to a post-flashover fire, even in larger compartments. Therefore, the internal heat release rate and heat imposed on the structure will be significantly larger for exposed timber enclosures 23,24 . Fire spread due to a larger external flame External flaming is a physical phenomenon that can occur in combustible and non-combustible structures. It consists of a flame emerging from openings in a building and it is most pronounced during the fully developed fire stage. In ventilation-controlled compartments (ie the openings are relatively small and the fire is limited by oxygen) the unburned gaseous fuels flow through the opening to react with the oxygen available outside the compartment. In fuel-controlled compartments (ie the openings are large, and the fire is not limited by the amount of oxygen) it is also possible to have external flaming due to flow dynamics. Common Perception: In a fire, the entire timber structure of any mass timber building will be consumed, leading to total building collapse. Reality: As is also the case for buildings with non-combustible structures, mass timber buildings with higher consequences of structural failure are typically / should be designed (by competent engineers) to survive burnout, i.e. to maintain their structural integrity during and beyond a fire event; they are not designed to collapse. In buildings with exposed mass timber structural elements, in the event of fire, the structure may ignite. However, depending upon the configuration of the fire enclosure, the amount of exposed surface area, the ventilation conditions and the mass timber specification (inclusive of the adhesive), burning can stop without third-party intervention and the structure continue to support the load. Additionally, fire safety measures like spandrels, compartmentation and sprinklers are used to control and contain the fire, lessening the prospect of fire spread to multiple floors.
In several large-scale experimental campaigns, representing different types of timber compartments, the exposed timber structure has been observed to have a significant influence on the size of the external flame 19,32. The larger the area of the exposed timber inside a compartment, the higher the production of flammable pyrolysis gases. Thus, it is reasonable to expect that in ventilation-controlled compartments, the excess of unburned pyrolysis gases flows out of the compartment and combusts outside, triggering a much larger external flame. In fuel-controlled compartments, the increased amount of pyrolysis gases can increase the forces driving the flow dynamics or transition the fire from a fuel-controlled to a ventilation-controlled regime. In either case, it should be foreseen that greater external flaming is likely to be present and that this should be adequately addressed by the fire strategy as this has the potential to facilitate vertical fire spread (envelopment) and fire spread across boundaries (building-to-building). Envelopment: Envelopment is usually addressed through a combination of external wall materials that do not readily support fire spread and internal fire separations, eg vertical compartmentation. If external flaming occurs, it has the potential to ignite external wall combustible materials should they be present, and / or circumvent compartmentation by heating openings on the storey above, facilitating re-entry of the fire. The envelopment hazard should be mitigated through appropriate consideration of the external wall design. Building-to-building fire spread: In the case of mass timber buildings, a larger external flame can result in higher heat fluxes to the neighbouring buildings. This needs to be considered when establishing safety distances between buildings to mitigate the risk of fire propagation to other buildings.
STRUCTURAL STABILITY DURING AND AFTER A FIRE
As discussed on page 64, timber will dry, pyrolyse, combust, and char when exposed to heat. Therefore, the effective cross-section of the structural elements will keep
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