Predicting intumescent coating protected steel temperature in fire using constant thermal conductivity

• The concept of effective constant thermal conductivity of intumescent coating was introduced to calculate the protected steel temperatures in fire.
• An extensive fire testing programme has been conducted to quantify the effective constant thermal conductivity.
• The effective constant thermal conductivity changes with coating thickness and steel section factor.
• The effective constant thermal conductivity concept may also be applied to thin-walled steel sections.

This paper presents the results of a study to investigate the feasibility of using a constant thermal conductivity for intumescent coating when calculating protected steel temperature in fire, based on analysing a series of fire tests on intumescent coating protected steel sections with a range of section factors and intumescent coating thicknesses. Having a constant thermal conductivity enables simplified analytical equations to be developed for design purpose. The constant thermal conductivity is calculated as the temperature averaged value within the temperature range of interest for fire resistance design. The constant thermal conductivity is then used to calculate the protected steel temperatures and the calculated steel temperatures are compared with the measured values from the fire tests. The results of this comparison suggest that it is feasible to use a constant thermal conductivity for intumescent coating. The constant thermal conductivity value would be dependent on the intumescent coating thickness and the steel section factor. However, this issue may be dealt with by obtaining the intumescent coating thermal conductivity from fire tests on steel plates, because a comparison between results based on steel plate fire tests and those based on steel section fire tests shows close agreement. The results also indicate that the constant thermal conductivity of intumescent coating converges to a fixed value, independent of the steel section factor, when the steel section factor is high, suggesting that the constant thermal conductivity method is particularly applicable to thin-walled sections with high section factors.