Large-scale compartment fires to develop a self-extinction design framework for mass timber-Part 2: Results, analysis and design implications

Abstract

A bstract This paper seeks to provide key fundamental knowledge underpinning the use of self-extinction principles as part of a design framework for buildings with engineered mass timber structures. The results from six compartment fire experiments in a cross-laminated timber (CLT) enclosure with different ratios of exposed timber are presented and analyzed to establish the effects of timber exposure on the dynamics of a fire and on the potential of the fire to self-extinguish. The results show the relevance of four key parameters that need to be considered concurrently when assessing self-extinction in mass timber compartments: (a) the characteristic time for burnout of the movable fuel load, (b) the characteristic time for the occurrence of char fall-off, (c) the characteristic time for the occurrence of encapsulation failure, and (d) the heat exchange within the compartment after consumption of the moveable fuel. Self-extinction was attained only when the characteristic time for the occurrence of char fall-off was longer than the characteristic time for burn-out and the heat exchange after burn-out resulted in a heat flux below a well-defined threshold. The position of the exposed timber surfaces affected the magnitude of the threshold heat flux. If the characteristic time for burn-out was greater than the characteristic time for encapsulation failure, self-extinction was not observed to occur. • Six large-scale CLT compartment fires to establish self-extinction conditions. • Fuel burnout before encapsulation failure or char fall-off was paramount. • Oxidation of charring fuel created thermal feedback loop, hindering self-extinction. • Self-extinction threshold heat flux for timber surfaces dependent on orientation. • Char fall-off occurred in slim range unsuitable for unitary surrogate temperature.

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