MODELING OF HEAT AND MASS-TRANSFER IN WOOD STRUCTURES DURING FIRE

One fundamental prerequisite for analytical treatment of the design process for wood structures exposed to fire is the development of an analytical model for the essential physical process in heated wood material. The model must be capable of treating transient temperature and moisture states in both uncharred and charred portions of the cross-section, as well as the growth of the charred layer in combination with its oxidation at the surface due to variable thermal action. Material data for such a model must also be produced. The calculations in this work are based on the model developed by the author. Simulations have been made with the one-dimensional computer program WOOD 1. The input data which have been varied in the calculations are the thickness, density, moisture content and thermal exposure of the material. These few and simple input data are of a type that are known in conjunction with a fire engineering design. The results of all calculations are presented in diagrams in which the three different calculated quantities have been set out as a function of the distance from the surface exposed to fire at different times. The presentation comprises temperature profile, density distribution and moisture profile. Assessment of the fire resistance of the board material is based on the same performance requirements as those set out in the internationally accepted test method, ISO 834, or its Swedish counterpart SIS 02 48 20. This implies for the calculations that the fire resistance is taken to be the time when the insulation requirement is no longer complied with, i.e. when the rise in temperature on the unexposed face exceeds 140-degrees-C. A comparison of the calculated fire resistances and those measured in the tests shows very good agreement. The maximum difference between the tests and the calculations is only about 1 min for boards. For the wall constructions the maximum difference is larger, but normally not more than 10 min.