Performance-based fire modelling of intumescent painted steel structures and comparison to EC3

• Intumescent coated steel structures are studied experimentally and numerically.
• Thermal conductivity of the coating is derived for different section factors.
• Performance-based fire response of an intumescent painted steel assembly is obtained.
• Results are compared to results of a frequently used conventional design procedure.
• The conventional design procedure is found to be on the unsafe side for some cases.

Following the present directives for building products, the manufacturers of intumescent paints for fire protection of structural steel typically offer tables for presentation of the product's insulating efficiency showing its dry film thicknesses (DFTs) required to sufficiently delay the defined steel critical temperature. From these tables only the final time of reaching this critical limit is obtained clearly, but the overall time evolution of steel temperature remains unknown. In connection to EC3, these data are only sufficient for the use of a simplified design procedure, mostly the method of critical temperature (MCT). Although of a questionable adequacy for cases of restrained thermal dilatations this method is, thus, often applied regardless of the specifics of the structural assembly. The severity of the error implemented by such a simplification is taken as the principal focus of the paper. In providing an answer, fire test results are presented for 13 different steel members protected by different DFTs of a selected intumescent coating. Then, the time-dependent thermal conductivity of the coating is derived and numerical procedure is composed for obtaining the real fire response of intumescent painted assemblies. The method is implemented into a parametric study. The calculated critical times are compared against the critical times suggested by the MCT revealing the MCT being severely on the unsafe side for some cases. The paper thereby confirms the initial hypothesis that the present way of presenting the fire-protective intumescent paints for structural steel is insufficient and will need to be upgraded in the future.