ABSTRACT
When using tensile membrane action in composite floor slabs, it is possible to justify elimination of fire protection to the internal secondary beams. If the entire secondary beam is unprotected, an issue arises 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’. At the moment industry has no clear guidance how the ‘coatback’ distance may be affected by various factors. This paper presents the results of a numerical investigation to calculate the ‘coatback’ distance under different conditions, including coating thickness, steel section factor, steel section depth, intumescent coating thermal conductivity and limiting temperature of the primary beam. In this study, only the secondary beam with ‘coatback’ is included in the numerical heat transfer modelling. To determine the ‘coatback’ distance, the temperature of the totally protected secondary beam is compared with the temperature of the secondary beam with ‘coatback’ at the protected end of the secondary beam. The secondary beam temperature at the protected end is assumed to be the temperature of the connected primary beam. The determined ‘coatback’ distances ensure that the additional temperature increase in the protected primary beam due to local conductive heat transfer from the secondary beam is no more than 2% of the temperature of the protected primary beam if without the additional conductive heat transfer. In the analysis, the thermal properties of one intumescent coating formulation were obtained from an available fire resistance test. The results show that the ‘coatback’ distance varies between (400–1100) mm. Among the factors investigated, the results suggest the ‘coatback’ distance is longer with thicker coating and smaller secondary beam section factor because of the larger differences in temperatures between the unprotected secondary beam and the target primary beam limiting temperature. The results also show drastic differences in ‘coatback’ distances for different coating thermal conductivity-temperature relationships, being shorter for products with high thermal conductivity and vice versa. However, the results show that changing the depth of the secondary beam has little influence on the ‘coatback’ distance. Nevertheless, changing the limiting temperature of the primary beam within a range of temperature of practical interest (450°C–700°C) results in a relative small change of approximately 100 mm in ‘coatback’ distance. Therefore, the ‘coatback’ distance for the highest limiting temperature can be used.
