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When wood is exposed to high temperatures it will decompose to provide an insulating layer of char that retards further degradation of the wood. The rate of char is initially fast but as the depth of char increases, the rate of char slows because of the increasing insulation provided. The rate at which timber chars varies between species and is predominately dependent on density and moisture content. The predictability of timber charring is used by building regulators to develop Fire Resistance Levels for building elements like large cross-section timber columns and beams. The Australian Standard AS 1720.4 Timber Structures : Fire-resistance of structural timber members provides a method of calculating fire resistance levels for solid timber. Large cross-section timber is able to carry load during a fire event as the char occurs on the outside of the element and the effective cross section of the timber is only slowly reduced. The insulating qualities of timber mean that although the temperature at the char layer may be 300oC, the temperature of the inner wood is considerably lower. The remaining uncharred cross-sectional area of a large wood member remains at a low temperature and can continue to carry a load. Calculating Depth of Char The Australian Standard AS 1720.4 Timber Structures : Fire-resistance of structural timber members provides a method of calculating a conservative char depth. By substituting appropriate values in the equations below, the residual cross-section may be calculated with sufficient accuracy for design of the elements and consequent compliance with the requirements of the 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 (mm) C = notional charring rate in millimetres per minute (mm/min) as calculated t = period of time, in minutes (min) 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. Example of calculating the fire resistance level (FRL) of a structural element Suppose that to provide structural support under fire conditions a blackbutt timber post is required to measure at least 50 x 50 mm for 60 minutes of exposure to fire. Blackbutt at a M.C. of 12% has a density of 900kg/m3 so the notional charring rate given by the equations above would be 0.5mm/min. This would result in a charring depth of 37.5 mm on each side of the post. The size of post required, assuming that the fire will affect all four sides of the post, would be 50 + 37.5 + 37.5 = 125 x 125 mm2. If this size is not available then the next largest size should be used.
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