Timber under Real Fire Conditions – the Influence of Oxygen Content and Gas Velocity on the Charring Behavior

Abstract

As for any building material, verification of fire resistance is mandatory for separating and loadbearing timber members. While non-standard fire design for steel members has long tradition, the corresponding possibilities for timber members are limited. Reasons for this can be found in the degree of complexity of the material and the limited research done in the field. This paper summarizes selected outcomes of tests investigating the influences on the charring behavior varying the oxygen content and the gas velocity. Besides the charring rate the char layer depth was the focus of this study to investigate char contraction (consumption of the char layer). In general, measurements are in line with previous results reported in literature. Results show that charring is predominantly depending on the fire compartment temperature. Results show further that for gas oxygen contents below 15 percent the gas velocity has no influence on the charring. However, at higher oxygen rates char contraction was observed affecting the protective function of the char layer. Thus, the charring and the temperature distribution was affected and the residual cross-section was decreased. In fully developed fires increased charring due to char contraction may not be observed due to the low oxygen contents. Contrary, in travelling fires or in the decay phase char contraction may be considered. This may have significant impact to Performance Based Design using non-standard temperature fire curves where the complete fire duration has to be taken into account. The work was performed within a FORMAS project “Natural fire design of timber structures†and the test series presented here were conducted within a Master Thesis in collaboration with the University of Trieste by A. Santomaso.