The response of exposed timber in open plan compartment fires and its impact on the fire dynamics

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Fires in open plan compartments have been the subject of much research over the past decade. The impact of exposed CLT on the fire dynamics in such compartments is also receiving a great deal of interest as trends in timber buildings include also open plan buildings. This article presents some initial results from an experimental study which was conducted to explore the role of a timber ceiling as well as ceiling intrusions on the flame spread rate of the fuel-bed on the floor. The reduced scale testing methodology is validated by contrasting observed fire behaviour with large-scale compartment fire experiments reported in the literature. The reduced-scale experiments highlight the complexity of the fire dynamics, with ceiling intrusions dramatically increasing spread rates. Spread rates of the fuel-bed with a timber ceiling are initially slower than configurations without a timber ceiling. Once ignition of the timber ceiling occurs, flames spread across the ceiling almost instantaneously. The additional feedback from the flames, smoke layer, and charring timber surface induces a very rapid flame spread across the fuel-bed. Self-extinction of the timber is obtained in all cases once the complete burn-out of the fuel-bed is attained. The results demonstrate that idealisations of a travelling fire for open-plan compartments may not be valid when considering complexities such as ceiling intrusions or combustible timber ceilings.