Page 57 - Build 165

Looking back for 9 to 5 basis
The original basis for the 9 to 5 rule was unclear, so BRANZ undertook a research project to investigate.
The 1992 Acceptable Solution does not provide many clues on where the rule came from. Preceding editions of NZS 1900 Chapter 5 (which were replaced with the Acceptable Solutions in 1992) did not include a similar requirement.
Draft standard DZ 4226, an intended replacement for NZS 1900 Chapter 5 but never implemented, included a 10 to 6 rule. The requirement would have been to re-rate 10 m of the wall above the roof or the roof within 6 m of the wall. It is possible that this was used as the approximate basis for the 1992 Acceptable Solution requirements.
DZ 4226 is quite explicit in describing the basis for the 10 to 6 rule, citing NFPA 80A as the source. However, the NFPA 80A requirements are actually slightly di erent and are linked to the number of storeys on re contributing to the ames from the roof.
Rather than requiring a greater wall protection height than the roof protec- tion, the NFPA 80A distances are equal (see Table 1). The rationale given for this is that a moderate wind could be expected to tilt ames and extend them a horizontal distance about the same distance as they would extend vertically with no wind.
The NFPA 80A requirements were based on a study of maximum ame heights from fully involved building res.
The maximum flame heights listed in Table 2 were found to be the same for di erent building occupancies.
Potential heat impact from roof re
The next step of the project was to use engi- neering analysis to evaluate the potential heat impact from a roof re on a higher adjacent wall. Using the NFPA 80A ame height data, heat transfer modelling was used to estimate the envelope where igni- tion of combustible items in an unprotected area, for example through non- re-rated windows, could occur.
Shaded area of external wall must not have unprotected areas if the lower level roof is not protected from re spread from below
Medium-density housing
If the shaded area of external wall is not protected against re spread from below, the roof must be protected by:
• 5.0 m wide FRR to roof, or
• providing sprinklers in the
Upper recells
9.0 m
5.0 m
5.0 m
recell below the roof
Firecell below roof
Figure 1: Acceptable Solution to prevent re spread from lower roof (from C/AS2 Figure 5.6).
2005 Bracken Court re in Dunedin.
Some ordinary combustibles will ignite at a minimum radiant heat intensity of 12.5 kW/m2, but New Zealand Building Code clause C3.6 allows a maximum received radi- ation of 16 kW/m2 based on assistance from the Fire Service to prevent re spread. For comparison, the heat intensity reaching the atmosphere from the sun is about 1.4 kW/m2. Contours where the radiant heat intensity drops to the maximum allowed for a range of re venting through square roof openings are shown in Figure 2.
Modelling results varied with configuration
For vertical ames (no wind e ects), the envelope where the heat intensity is above
16kW/m2fora15m×15mroofopening was 9.2 m vertical and 4.6 m horizontal or approximately equivalent to the 9 to 5 criteria.
Adding the e ect of wind tilting the ame to 45° increased the horizontal envelope to 6.1 m.
Increasing the number of storeys contrib- uting to the re to four resulted in the enve- lope reaching 12.9 m vertically and 9.9 m horizontally.
Reducing the size of the roof vent with a single storey contributing reduced the enve- lope to 5.4 m vertically above the re and 4.9 m horizontally from the roof opening.
5.0 m
Build 165 — April/May 2018 — 55
FEATURE SECTION
PHOTO – TED DANIELS