5.3
ENGINEERED TIMBER THERMAL RESPONSE AND FIRE SAFETY IMPLICATIONS
To ensure the safety of commercial timber buildings, designers must address the fire hazards associated with timber structures if they contribute as a source of fuel. Identifying and understanding these hazards is crucial, as this informs the development of fire strategies to mitigate them effectively and thus what evidence should be developed to support compliance with the Building Regulations and the attainment of other stakeholder goals.
from the surface will reach the pyrolysis front, slowing down the thermal decomposition and the mass loss rate of the wood below. Thus, the burning of timber is a time-dependent process as presented in page 69 figure, with the charring rate and thickness of the char layer evolving with time. 6. Above 350 °C char oxidation occurs and the surface starts to regress initiating a steady-burning phase. The rate of charring will depend on the external heat flux and the heat from the flame. At the point where steady combustion has been established, different scenarios are possible. Depending on the amount of external heat flux and the design of the engineered timber product (ie type of glue, lamella thickness, etc) the timber can continue burning, self-extinguish or smoulder. Self extinguishment of flaming combustion Small-scale experimental studies have shown that to sustain continuous flaming combustion it is required to have a critical mass loss rate between 3.0 and 4.0 g/s.m² 15,16 is required. Otherwise, insufficient pyrolysis gases are generated to feed the flame at the surface and the timber undergoes self-extinguishment of flaming combustion 17 . To achieve this critical mass loss rate, small-scale studies indicate that the external heat source should be in the range of 30-45 kW⁄m² 16,17 . Additionally, medium and large-scale experiment compartments presented in 14,18,19 that indicated where the external heat flux to the timber boundaries dropped below 45 kW⁄m² self-extinguishment was achieved. Thus, this compartment research indicates that the threshold values established in small-scale tests provide an acceptable starting point to create design tools with conservative acceptance criteria that will help to predict whether self-extinguishment will happen or not in a given scenario.
THERMAL RESPONSE OF ENGINEERED MASS TIMBER
Engineered timber elements undergo distinct changes when exposed to heat, leading to physical, chemical, and structural alterations, as per Figure on facing page: 1. At the very initial stages of a fire, the cross-section of the timber element is still close to ambient temperature and its mechanical properties are preserved. 2. As the temperature rises to circa 100 °C, the timber element starts losing mass as per page 69 figure, due to dehydration. At this temperature, timber loses between 50% and 65% of its strength 14 depending upon the nature of the action (compression, tension and shear). 3. Between 200°C and 350°C pyrolysis occurs. This is a thermal decomposition of timber resulting in the release of pyrolysis gases and the formation of a char layer. At this stage, at approx. 300°C, timber loses 100% of its strength and stiffness 14,15 . 4. Pyrolysis gases are combustible volatiles, which are released out of the timber and mix with air at the surface. If the conditions are appropriate, ignition of the pyrolysis gases can occur and flaming combustion starts. This stage is characterised by a rapid loss of mass as indicated in page 69 figure. 5. As the pyrolysis front progresses deeper into the timber, it leaves behind a char layer. This char layer has insulating properties. The thicker it is, the less heat
After self-extinguishment, the heat accumulated at different depths of the timber will dissipate in all directions. This
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