Background
It is generally the architect's aim to maximise land utilisation and fill every square meter of the site with functional/usable space for the client. Where this can land them in hot water is when trying to address the National Construction Code 2019 (NCC) requirement to ensure external walls are provided with a minimum Fire Resistance Level (FRL). This is a problem in of itself (and shall be addressed in a future post) however, the focus of this post is to discuss what happens when openings are introduced into these walls required to have an FRL.
NCC 2019 Amendment 1 requires the following distances from the opening to a fire source feature (as defined by Clause C3.2):
3 metres separation from a side or rear boundary of an allotment; or
6 metres separation from the far boundary of a road, river, lake or the like adjoining the allotment, if not located in a storey at or near ground level; or
6 metres from another building on the allotment that is not Class 10.
What's the problem?
Moving openings away from boundaries or even omitting them altogether to satisfy the above criteria introduces quite the restriction to architectural creative license (although this the code's method of ensuring risk of fire spread between property openings is minimised) and has a negative impact on other items such as natural ventilation and natural lighting into the building.
The code's resolution to this problem (via clause C3.4) is categorised into the following:
Self-closing fire-rated doors/windows
Self-closing doors/windows (no fire rating) protected with drenching sprinkler heads
Whilst the above solutions may work for some buildings, the reasons why they're not suited to all buildings may be one of or a combination of the below:
Cost prohibitive (fire-rated doors/windows)
Does not match the intended visual theme of the space/building
Available water supply to serve the drenching sprinklers is not readily available
So what options are available? You guessed it. Fire engineering.
Performance Solution
Fire engineers typically approach this problem by considering what type of fire the openings are exposed to. Fortunately, the NCC provides a Verification Method which as summarised below.
So what does this all mean? Using the above criteria, fire engineers need to determine the level of heat received at and transmitted from the opening in question with the main aim of demonstrating that ignition won't occur from the fire source to the adjacent property.
How to Justify?
Without revealing too many tricks of the trade, fire engineering practitioners such as ourselves assign appropriate values for the following parameters:
Temperature of fire source
Heat flux at which ignition will occur (Acceptance Criteria)
Distance from the fire source feature to the property
The foundation of the heat flux calculation is built on the below heat transfer equation:
The intent of this post is not to delve into the weeds but more so to provide a general overview of the concepts. There are plenty of resources out there such as this one which can assist with understanding heat radiation.
As mentioned above, there are two parts of the justification:
Calculating heat received at the openings when a fire occurs on the adjacent property
Calculating heat transmitted from the opening to the adjacent property
Each of these two calculations yield a heat flux (kW/m2) which must be compared to the threshold of ignition (Acceptance Criteria). If all goes well, the heat flux will not exceed the minimum value required to ignite items.
Impact on Design
This is the question on everyone's mind! In most cases, the heat flux initially calculated above will likely result in ignition occurring thereby falling short of our Acceptance Criteria. Fire engineers may be able to get past this by introducing:
Toughened glass on windows which resist higher levels heat flux before breaking whilst also transmitting a reduced level of heat flux through to the interior of the building.
Heat-absorbing stainless steel mesh (e.g. CrimSafe) in front of windows
Solid wall construction between the boundary and the opening to increase the distance between the fire and the building thereby lessening the intensity of the heat flux.
The fire engineering process for this Performance Solution is generally iterative and concessions may have to be accepted on the design in order for the Acceptance Criteria to be satisfied. We will endeavour to work very closely with the design team as the fire engineer to ensure the right outcome is achieved.
Conclusion
This may be a common Performance Solution for buildings incorporating a lot of glass or buildings which seek to maximise every single sliver of land on the allotment. Regardless, our fire engineers are trained to work as hard as they can to try and get the outcome you desire.
Please contact us to discuss opening protection for your project. This post did get a little technical but that's not something for you to worry about. The technical part is for us to worry about!