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TEMPERATURE DISTRIBUTION IN INTUMESCENT COATING PROTECTED STEEL SECTIONS
[Thesis]. Manchester, UK: The University of Manchester; 2017.
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Abstract
Intumescent coating is the most common steelwork fire protection in the UK. Despite popularity of using intumescent coatings, there is a lack of understanding of their effects on protected steel sections in fire. Hence, intumescent coating performance for different applications is assessed by means of fire testing that is a labour consuming and extremely expensive process. A particular issue is related to coatback. When using tensile membrane action in composite floor slabs, it is possible to justify elimination of fire protection to the internal secondary beams. However, an issue arises when an unprotected steel beam is connected to a protected steel beam because there will be additional conductive heat transfer from the secondary beam to the connected protected primary beam, which may endanger safety of the primary beam if the primary beam has been designed on the basis of total fire protection. To rectify this problem, a short length of the secondary beam requires fire protection to minimise the local heat conduction from the secondary beam to the primary beam. This short length of fire protection is termed coatback. This study has found that the coatback distance changes with a number of design parameters, including: thermal conductivity and thickness of intumescent coating, secondary beam section factor and primary beam limiting temperature. Therefore, it is not safe to have a fixed value as recommended in code of practice by the UKs Association for Specialist Fire Protection. In addition, this thesis recommends a less time consuming and inexpensive test solution for the determination of coatback distance. There are also uncertainties on the assumed temperature distribution in a fire protected steel cross-section. In EN 1994-1-2, the temperature distribution is assumed to be uniform if the steel section depth does not exceed 500mm. However, if the depth of the steel section is more than 500mm, the temperature distribution is considered to be non-uniform. The web of an I-section is much thinner than the flanges. Therefore, the EN 1994-1-2 assumption for deep beams leads to much higher temperatures in the web than in the lower flanges. This has important consequences on limiting temperatures of steel sections and hence specification of their fire protection thicknesses. The accuracy of the Eurocode assumption was assessed using the relevant experimental data, from the database of Exova Warringtonfire standard fire tests. The commercial fire test results for 64 intumescent coating protected steel sections suggest that the steel section temperature can be assumed to be uniform independent of the steel section depth and EN 1994-1-2 recommendation should be changed.