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Adding Timber Size to Insulate Against Fire
Fire confronts the performance attributes of any structure. Many people don’t realise that large solid timber sections perform well in fires – even better than steel. Timber chars slowly which has an insulating effect on the unburnt timber beneath. This makes it possible to design timber to resist burning or collapse for a given period of time. For instance, the rate of charring and hence the rate of reduction of load carrying capacity due to loss of cross section, can be calculated. As a result it is possible to design slightly larger members in order to resist fire. The need for this depends on the class of building as defined in the Building Code of Australia (it tends to be unnecessary for many residential buildings). The means for establishing the additional size required for charring is determined using AS 1720.4 which addresses fire resistance for structural timber members. For instance, by substituting appropriate values into the equations below, the residual cross section for construction elements can be calculated with sufficient accuracy to meet the compliance requirements of the BCA (refer Specification A2.3 Clause 3).

Notional Charring Rate

C = 0.4 + (280/D)2

where
C = notional charring rate, in millimetres per minute (mm/min)
and
D = timber density at a moisture content of 12% in kilograms per cubic metre (kg/m3)

Note: species of higher density char more slowly (i.e. reduction in charring is roughly inversely proportional to increase in density)

Effective Depth of Charring (in millimetres)

dc = Ct + 7.5
where
dc = calculated effective depth of charring in millimetres (mm)
and
C = notional charring rate in millimetres per minute(mm/min) as calculated
and
t = period of time, in minutes (min)
Note: The effective depth of charring will depend on the number of faces of the member exposed to the fire – 1, 2, 3, or 4. If all faces are exposed, then charring depth must be added to all faces of the structurally required cross section size.

Sample: calculation to determine the size of a 60/-/-FRL Blackbutt post

Step 1
Determine the size of the post required to support the floor using timber design methods in AS 1720.1. Reference to AS 1720.4 (Clause 2.8) may also be useful as it defines 'fire limit state' load conditions. This state may allow lower load conditions than when a fire is absent. As a result, the increase in timber size for charring may go part/full way to meeting sizes requirements for normal load conditions. Say a 55 x 55 mm post is required for the example.

Step 2

Step 3

Step 4

Thus for a Blackbutt post with an FRL of 60/–/–, the most practical size is probably 150 x 150 unseasoned timber which will season in-situ (i.e. this is being selected as a substitute for seasoned Blackbutt, which is hard to obtain). Another option is a glue-laminated Blackbutt post at least 130 x 130mm.