How is a BAL Rating
In our previous articles we have discussed what a BAL rating is. If you haven’t seen that article you can read it here: What is a BAL rating. But you may ask, how is a BAL rating calculated?
A BAL rating is calculated using the calculation methods in AS3959. There are two different methods cited in AS3959; The simplified procedure (Method 1) or Detailed Procedure (Method 2). In most cases, it is only necessary to use method 1 and this is what we will be concentrating on in this article.
There are fives steps to calculating a BAL Assessment.
Step One: Determine your Fire Danger Index(FDI). You are probably familiar with the term FDI. This is updated dailyand often seen on signs next to major roadways, as shown below. While the sign may change daily, AS3959 sets the relevant FDI based on the location of the project. For Western Australia this is set at an FDI of 80.
Step Two: Determine the vegetation that will impact the site. All vegetation within 100 metres of the proposed building needs to be classified and either excluded as low threat or classified into one of the seven different vegetation categories in AS3959. The type of vegetation can have a large impact on the intensity and speed of a bushfire so it is critical vegetation is classified correctly. These classifications include not only trees, but also grass, shrubs and scrub.
Step Three: Calculate distance between the building site to vegetation. How close the vegetation is to the proposed building will influence the impact a bushfire will have. The measurement is taken from the closest wall or supporting column to the vegetation, on the horizontal plane. Eaves and roof overhangs are not included in the measurement.
Step Four: Determine the slope UNDER the vegetation. The greater the slope under the vegetation the faster a fire will move, with the rate of spread of a fire doubling with every 10 degrees of up-slope.
Step Five: We now have enough information to calculate the BAL rating for the project. Using the calculation tables in AS3959 we can put this information together and calculate the BAL rating for each vegetation plot. The vegetation plot and associated BAL that will have the most detrimental effect on your project is then used as your overall BAL rating.