Steel Buildings in Europe

Part 7: Fire Engineering 7 - 17 a  is the unit mass of steel [kg/m 3 ] h  net,d is the net heat flux per unit area [W/m²] Solving the incremental equation step by step gives the temperature development of the steel element during the fire. In order to assure the numerical convergence of the solution, some upper limit must be taken for the time increment  t. In EN 1993-1-2, it is suggested that the value of  t should not be taken as more than 5 seconds. The thermal actions are determined by the net heat flux net,r h  absorbed by the steel member during the fire exposure. It is expressed in terms of the hot gas temperature as the sum of two distinct fluxes: a convective component net,c h  and a radiant component r h net,  . Convective heat flux is expressed as: ) ( c g m net,c      h  (5) where: c  is the coefficient of heat transfer by convection [W/m²K] g  is the gas temperature [°C] m  is the surface temperature of the member [°C] Radiant heat flux is given by: ) 273) 273) ( (( 4 m 4 r 0 m net,r         h  (6) where:  is the configuration factor, including position and shape effect (<1) m  is the surface emissivity of the member r  is the radiation temperature of the fire environment [°C] (  r ≈  g) m  is the surface temperature of the member [°C] 0  is the Stephan Boltzmann constant [= 5,67 10 -8 W/m 2 K 4 ] According to EN 1991-1-2, for many practical cases the configuration factor may be taken equal to unity. The coefficient of convection ( c  ) varies from 25 W/m²K (standard fire conditions) to 50 W/m²K (hydrocarbon fire conditions). The emissivity of carbon steel and composite steel and concrete members may be taken as 0,7 m   .