Fire design of timber slabs made of hollow core elements

The fire design of timber structures usually take into account both the loss in cross-section due to charring of wood and the temperature-dependent reduction of strength and stiffness of the uncharred residual cross-section. The fire behaviour of timber assemblies made of hollow core elements is characterised by different charring phases. After the fire exposed timber layer is completely charred and the char-layer has fallen off, the thin vertical timber members are exposed to fire on 3 sides, leading to very irregular residual cross-sections with charring depths much greater than for heavy timber structures. Based on an extensive experimental and parametric study, a simplified calculation model for the fire resistance of timber slabs made of hollow core elements has been developed. The calculation model bases on the reduced cross-section method and takes into account two different charring phases. The paper first describes and discusses the simplified calculation model, and then compares the test results to the calculation model.