6. PERFORMANCE OF LVL IN FIRE 6.1 WOOD AND FIRE Wood and wood products, including LVL, are combustible materials. They start to burn at the surface at a temperature of 270 °C when exposed to flame. However, self-ignition does not occur at temperatures below 400 °C. According to structural fire design part of Eurocode 5 (EN1995-1-2), the start of charring is defined as the point at which the temperature of timber surface reaches 300 °C. The resistance to fire behaviour of wood products is highly predictable and can be calculated according to the structural fire design specifications of Eurocode 5. Burning creates a char layer on the surface of wood products. The char layer acts as an insulation layer, which slows the burning and protects the rest of the cross section, see Figure 6.2. However, the high temperature prior to burning reduces the strength and stiffness properties of the wood even before charring, see Figure 6.3, which must be taken into account in structural fire design. Figure 6.3. Influence of temperature on the mechanical properties of softwood. Left: Reduction of modulus of elasticity parallel to grain, Right: Reduction of strength parallel to grain (EN1995-1-2:2004, Figure B.4 and B.5). Figure 6.2. Temperature gradient in burning wood. The temperature drops significantly behind the charring layer. 15 mm from the charring zone the temperature is below 100 °C 36. (Schaffer, FPL, US 1967 and Brandsäkra trähus 3, SP Trä, Sweden 2012) Reaction to fire requirements are specified for wood surfaces to control the risk of flame spread in buildings. They set boundary conditions for the use of visible wood in claddings and structures. In some cases fire retardant treatments or sprinkler systems can allow more visible wood structures to be used in architectural design. 6.2 REACTION TO FIRE In the European classification system defined in EN 135011, the reaction to fire performance class of LVL is D-s2, d0. This class can be applied to LVL without further testing when the density is at least 400 kg/m3 and the product thickness is ≥ 18 mm (European Commission Delegated Regulation (EU) 2017/2293) 37. LVL Handbook Europe 159
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