Effective cross-section of a timber beam exposed to natural fires

Abstract According to the Eurocode standards, advanced numerical models or the reduced cross-section method can be used to determine the mechanical resistance of timber structures under fire conditions. The method assumes a rectangular effective cross-section with full strength and stiffness and the cross-section of effective thickness without strength and stiffness after fire exposure. The effective thickness is the sum of the charring thickness and the zero-strength layer thickness, which are specified in the standards only for standard fire exposure. In the present paper, the analyses are combined with the reduced cross-section method and numerical fire, hygro-thermal and mechanical models to determine the charring thickness, zero-strength layer thickness and effective thickness of a simply supported timber beam under bending exposed to different natural fires. The results show that the three thicknesses increase with fire duration until the end values are reached at which the timber beam can withstand a prescribed natural fire. The end values for the charring thickness are between 18.2 mm and 72.1 mm, for the zero-strength layer thickness between 14.9 mm and 27.4 mm and for the effective thickness between 39.9 mm and 89.9 mm. The results also show that the end values of the charring thickness and the effective thickness depend on the maximum gas temperature. The end values of the zero-strength layer thickness depend on the linear cooling rate. Simple linear equations are proposed to account for these dependencies. Piecewise linear equations are also proposed to describe the temporal development of the three thicknesses.