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AS 3959:2018Australia

Bushfire Attack Level (AS 3959)

Australian structural engineers and building practitioners determining Bushfire Attack Level for site classification and construction requirements under AS 3959:2018. Supports both the simplified lookup method and the detailed Appendix B method using fire danger index, vegetation type, effective distance, and slope.

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What it calculates

Determine Bushfire Attack Level (BAL) for building sites to AS 3959:2018. Supports both simplified and detailed calculation methods using fire danger index, vegetation classification, distance, and effective slope.

Code standards

  • AS 3959:2018

How it calculates

The Bushfire Attack Level calculator determines BAL for a site per AS 3959:2018. BAL quantifies the radiant heat flux exposure from bushfire to which a building may be subjected, expressed as a level that maps directly to construction requirements.

Simplified method (AS 3959:2018, Tables 2.4.2 and 2.4.3)

The simplified method looks up BAL from AS 3959:2018 Table 2.4.2 (for sites in FDI 80 fire weather regions) or Table 2.4.3 (for sites in FDI 100 fire weather regions). Inputs are:

  • FDI - Fire Danger Index for the region (80 or 100), derived from AS 3959:2018 Figure 1.5 or the relevant state government authority
  • Vegetation class - Grassland, Scrub, Shrubland, Mallee/mulga/heath, Rainforest, or Forest
  • Distance - horizontal distance in metres between the site and the nearest edge of classified vegetation
  • Effective slope - slope under the vegetation in degrees (upslope increases fire intensity; downslope decreases it)

The table returns one of six BAL ratings: LOW, 12.5, 19, 29, 40, or FZ (Flame Zone), increasing in severity with higher heat flux.

Detailed method (AS 3959:2018, Appendix B)

The detailed method calculates the radiant heat flux (kW/m²) at the site directly from fuel and weather parameters using the McArthur fire spread model:

  • Fuel age and fuel moisture content - derived from climate data or entered directly
  • Wind speed at 10 m height - from the design fire weather scenario
  • Ambient temperature and relative humidity - from the design weather event
  • Fuel heat of combustion - for the vegetation type

The calculated flame height and heat flux are compared to the BAL threshold values in AS 3959:2018 Table 2.4.4 to determine the governing BAL. The detailed method can produce a less conservative result than the simplified method for sites where table inputs do not closely match actual conditions.

BAL thresholds and construction requirements

| BAL | Radiant heat flux | Construction section | |-----|-------------------|----------------------| | LOW | less than 12.5 kW/m² | No specific requirements | | BAL-12.5 | 12.5 kW/m² | Section 3 | | BAL-19 | 19 kW/m² | Section 4 | | BAL-29 | 29 kW/m² | Section 5 | | BAL-40 | 40 kW/m² | Section 6 | | BAL-FZ | greater than 40 kW/m² | Section 7 (Flame Zone) |

The calculator reports the governing BAL and the applicable AS 3959:2018 section, so construction specifications can be drawn directly from the result.

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Frequently asked questions

What standard does this calculator follow?
The calculator implements AS 3959:2018 - Construction of buildings in bushfire-prone areas. It determines Bushfire Attack Level (BAL) using either the simplified method (Tables 2.4.2 and 2.4.3) or the detailed method (Appendix B). The BAL result directly determines which construction requirements in Sections 3 through 9 of AS 3959:2018 apply to the building.
What are the key inputs?
Key inputs are calculation method (simplified or detailed), Fire Danger Index (FDI) for the region, vegetation classification (Grassland G, Scrub S, Shrubland H, Mallee/mulga M, Rainforest R, Forest F, or no vegetation), horizontal distance between the site and the classified vegetation boundary, and effective slope under the classified vegetation.
What does the calculator output?
Outputs include the Bushfire Attack Level (BAL-LOW, BAL-12.5, BAL-19, BAL-29, BAL-40, or BAL-FZ), the applicable AS 3959:2018 sections for construction requirements, and a description of the predicted bushfire attack for the site. The result is provided for each classified vegetation type present on or near the site.
What is the difference between the simplified and detailed methods?
The simplified method uses tabulated FDI, vegetation type, distance, and slope look-up tables (AS 3959:2018 Tables 2.4.2 and 2.4.3) to determine BAL directly. The detailed Appendix B method calculates flame height and heat flux from first principles using fuel moisture, wind speed, ambient temperature, and relative humidity, which can sometimes produce a lower BAL for sites where simplified tables are conservative.
What vegetation classifications are covered?
The calculator covers all AS 3959:2018 vegetation categories: Grassland (G), Scrub (S), Shrubland (H), Mallee/mulga/heath (M), Rainforest (R), and Forest (F). If no classifiable vegetation is present within the required separation distance, a BAL-LOW result is returned. Multiple vegetation types at different distances can be assessed, with the most severe BAL governing.

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