Charring rate
Charring rate, refers to depth and speed that materials burn on exposure to fire. It is a key part of calculating the fire resistance of different timber elements in buildings, particularly structural elements. The charring rates are calculated by measuring the char depth, in comparison with the time material is exposed to the heat load.
EN 1995-1-2:2004 provides charring rates for different timber materials for the purposes of informing initial design. The charring rates apply to each face of the timber that is exposed to fire. For example, if a softwood element is exposed on all four of its sides to 20 minutes of fire: 2 x 20 x 0.8 = 32 mm
Example different material charring rates ßn (mm/min)
- Softwood timber 0.8
- Softwood glulam and laminated veneer lumber (LVL)0.7
- Western Red Cedar (390 kg/m³) 0.8
- Hardwood timber and hardwood glulam 0.55
- Oak and Sapele (>640 kg/m³) 0.5
It is important to note that the process of charring releases air and oxygen and chemically restructures timber to actually make it more fire resistant, this process can also be carried out mechanically where wood is burned evenly and at a constant rate to improve performance.
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