fence). Ember attack is the primary cause of house loss (around 80 per cent), with the weakest link often being timber decks and/or verandas, followed by sparks entering into roof cavities, most typically from gutters and at the eaves. Radiant heat: This may arise directly from the fire front or combustion nearby (ie a garage/carport or adjacent house). It may super-heat building components (inside or outside) so they ignite from flammable gases, or from drying the building materials so they become more readily flammable. The greatest bushfire risk to human life and wellbeing is exposure to radiant heat in the open, followed by exposure to radiant heat when trapped within burning buildings.
Environment – ISSUE 05 OCTOBER 2019 6 Flame contact: This risk is influenced by the nearby fuel load, slope and fire intensity; hence siting of buildings and combustibility of building materials is crucial, as is avoiding fine fuels (ie garden, mulch) and nearby items (ie decks, fences and buildings) close to the building. Approximately 85 per cent of house losses occur within 100 metres of bushland (Leonard, cited within Victorian Bushfires Royal Commission, 2010b). Convective heat: Hot air from an approaching bushfire pre-heats and dries out both vegetation and buildings, creating hot gases that increase the likelihood of ignition. Strong winds: Bushfire weather is associated with strong/extreme winds. Aside from winds driving ember attack and spotting, the internal/ external pressure difference also creates conditions for building failure (ie falling branches, glazing failure, roof dislodgement). And bushfires create their own extreme weather (Victorian Bushfires Royal Commission, 2010a and 2010b). Assessing the level of bushfire attack – FDI As discussed, the FDI is an overall index that approximates the potential fire danger – and it’s increasing. The current bushfire regulations (AS 3959:2018) are conservatively based on 2009 FDIs, rather than responding to the higher risk scenario already established (Figure 5). In NSW, the Rural Fire Service (RFS) has sought to raise the FDIs to 100 across the state (except alpine regions). Measuring risk – Bushfire Attack Levels (BALs) ratings From 2009, the Australian Standard AS 3959 Construction of Buildings in Bushfire-Prone Areas has adopted six BAL ratings as a means of assessing a building’s potential exposure to bushfire attack (Figure 6). The BAL is largely based upon radiant heat flux expressed in kW/m 2 , with each step representing a higher risk, from 12.5kW/m 2 (BAL—12.5) through to 40 kW/m 2 (BAL—40). Up to BAL—29, most common building materials and construction is likely to be acceptable. At BAL—40 the range of acceptable materials and components gets more restrictive and expensive (tested to AS 1530.8.1, although the generic roofing solutions within the AS 3959 Appendices have not been tested but are still accepted).
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- Spring '15
- Aminul Islam
- Wind, Bushfire, Ash Wednesday fires