providing horizontal stability is “cantilevered from a structural base
having an FRR of no less than that of the building element concerned” and the definition for “Structural adequacy” (page 31), refers the reader back to B1/VM1.

The main problem I’m having is rationalising all this with NZBC C6 – Structural Stability, clause C6.4 which states “Collapse of building elements that have lesser fire resistance must not cause the consequential collapse of elements that are required to have a higher fire resistance.“

I’m not certain that the 0.5kPa face loading in B1/VM1 was intended to approximate the loading from elements attached to the fire rated boundary wall collapsing and pulling, or pushing, the FRR wall over.

Perhaps I am missing some guidance somewhere… does anyone have deeper knowledge on this that they can share? 

Based on my current understanding and interpretation, I’m inclined to tell the architects that they need to fire rate the roof (to comply with C/AS2 clause 5.7.4 b), and any walls supporting the roof also need to be fire rated (to comply with C/AS2 clause 5.7.4), which seems to align with the expectation of preventing consequential collapse… but seems over the top.