Timber's Performance Against Fire

As the world increasingly turns to timber in construction for the many benefits, there remains one characteristic of timber that needs further understanding. Many people assume that timber buildings perform poorly in fire situations due to the nature of wood. However, even though timber does burn, it actually has significant insulating properties and burns in a slow and predictable way.
Fire Resistance

When timber is exposed to the heat of a fire, it goes through a thermal breakdown process and a layer of charcoal forms on its surface. This charred layer is the key to timber’s fire resistance, acting as an insulator that protects the inner core. Although the temperature at the char layer may be 300°C, the temperature of the inner core is considerably lower.

Large timber members (sawn timber or glulam), can be naturally fire resistant thanks to the unique charring properties of the wood. The self protecting nature of the charring layer increases the likelihood of a timber structure surviving fire as the uncharred inner core remains unaffected, maintaining its structural strength and stability. Therefore, glue laminated timber will not twist or spall in a fire event.

Sustainable Development

Hyne Timber uses both sustainably grown hardwood and softwood to engineer beautiful glue laminated (Glulam) timber beams.

LGL and Beam 17 come from Australian plantation pine. The fire properties of this species are listed in Material Group Number 3, which indicates that the material can reach flashover in more than 120 seconds but less than 600 seconds after exposure to 100kW. Pine wood is resistant for areas where Bushfire Attack Levels* are considered BAL-12.5 and 19.

Beam 18 is made of Victorian Ash. This hardwood is listed in the Fire Properties Group Number 3 and its bushfire resistance makes it suitable for BAL-12.5 and 19 (door and window joinery only).

Beam 21 is available in both Forest Reds and Spotted Gum. Spotted Gum is considered a bushfire resistant timber up to and including BAL-29 while Forest Red Gum is rated for BAL categories of 12.5 and 19. Based on the Building Code of Australia’s Early Fire Hazard indices, its spread of frame and smoke develop indexes sit low at 3 in a scale of 10.

* In the Australian Standards AS 3959, a home may experience different bushfire intensity levels. These are referred to as Bushfire Attack Levels (BAL). There are six levels in total: BAL Low, BAL-12.5, BAL-19, BAL-29, BAL-40 and BAL Flame Zone.

Fire Tests

Many Australian standard tests exist to cover fire concerns. A summary of these can be found below:

 

Fire CharacteristicVerification MethodApplication
Average specific extinction areaAS/NZS3837Used for wall and ceiling linings and is species specific
Bushfire attack level (BAL) suitabilityAS1530.8.1 and / or AS1530.8.2Used for external building elements e.g. glulam columns. Is species specific up to & including BAL-29. AS1530.8 Parts 1 & 2 are applicable for testing products for BAL-40 & BAL-FZ respectively
Critical radiant flux (CRF)AS/ISO9239.1Floor linings and floor coverings
Fire resistance for structural adequacyAS1720.4 or AS1530.4

AS1720.4 engineering char calculation method is applicable for resorcinol/phenol type glues only. AS1530.4 is a full scale fire test method that is used for "system" testing e.g. glulam floor / ceiling system

Flammability IndexAS1530.2Used for sarking-type materials
Material group numberAS5637.1Used for wall and ceiling linings and is species specific. Timber generally achieves a Group Number 3
Smoke developed indexAS/NZS1530.3Used for materials and assemblies for specific locations e.g. theatres, public halls
Spread of flame indexAS/NZS1530.3Used for materials and assemblies for specific locations e.g. theatres, public halls
Smoke growth rate indexAS5637.1Used for wall and ceiling linings and is species specific
Fire Retardant Treatment

Timber can be treated with chemicals that can modify its performance in fire. A number of treatments are commercially available in Australia. Hyne Timber does not offer fire retardant treated timber and advice should be sought directly from the companies offering these treatments.

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