Fire Safety Science Digital Archive

A two-dimensional finite-element model has been developed to simulate the heat and mass transfer in both gypsum board and wood in order to predict the thermal response of a wood-frame floor assembly exposed to fire. Both volatile pyrolysis products in wood and water vapour in wood and gypsum board are considered in the mass transfer analysis. Calcination of gypsum board and pyrolysis of wood are modelled using Arrhenius expressions. The evaporation of water is modelled assuming the partial pressure of water is equal to the equilibrium vapour pressure. The gas in the cavity is assumed to be fully transparent, allowing radiant heat transfer between all surfaces in the cavity, thus leaving convective heat transfer to heat the gas inside the cavity. Comparisons are made to two full-scale fire resistance tests. One test was carried out using the standard temperature exposure while the second test used a non-standard exposure that was based on measurements taken in experimental fires in wood frame houses. Comparisons between experiment and model predictions show good agreement on the unexposed side of each of the two layers of gypsum board protecting the assembly. The cavity temperature is under-predicted resulting in an under-prediction of the temperatures in the floor joist and sub-floor. The model currently does not account for the fall-off of gypsum board which limits the models ability to predict the results for the non-standard exposure since the gypsum board failed very early in the test.