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The Australian Standard for construction of buildings in bushfire-prone areas (AS3959) specifies requirements in order to improve the resistance of buildings against bushfire attack from burning embers, radiant heat, direct flame contact, or a combination of these three factors. In particular, AS3959 informs the design and construction of buildings in bushfire-prone areas through the concept of Bushfire Attack Level (BAL). In principle, BAL can be determined through application of the Standard to any site in the landscape, thereby providing the appropriate design and construction measures required to reduce the risk of bushfire igniting a building situated at the site. The calculation of BAL at a site relies on the following four main factors: • fire danger; • vegetation type; • distance of the site from the classified vegetation; and • the topographic slope on which the vegetation is situated. Moreover, BAL is calculated based on the assumption that a fire will behave in accordance with the quasisteady assumption. In this paper we examine the validity of AS3959 in situations where the quasi-steady assumption is violated. In these situations the expectation is that fires will exhibit dynamic modes of fire spread such as vorticity-driven lateral spread, eruptive fire spread and ember storms. As such, this study constitutes a critical review of the current models and methods informing the construction of buildings in bushfire prone regions, and that underpin bushfire risk assessment on the urban interface more broadly. We find that there are a number of recent insights into dynamic fire behaviour that are not adequately addressed in the modelling framework that underpins AS3959. For example, current calculation of BAL is based on radiation thresholds derived using (scalar) distance to vegetation as a key input, without any acknowledgement of directional influences. A case study is presented that highlights the fact that dynamic modes of propagation could produce a strong directional bias on bushfire attack levels. The overall implication is that construction standards and bushfire risk more generally, as assessed based on the principles espoused in AS3959, is likely to be under-predicted in the presence of dynamic fire propagation. The insights gained from recent dynamic fire modelling efforts provide the basis for further review and refinement of AS3959.

Reassessing the validity of AS3959 in the presence of dynamic bushfire propagation