Heating rate effect on the thermophysical properties of steel in fire

Steel structures subject to fire require fire resistance design to protect the lives of occupants and reduce the structural damage and failures. The fire resistance design incorporates the estimation of the thermomechanical behaviours of the structures based on the development of temperatures of the structural elements subject to fire in which the structures are heated at varying rates. Thermal properties of steel, including the emissivity and specific heat capacity, are particularly required for predicting the temperatures of steel structures under fire condition. Since the heating rate under fire condition varies, it is uncertain how the heating rate may affect the thermal properties of steel as, to date, there is no such investigation carried out. In this study, the thermal properties of steel under the heating rate effect were investigated. The emissivity of steel was measured in experiments and found to be strongly dependent on the heating rate. A new emissivity model based on the kinetic theory was developed and provides an accurate estimation of the property at different temperatures under different heating rates. In addition, the specific heat capacity of steel during phase transformation was also found to be dependent on the heating rate and its formulation was proposed in kinetic modelling. This study provides new insights into the material properties of steel and new models for estimating the thermal properties used for achieving robust fire resistance designs.