EWPAA Structural Plywood and LVL Design Manual - Engineered ...

16 Resistance to Fire, Decay and Bugs16.1 Fire & WoodThree components are required for a fire, i.e. fuel, heat and oxygen. This knowledge is essential whenconsidering containment which requires eliminating one of these three components from the other two. Thatis, to extinguish the fire requires removing:• heat by wetting;• fuel by eliminating the source;• oxygen by smothering the fire.Wood is composed of a mixture of cellulose, hemicellulose, and lignin bound together in a complex network.Heating wood above 280°C causes decomposition or pyrolysis converting it to gases, tar and charcoal. Attemperatures above 280°C the gases will flame vigorously but the charcoal requires temperatures of about500°C for its consumption. A build-up of char tends to protect the unburnt wood from rapid pyrolysis. Theunburnt timber, being a good insulator, results in the timber close to the char edge being unaffected by thefire. FIGURE 16.1 shows a schematic representation of burning wood.FIGURE 16.1: Zones of burning wood16.2 Fire Hazard Properties – Test MethodsThe most dangerous period with regards to human safety is usually at the initial stages of the fire prior toflashover. Hence the need for provisions in the BCA which limit the spread of fire and the development ofsmoke until the building occupants have time to evacuate. Tests have been developed which simulate afire in a building or are done on test specimen to facilitate the generation of relevant design data.AS 1530 Part 3Early fire hazard tests to AS 1530 Part 3 (which has now been superseded by ISO 9239.1, ISO 9705 andAS/NZS 3837 see FIGURE 16.2) were performed to assess the surface burning characteristics ofmaterials. The data generated through these tests is still valid for sarking type materials, i.e. reflectivefoil or other flexible membranes for waterproofing, vapour proofing or thermal reflectance in Class (2) to (9)buildings. However, it does not apply to:• floor materials and floor coverings;• wall and ceiling linings fire hazard properties.The test sample parameters quantified in the AS 1530 Part 3 test are:• tendency to ignite through assigning an ignitability index;• tendency to propagate flame through assigning a spread of flame index;• ability to release heat once ignited through assigning a heat evolved index;• tendency to produce smoke while burning through assigning a smoke developed index238