FIRE HAZARDS INDUCED BY STRUCTURAL TIMBER
polyurethane 29 . Several small and large-scale experiments indicate that CLT manufactured with modified PUR glues undergo less delamination 23,30 . However, delamination in modified PUR can still occur if the load is sufficiently high 28 . At a high temperature gradients, the moisture front moves faster and creates pressure on the bond line. When this is combined with a high load, the bond line can fail at lower temperatures (eg 100-120 ⁰ C). If the gradient is lower, the moisture front traverses through the bond line and the failure occurs at higher temperatures (ie >160 ⁰ C) 28 . Since the application of structural load, moisture conditions, and heating condition, all influence the bond line temperature at which HID occurs, a consistent critical temperature for delamination across different adhesives is not evident, making it difficult to be modelled and predicted. Common Perception: Heat-induced delamination is fully prevented by applying modified PUR glues (ie with added fire retardants). Reality: There is experimental evidence that heat-induced delamination in modified PUR can still occur, for example when the structural load is sufficiently high. The application of structural load, moisture conditions, and heating conditions, all influence the bond line temperature at which heat-induced delamination can occur. Hence, delamination across different adhesives is a complex phenomenon, difficult to model and predict. Nevertheless, it is noted that preventing heat- induced delamination is not always a pre-requisite for self-extinguishment of flaming combustion, as it depends on the configuration of which surfaces are exposed.
The fire hazards outlined below have been identified by understanding the burning behaviour of timber and by conducting large-scale experiments representing commercial and residential enclosures. To date, several large-scale experiments representing commercial enclosures have been conducted 16,17,21,23,24,31,32 . These experiments usually consist of large open floor spaces, with high ventilation factors (ie large openings), and typically an exposed CLT ceiling is present. Although each experiment is different, the list of identified hazards presented below has, to varying extents, been reflected in these experiments. Different hazards appear at different stages of the fire. Therefore, they challenge different parts of a fire safety strategy. The following list of hazards is organised following the timeline of development of a typical fire: ignition, fire spread, fully developed fire, decay stage and post-fire stage. Internal fire spread and change in fire dynamics In several large-scale experiments, like in 23,24 it has been noted that in the initial growing phase of the fire, the spread rates of the fire are comparable to those within non-combustible compartments. However, this research concludes that as soon as the fire ignites a large exposed timber area, like an exposed CLT ceiling, a quick change in the fire dynamic occurs. It drastically increases the fire spread rate within a compartment. This phenomenon can challenge the early stages of horizontal evacuation, while occupants are evacuating the compartment of fire origin. To mitigate this, the flame spread over the combustible timber surface must be considered. This can be controlled by applying different treatments and coatings, the inclusion of automatic suppression systems or by breaking the continuity of the timber surface with beams or other elements.
Additionally, the ignition of the ceiling usually accelerates the ignition of the fuel located on the floor. This often leads
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