Bushfire Attack Level ("BAL")

Bushfire Attack Level ("BAL")

Bushfire Attack Level is a measure of the severity of exposure of a building or structure to ember attack, radiant heat and direct flame contact expressed in kilowatts per metre (kW/m2). The measure of the bushfire attack level of a building or structure may then used as a basis for determining the requirements for construction to improve protection of building elements from attack by bushfire.

Defined in Australian Standard AS 3959:2018 — Construction of buildings in bushfire-prone areas, there are two methods of determining the Bushfire Attack Level for a building or structure;

Method 1

Method 1 is a simplified procedure that involves five steps to determine Bushfire Attack Level.

  • Step 1 - Determine the relevant Fire Danger Index (FDI);
  • Step 2 - Determine the vegetation classification types;
  • Step 3 - Determine the distance of the site from classified vegetation;
  • Step 4 - Determine the effective slope of land under the classified vegetation;
  • Step 5 - Determine the Bushfire Attack Level.

Method 2

Method 2 is a detailed procedure, to determine the Bushfire Attack Level on a detailed. basis for all circumstances where the effective slope under the classified vegetation is no more than 30 degrees downslope (or 15 degrees upslope) and the slope of the land between the site and the classified vegetation is no more than 20 degrees, regardless of slope type;

  • Step 1 - Determine the relevant Fire Danger Index (FDI);
  • Step 2 - Determine the vegetation classification, fuel loads and vegetation height;
  • Step 3 - Determine the effective slope under the classified vegetation;
  • Step 4 - Determine the slope, in degrees, of the land between the site and the classified vegetation;
  • Step 5 - Determine the distance of the site from classified vegetation;
  • Step 6 - Calculate the flame length;
  • Step 7 - Determine flame width;
  • Step 8 - Determine the elevation of receiver;
  • Step 9 - Calculate the radiant heat flux;
  • Step 10 - Determine the Bushfire Attack Level.

 

Disclaimer

In preparing this definition, we have drawn from various sources including Legislation, Codes, Standards and industry information, research and knowledge.  Like the english language, these definitions may subtly change from time to time. As such these definitions are provided solely on the basis that users will be responsible for making their own assessment of the definition and and are advised to verify all relevant representations, statements and information.