BUILDING PHYSICS 34 binderholz CLT BBS in the event of a fire Wood has the capacity of building up a protective layer of carbon in the event of a fire. It has an insulating effect, delays burn-off and counteracts fire propagation. The burn-off speed of binderholz CLT BBS has been determined by comprehensive testing at accredited testing institutions. The carrying capacity of CLT BBS components in the event of a fire can therefore be calculated with high accuracy. Thus, it is understandable that firefighter prefer deployments in plywood board structures over fighting fires in buildings with other construction designs. This is because they know how long they can stay in them without putting themselves in danger. Classification of the fire resistance of CLT BBS components To determine the fire resistance, comprehensive fire tests have been conducted on CLT BBS elements at various independent and accredited testing institutions. In the fire tests, not only large-area CLT BBS elements have been tested by themselves but also different connecting joints. These are, like the component itself as well, smokeproof and gastight and they accordingly do not permit any burn-through (see Figure 19). The classification of the assemblies in the Solid Timber Manual 2.0 was made by IBS Linz and MFPA Leipzig on the basis of the fire tests explained above. The classification is also shown on the individual datasheets. Figure 19 – Smoke-tight butt joint through caulking strip and wooden riser Burn-off behaviour of unprotected plywood boards according to EN 1995-1-2 The burn-off speed or the burn-off rate ß0 for pinewood according to EN 1995-1-2 is 0.65 mm/min and remains constant through the formation of a carbon layer on the surface. The glued layers in the plywood boards lead to local small-area chippings in the carbon layer through the softening caused by the temperature. Before the next layer on which the fire load is working has built up a carbon layer again (25 mm), this effect causes the burn-down rate to double to 1.3 mm/ min. Thus, the first 25 mm of each new plywood board layer after a glued joint burns off at this increased speed (Teibinger and Matzinger, 2013). Tested burn-off rates of binderholz CLT BBS The burn-off rate β0 indicates how many millimetres of wood in a large-surface application burn down per minute of the fire duration. To determine this parameter of binderholz CLT BBS, representative plywood cross sections with and without fire protection planking have been treated with flames in the standard tests at accredited testing institutions. Wall and ceiling components that are or are not exposed to load have been tested this way. Based on the results of these individual tests, the following burn-off rates can be expected on the safe side for the engineering measurement of the carrying capacity of binderholz CLT BBS in the event of a fire. Any potentially existing planking layers have no negative effects on the burn-off rates, which is why the burn-off rates for CLT BBS with and without initial protection by planking are considered to be equal.
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