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Abstract
Thermal action on timber causes it to degrade through combustion of its chemical components, which leads to the release of vapors, combustible gases and surface char. This diminishes its load capacity, due to the reduction of its cross section by charring and to changes in its mechanical properties of strength and stiffness as a function of its exposure to high temperatures. This paper reports the charring rates observed on Eucalyptus structural beams and presents a numerical and experimental study of the behavior of these beams when exposed to fire, in which the properties of strength and stiffness were evaluated as a function of rising temperatures, allowing an analysis of the effect of the section factor on the internal rise in temperature of structural Eucalyptus beams.
At room temperature, the mam actions causing structural strains are those of gravity and wind. At high temperatures, new strains are generated which further reduce the timber's load capacity and trigger additional strains.
In timber, the reduction of load capacity is due mainly to the gradual decrease in the cross section, which is converted into char, and to the reduction of its mechanical properties of strength and stiffness. In the study of timber as a structural material, the formation of char is thus a crucial parameter in view of the loss of strength of the structural member resulting from the reduction of the cross section.
Studies by Truax (1959) allowed, for estimation of the reference temperature at the base of the charred layer, offering an invaluable aid to determining the rate of conversion of wood into char, i.e., the charring rate. Truax suggested that the charring temperature ranges from 615 to 550 °F (324 to 288 °C). For the purpose of simplification, this temperature is assumed to be 290 °C based on proximity to previous researches of White (1988) and EN 1995-1-2 (CEN 2004).
The charring rate is therefore an important design factor for evaluating the stability of structural wood elements and their load capacity when subjected to fire. Although the wood inside the beam is not exposed to sufficiently high temperatures to cause charring, it is nevertheless affected by the temperature, which reduces its mechanical properties by degrading its chemical compounds.
Several studies have been carried out...