Modelling the charring behaviour of structural lumber

Abstract

Charring rates for large-section timber based on experimental data have been generally established. The established rates may not be appropriately used for the prediction of failure times of lumber members which are small by comparison. It is questionable whether a constant rate can be safely assumed for lumber members since the rate is likely to increase once the centre-point temperature of the members starts to rise. This paper presents an empirically based model of charring rates for Spruce-Pine-Fir (SPF) Machine-Stress-Rated (MSR) 2X4 lumber subjected to a constant-temperature exposure of 500 degrees C, on the basis of test results on 55 specimens. In order that the model can be used with reliability analysis, one of the two model parameters was treated as a Lognormal random variable to explain variations observed in the charring rates. Furthermore, the model has been extended to permit evaluations for other lumber sizes and under exposures such as ASTM E119 fire conditions. The model was compared with existing models and used to predict char data found in the literature.