Safety Engineering

AS 3959, the Australian Standard that provides the requirements for construction in Bushfire Prone Areas in Australia, provides two methods of assessment of the required construction levels.

Method 1 is a basic method which is based on tables developed from a series of engineering formulae which together form the View Factor, a very involved calculation which tales into account the factors involving a bushfire over a 100m front, and which when combined with a series of other factors including fuel load, land slope, atmospheric transmissivity, distance from the fire, enables the amount of radiant heat flux that is impacting on the building to be assessed.

The drawbacks in using Method 1 is that it is limited to 20 degrees of slope, uses default fuel loads, and slope is measured in blocks of 5 degrees.

Utilising Method 2 of the AS 3959-2009, which is a computer based bushfire modelling and heat output program contained within the AS 3959-2009, we commence by taking site specific fuel loads iusing a methodology taught in both the University of Western Sydney and by the Rural Fire Service Queensland.

This generally produces substantially lower fuel loads than the default fuel loads in the Standard, which are based on southern guidelines, where the climate caused much higher accumulation of fuel and different growth characteristics of bark, which is one of the elements addressed.

Slope is measured, using a survey instrument, or calculated from contours, an calculated at one degree intervals. This impacts on the results substantially, as fire doubles with speed and intensity for each 10 degrees of slope.

The result of correctly using Method 2 can be that the distances that the various heat loads determining Construction Levels occur can be greatly reduced, resulting in lower building costs without sacrificing safety.