The Variability of Critical Mass Loss Rate at Auto-Extinction

Abstract

The conditions under which auto-extinction of timber can occur are of particular importance for mass timber structures where elements of structural timber are exposed, or become exposed, during a fire. Using experiments on 95 mm cubes of timber, the sensitivity of auto-extinction to sample orientation, and an imposed flow across the surface of the sample was investigated. It was found that the variation in burning rate between the vertical and horizontal orientation was 8% but that, for samples in a vertical orientation, auto-extinction occurred at a higher value of mass loss. It was found that while the critical mass loss rate for extinction was relatively consistent in a given orientation (coefficient of variation, 16%), the variability in the time to extinction, and thus the total mass of timber consumed, was significant (coefficient of variation, 40%). Critical mass loss for extinction was linked to the square of the flow velocity at the surface of the sample.