An improved Analytical Formula for Predicting the Temperature of Heavily Protected Steel Sections

Fire verification might be particularly demanding for steel structures and insulation is a common option to slow down the temperature increase in the steel elements without modifying the original structural design. Simple analytical formulae, as provided for instance in EN1993-1-2 design standard, allow a quick estimate of the temperature of insulated steelwork, without determining the thermal field inside a steel cross-section by performing in-depth experimental or numerical analyses. However, the EN1993-1-2 formulation considers heat transfer with temperature boundary conditions, rather than more realistic conditions on the heat flux, and is inaccurate for heavily insulated steel sections, in which protective solutions with high heat capacity are adopted. In this paper a new analytical formula aimed at estimating the temperature of protected steel members is proposed. Its accuracy is assessed by comparing the predictions of the proposed and the EN1993-1-2 formulations with the results of a parametric analysis consisting of 1-D models. Several steel thicknesses, insulation materials and thicknesses and an exposure to the ISO 834 heating curve are considered in the analyses. It is shown that the EN1993-1-2 can be both conservative and unconservative depending on the ratio between the insulation and the steel heat capacities mu and is not suited for heavily insulated steel sections with high values of mu. On the contrary, the proposed formulation results in being always safe and particularly suited for heavily insulated steel sections.