New national bushfire building standard
AS3959-09: Construction of Buildings in Bushfire Prone Areas has been accepted by the Australian Building Codes Board (ABCB) and will replace the 1999 standard currently referenced in the Building Code of Australia (BCA) when it is next updated in 2010.
A new national bushfire standard designed to improve the resistance of homes in bushfire-prone areas to better withstand bushfire attack, was finalised by Standards Australia on 5 March 2009. AS3959-09: Construction of Buildings in Bushfire Prone Areas has been accepted by the Australian Building Codes Board (ABCB) and will replace the 1999 standard currently referenced in the Building Code of Australia (BCA) when it is next updated in 2010.
While for most states and territories compliance with the standard won’t be mandatory until the BCA is updated next year, the Victorian and ACT Governments have chosen to implement the standard immediately as an amendment to their own building regulations. Therefore, residential buildings constructed from 11 March 2009 onwards in Victoria and the ACT must comply with the standard. The standard applies to new homes, as well as renovations and additions, and also to structures attached to or sharing a common roof space with the building classified as having to comply with the standard, such as garages, carports and verandas. The standard is based on a 1090K flame temperature, an increase from the 1000K benchmark used in the previous standard, meaning that it is likely more homes will be fall under the new standard.
The new standard uses a five step science-based method for assessing the likely threat to life and property of bushfire at building sites and to determine a site’s Bushfire Attack Level (BAL). The BAL system is based on the potential danger of the site and construction materials to heat flux exposure, expressed as kW/m2. In determining a site’s BAL, the Fire Danger Index (FDI), vegetation type, distance of the site from vegetation and the effective slope under the vegetation are taken into account. Based on an assessment of these variables, a site is assigned one of six Bushfire Attack Levels (BAL) ranging from low-risk BAL-LOW to very high-risk BAL-FZ (Flame Zone), where a site assessment of BAL-12.5 or higher mandates bushfire protection measures.
The six categories and their predicted risk levels are:
- BAL–LOW – There is insufficient risk to warrant specific construction requirements.
- BAL – 12.5 – Ember attack.
- BAL–19 – Increasing levels of ember attack and burning debris ignited by windborne embers together with increasing heat flux between 12.5 and 19kW m2.
- BAL-29 – Increasing levels of ember attack and burning debris ignited by windborne embers together with increasing heat flux between 19 and 29 kW m2.
- BAL–40 – Increasing levels of ember attack and burning debris ignited by windborne embers together with increasing heat flux with the increased likelihood of exposure to flames.
- BAL–FZ – Direct exposure to flames from fire front in addition to heat flux and ember attack.
GLAZING REQUIREMENTS UNDER AS 3959-09
Construction specifications for new buildings are required to reflect their location’s assessed level of bushfire exposure. AS3959-09 sets out construction requirements, including those for glazed elements, according to a site’s BAL. The requirements for windows and doors featuring glazing are summarised in the table below. The requirements are applicable to glazing that is less than 400mm from the ground or other similarly flat surfaces such as decks, carport roofs and awnings. Where double glazing is used, the below requirements apply only to the external face.
The screens referred to below must be mesh with apertures no larger than 2mm. For buildings with a risk level of BAL-12.5 to BAL-29, screens may be made of aluminium, steel or bronze. For home on a site designated BAL-40 or BAL-FZ, screens must be steel or bronze only.
If a wall is shielded by other parts of the building and not directly exposed to the source of the bushfire attack , it need not comply with the assigned BAL but rather the next lower category, but no lower than BAL-12.5, except where the exposed elevations have been determined as BAL-LOW.
Glazing Requirements for external windows, doors and roof penetrations (I.e. roof lights/skylights):
- BAL–LOW – No specific requirements.
- BAL – 12.5 – Windows: minimum 4mm Grade A safety glass or glass blocks with openable portion screened
- Doors: side hung doors as for windows, sliding doors must comply with AS1288.
- Roof penetrations: Grade A laminated safety glass OR polymer with a Grade A safety glass diffuser installed under the glazing. For IGUs, minimum 4mm Grade A safety glass must be used in the outer pane.
- BAL–19 – Windows: minimum 5mm toughened glass with openable portions screened or glass blocks OR annealed glass completely screened
- Doors: minimum 5mm toughened glass
- Roof penetrations: as for BAL – 12.5.
- BAL-29 – Windows: minimum 5mm toughened glass with openable portions and portions within 400mm of the ground or similar screened.
