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Requirements

The National Construction Code – Building Code of Australia (NCC-BCA), the Australian Standard (AS3959 – Construction of buildings in bushfire prone areas) along with the relevant State’s Bushfire Planning codes, setout the minimum requirements for assessing, calculating and assigning the BAL ‘Bushfire Attack Level’ of a structure within the states minimum distance of potential Bushfire Prone Land.

The Australian Standard (AS3959-2018) is primarily concerned with improving the ability of buildings in designated bushfire-prone areas to better withstand attack from bushfire thus giving a measure of protection to the building occupants (until the fire front passes) as well as to the building itself.
Improving the design and construction of buildings to minimize damage from the effects of bushfire is but one of several measures available to property owners and occupiers to address damage during bushfire. Property owners should be aware that the Standard is part of a process that aims to lessen the risk of damage to buildings occurring in the event of the onslaught of bushfire. Other measures of mitigating damage from bushfire fall within the areas of planning, subdivision, siting, building design, landscaping and maintenance.
AS3959 specifies requirements for the construction of buildings in bushfire-prone areas in order to improve their resistance to bushfire attack from burning embers, radiant heat, flame contact and combinations of the three attack forms.
Although the Standard is designed to improve the performance of buildings when subjected to bushfire attack in designated bushfire-prone areas there can be no guarantee that a building will survive a bushfire event on every occasion. This is substantially due to the unpredictable nature and behaviour of fire and extreme weather conditions.
The BAL applies to the entire building and to any attached or adjacent structures within 6 metres of the building. It is determined by using either; a simplified procedure described as (Method 1); or detailed procedure described as (Method 2).

Calculating the Bushfire Attack Level (BAL)

There are two methods for determining BALs: Method 1—A simplified procedure that involves five steps listed in AS3959 or PBP2019 to determine BALs, which is subject to limitations on the circumstances in which it can be used. Method 2—A detailed procedure, set out in Appendix B (AS3959), using calculations to determine BALs, appropriate where a more specific result is sought or where the site conditions are outside of the scope of Method 1. BALs are used to determine which, if any, construction requirements contained in Sections 3 to 9 of the Australian Standard are appropriate for a particular site.

Advising of the relevant BAL the building needs to meet

The Bushfire Attack Level BAL—LOW is based on insufficient risk to warrant specific bushfire construction requirements. It is predicated on low threat vegetation and non vegetated areas. * BAL—12.5 is primarily concerned with protection from ember attack and radiant heat up to and including 12.5 kW/m2 where the site is less than 100 m from the source of bushfire attack. * BAL—19 is primarily concerned with protection from ember attack and radiant heat greater than 12.5 kW/m2 up to and including 19 kW/m2. * BAL—29 is primarily concerned with protection from ember attack and radiant heat greater than 19 kW/m2 up to and including 29 kW/m2. * BAL—40 is primarily concerned with protection from ember attack, increased likelihood of flame contact and radiant heat greater than 29 kW/m2 and up to and including 40 kW/m2. * BAL—FZ is primarily concerned with protection from flame contact together with ember attack and radiant heat of more than 40 kW/m2. Construction in BAL—FZ may require reliance on measures other than construction. The requirements for construction of a building BAL—FZ may be regulated by the building authorities having jurisdiction in the States and Territories of Australia.

Assessing the Vegetation & Fuel Loads

Bushfire Prone Area is land that could support a significant bushfire or be subject to significant bushfire attack, it includes potentially hazardous vegetation with a Medium, High or Very high Potential Bushfire Intensity. Bushfires in these areas have the potential for high to extreme levels of flame attack, radiant heat and ember attack as a result of high potential fuel loads, slope and severe fire weather. Land that could be subject to significant bushfire attack from embers, flames or radiant heat is included in a Potential Impact Buffer with a default width of 100m from all areas of Medium, High or Very high Potential Bushfire Intensity. Fire-line intensity is a standardised measure of the rate that an advancing fire would consume fuel energy per unit time per unit length of fire front introduced by Byram (1959). One of the main benefits of the fire-line intensity metric is that it can be used to estimate the potential flame length (e.g. Byram 1959; Alexander and Cruz 2012) and thus the radiant heat expected at various distances from potentially hazardous vegetation. Radiant heat measures can in turn be used to estimate required minimum distance from hazardous vegetation needed to afford safety levels to people and/or buildings. Fireline intensity can also be used to derive estimates of flame length, radiant heat and other fire metrics to provide approximate estimates of Bushfire Attack Level (AS3959), which can provide a connection between landscape scale bushfire consultant metrics and site level decisions for building design. The Potential Fuel Load, is a key driver of fire behaviour, fire-line intensity and bushfire hazard. Potential Fuel Loads represent the approximate mass (measured in tonnes/ha) of combustible fuel material that would typically accumulate if vegetation is not regularly burnt or subject to fuel reduction practices. Potential Fuel Loads are assigned to vegetation categories (Vegetation Hazard Classes) formed by amalgamating land use and vegetation types with a moderately consistent fuel load and structure.