These Technical Design Guide documents, issued by the Forest and Wood Products Australia, provide information on fire-rated timber framing systems for use in Class 2 to 9 buildings (courtesy of woodsolutions.com.au):

• Townhouse buildings Class 1a
• Multi-residential buildings Class 2 & 3
• Commercial buildings Class 5, 6, 9a & 9b

In Australia, construction sites may be assigned a Bushfire Attack Level (BAL) as defined in AS 3959-2009 Construction of buildings in bushfire-prone areas. Designing a timber building to meet the requirements of a specific BAL rating could mean minimising the risk of ember, radiant heat and even flame damage.

A Technical Design Guide "Building with Timber in Bushfire-prone Areas" issued by Forest and Wood Products Australia is essential for understanding the requirements of AS3959 and the potential timber product solutions (courtesy of woodsolutions.com.au).

A design method based on the established relationship between wood density and char development is provided in AS1720.4 "Fire resistance for structural adequacy of structural timber members". This standard provides a computational method for determining the fire-resistance for structural adequacy of solid wood, plywood, laminated veneer lumber (LVL), and glued laminated structural timber (glulam) members as an alternative to testing in accordance with AS1530.4 "Fire-resistance test of elements of construction". This Standard also provides methods for protecting metal connectors from the effects of fire. Refer to AS1720.4 for further information.

The method of calculating a conservative char depth is detailed below. By substituting appropriate values in the equations, the residual cross-section may be calculated with sufficient accuracy for design of the elements and consequent compliance with the requirements of the NCC BCA, as allowed for in Specification A2.3 Clause 3.

The Notional Charring Rate is given by:

• C = 0.4 + (280/D)²

Where:

• C = Notional Charring Rate, in millimetres per minute (mm/min)
• D = timber density at a moisture content of 12% in kilograms per cubic metre (kg/m³)

The Effective Depth of Charring (in millimetres) is given by:

• dc = Ct + 7.5

Where:

• dc = calculated Effective Depth of Charring in millimetres
• C = notional charring rate in millimetres per minute as calculated
• t = period of time, in minutes

Note that the net effect of charring/fire will depend on whether 1, 2, 3 or 4 faces of the member are exposed to the fire.

Broadly speaking, the charring rate (C) is inversely proportional to the density (in kg/m³) of the timber element. The calculation of expected performance must however allow for charring (and hence reduction in cross section size and thus load carrying capacity) on 1, 2, 3, or 4 sides, as appropriate.