- Doors: side hung doors to have minimum 5mm toughened glass with portions within 400mm of the ground or similar screened, sliding doors minimum 6mm toughened glass OR completely screened.
- Roof penetrations: as for BAL – 12.5. Glazed roof penetrations in roofs with a pitch of less than 18 degrees to the horizontal must be protected by ember guards (mesh specifications as for windows).
- BAL–40 – Windows: minimum 5mm toughened glass completely screened.
- Doors: side hung doors must be minimum 6mm toughened glass with portions within 400mm of the ground or similar screened, sliding doors to have a fire resistance level (FRL, as defined in the BCA) of at least 30 minutes integrity only (-/30/-) OR completely screened.
- Roof penetrations: Glazed assemblies for roof penetrations must have an FRL of (-/30/-). Glazed roof penetrations in roofs with a pitch of less than 18 degrees to the horizontal must be protected b ember guards (mesh specifications as for windows).
- BAL–FZ – Windows: protected by a bushfire shutter OR openable portion screened and either: an FRL of at least 30 minutes integrity only (-/30/-) for the whole window system, or, tested to comply with AS 1530.8.2
- Doors: as for windows.
- Roof penetrations: must have an FRL of (30/30/30) or (-/30/30) when tested from the outside OR tested to comply with AS 1530.8.2
ALTERNATIVE OPTIONS: TESTING TO AS 1530
Any materials tested to and achieving the requirements of AS 1530 may be used in lieu of the applicable requirements of the relevant BAL. The main difference for the glazing resulting from the successful completion of this process is that screening for ember protection on the fixed windows need not be applied. However, screening on the openable parts of the windows and doors will still be required.
For materials proposed to be used in areas with a rating of BAL-12.5 to BAL-40, testing must be carried out in accordance with AS 1530.8.1: Tests on elements of construction for buildings exposed to simulated bushfire attack – Radiant heat and small flaming sources, to determine the glazing system’s BAL. BAL-FZ glazing must be tested to AS 1530.8.2: Tests on elements of construction for buildings exposed to simulated bushfire attack – Large flaming sources.
Note that the whole window system must be tested, not the glass alone. Both AS 1530.8.1 and AS 1530.8.2 illustrate two standard glazing system configurations, featuring a range of window styles. In order for a window system to be awarded general approval, both configurations must be tested.
Testing to AS 1530.8.1
AS 1530.8.1 outlines the methods for testing a glazing system to a particular BAL, up to a maximum of BAL-40. The external side of the system is exposed to radiant heat and cribs to simulate burning debris and the radiant heat flux recorded at regular intervals for a minimum of 60 minutes.
If a glazing system contains combustible elements that have the potential to compromise the performance of eaves or walls, the test must incorporate this detail. Any failure of either of these attributable to the glazing system will result in a failure of the glazing system.
If both configurations mentioned above are tested and approved, window systems can be used freely in the BAL threshold approved, up to a maximum height of 3000mm and an indefinite length, as long as individual panes are no longer than 2400mm. For a single configuration, the maximum size of the system intended for use can be tested up to 2400mm by 2400mm. Results of testing achieved at a particular peak heat flux level also apply to lower levels with the same or smaller sized cribs.
Glazed roof penetrations are to be evaluated separately for flat and pitched roofs. Flat roofs are those with a gradient of less than 15 degrees while assemblies intended for use in pitched roofs are tested in a representational roof with a gradient of 45 degrees. Once tested in a roof with a 45 degree gradient, the results will apply to roofs with gradients between 18 degrees and 75 degrees. The assembly must be tested at full size with a minimum test size is 2000mm by 1500mm.
Testing to AS 1530.8.2
Standard 1530.8.2 sets out methods for testing building materials exposed to direct flames. Testing of glazing systems is carried out in a fire resistant furnace, simulating exposure to direct flames. A 30 minute exposure period is followed by a 60 minute observation phase in which the performance of the system is monitored. The standard sets out seven key areas of performance and the results will indicate the period of exposure for which all criteria were satisfied.
The same differentiation between flat and pitched roof as discussed under AS 1530.8.1 applies to AS 1530.8.2 testing. The minimum test size for glazed roof penetrations is 1000mm by 1000mm.
Disclaimer: This information is presented as a means of providing an introduction to the glazing requirements of AS 3959-09: Construction of Buildings in Bushfire Prone Areas. It is not provided with the intention of giving a comprehensive understanding of all obligations in relation to glazing or any other construction process or material. Companies should make their own enquiries as to their obligations under the new standard.