Steel Buildings in Europe

Part 7: Fire Engineering 7 - 50  Reynolds-averaged Navier Stokes (RANS): The basic equations are averaged and all turbulent scales are modelled. The most frequently model used is   k model. The input data are the same as those required for a zone model but they have to be supplied with a higher degree of detail. They are the detailed room geometry, room construction (including all walls, floors and ceilings), number of vents (or holes) and their sizes, room furnishing characteristics, fuel/combustion characteristics, turbulence parameters, and radiation parameters. The output data are the smoke and heat movements, prediction of sprinkler and fire detector activation time, time to flashover, temperatures in the domain, velocities, smoke layer height, and species yield. Due to their complexity and the CPU time needed, field models are very little used for evaluating fire resistance of structures, particularly for fully developed fire. In the fire domain, the use of a field model is often reduced to specific cases with sophisticated geometry. 6.2 Thermal Models Advanced heat transfer models can be used to calculate temperature distribution in a structure in a fire. They are mostly based on either finite difference methods or finite element methods. They are often used to estimate temperature gradients through structural members primarily made of materials with a low thermal conductivity and/or high moisture content, such as concrete. Moreover, they can be applied to structural members under nominal fire conditions or natural fire conditions. Such methods have to take into account non-linearity due to temperature dependence of material properties and boundary conditions. As commonly assumed in fire design, heat transfer from fire to exposed surfaces is essentially by convection and radiation. Inside homogeneous materials such as steel, heat is only transferred by conduction. On the other hand, for porous materials such as concrete or where internal cavities exist, heat transfers are more complex. The three processes: conduction, convection and radiation can occur together, to which may be added mass exchange. However, by way of simplification, only the dominating process is explicitly introduced in thermal analysis, taking into account secondary processes through adequate adjustment. In fire design, it is usually assumed that concrete is a homogeneous material and that heat transfer occur mainly by conduction. Heat transfer by convection and radiation occurring in pores are considered as secondary processes and are implicitly taken into account in thermal properties available for concrete (conductivity, specific heat). Moreover, mass-exchange is generally neglected and only moisture evaporation in concrete is taken into account. The effects of moisture (assumed uniformly distributed in the concrete) is treated in a simplified way, assuming that when the temperature in a concrete part reaches 120°C, all of the heat transferred to that part is used to evaporate water. Moisture movements are rarely modelled. For composite members, contact between steel parts and concrete parts can be assumed to be perfect (no gap). Radiation in internal