Smouldering combustion after the fire Smouldering usually appears as a local effect, and although it is much slower than flaming combustion, it can be harder to detect and extinguish and its effects cannot be neglected. Studies conducted by Imperial College 21 , showing that smouldering can persist for several hours or days after flaming combustion has been extinguished, and although it can extinguish, it may also transition to flaming combustion. Thus, smouldering is a phenomenon that needs to be addressed to demonstrate safety in the hours and days after the fire. This is likely to require some form of intervention by the fire and rescue services. Common Perception: An R60 fire rating means that the building will fail after 60 minutes. Reality: The time in minutes used to express fire resistance ratings, for example R60 (60 minutes loadbearing capacity), relates to the time taken for an element to fail to fulfil one or more classification criteria under standardised furnace conditions . It is not the same as time elapsed during a real fire. For instance, in the fire test the isolated construction element is subjected to elevated temperatures that follow an indefinitely growing time-temperature curve which has been standardised for classification purposes, and broadly reflects the fully developed phase of a fire; whereas a real fire behaves differently, with growth, fully developed and decay phases, and does not grow indefinitely. Furthermore, by testing isolated construction elements both positive and negative behaviours associated with system performance in a real fire are not addressed.
reducing during a fire and potentially beyond. Currently, there are available models to estimate charring rates and effective cross-sections due to the changes in the mechanical properties, like the models in Eurocode 5 31 . However, these models are simplified and only applicable to certain conditions. Designers must understand the tools they use and ensure that the load-bearing capacity of the timber structure will be able to withstand the imposed loads for the required time. Additionally, temperatures deep within the cross-section of the timber elements will continue to rise as the thermal wave progresses after the fire. Consequently, the mechanical properties of deeper sections of the element will change since timber loses its strength at relatively low temperatures. This should be considered to ensure adequate stability after the fire has been extinguished 14 , with glulam columns particularly vulnerable to strength loss during the cooling phase of a fire and beyond. Design for burnout Higher consequence of failure buildings are generally afforded structural fire provisions with the expectation of maintaining their structural integrity during and beyond a fire event. For buildings with a non-combustible structure, this is often achieved with structural elements that achieve high levels of fire resistance when subject to standard testing that translates to often indefinite survivability when subject to a real fire. For a non-combustible structure to survive the full duration of a fire, it must withstand the thermal assault from the movable and permanent fire load in a building, eg furniture, linings, fixtures and fittings. However, in the case of a timber structure, the structural elements can continue burning after the movable and permanent fire load burns out and until the whole timber structure is consumed. Therefore, the only way to design for burnout (ie surviving the full duration of a fire) for these structures is by demonstrating self-extinguishment of flaming combustion of the timber elements on a case-by-case basis. Currently, simple models and experimental work exists 15,16,17,18,24 to help designers estimate the potential for self-extinguishment of timber in different configurations.
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