This paper presents a review of the fire safety risks that mass timber construction may introduce in buildings, with special consideration to medium- and high-rise buildings. These main risks are represented by an effect to the classical fire safety strategy for buildings: (1) Compartmentalisation, to ensure that the fire will not spread farther than the compartment of origin, (2) Preventing vertical flame spread to avoid façade and multi-floor fires. These requirements are essential if an adequate fire strategy has to be achieved. In order to study these newly identified risks in buildings with exposed mass timber (i.e. Cross-Laminated Timber); five medium-scale compartment fire tests were conducted of internal dimensions 0.48 m x 0.48 m x 0.37 m. Two tests had one sidewall and the ceiling of exposed CLT, while the other three were baseline tests (i.e. same geometry and the CLT completely encapsulated). Results consisting of incident radiant heat fluxes onto the compartment walls and the façade area above the window are presented herein. These are the parameters that will determine if self-extinguishment and vertical flame spread will occur. Self-extinguishment of the exposed timber occurred for all the tests with exposed mass timber after the fire consumed all the available fuel on the floor of the compartment. Therefore, after burnout, the heat flux onto the exposed walls was below the critical heat flux for self-extinction. Nevertheless, it was difficult to compare the heat flux data at the time of self-extinguishment with previous outcomes for self-extinguishment conducted on bench-scale testing, as the time scale differences induce a considerable delay before flameout is observed. The heat fluxes into the façade were as much as two times bigger when exposed timber was present.

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