Steel Buildings in Europe

Part 7: Fire Engineering 7 - 47 6 GUIDANCE ON THE USE OF MORE ADVANCED SOLUTIONS This chapter gives an overview of advanced calculation models available for fire modelling, thermal modelling, and structural modelling that can be used in fire engineering design [9,16] . 6.1 Fire models Two kind of numerical models are available to model the development of real fires: zone models and field models. These models and allow temperatures, smoke descent, flame spread, time to flashover and many other effects to be calculated. 6.1.1 Zone models The simplest model is a one-zone model for fully developed fires (post- flashover fires), in which the conditions within the compartment are assumed to be uniform and represented by a single temperature. Two-zone models may be used for pre-flashover situations, mainly in the growth phase of a fire. The model is based on the hypothesis of smoke stratification, separating the fire compartment into two distinct layers: a hot upper layer (containing most of the fire’s heat and smoke), and a cool lower layer (which remains relatively uncontaminated by smoke). A fire plume feeds the hot zone just above the fire. The temperature of each layer is calculated from conservation of energy; the amount of toxic combustion products in each layer is calculated from conservation of chemical species; and the size of each zone is calculated from conservation of mass. Simple rules govern plume entrainment, heat exchange between zones and mass flow through openings to adjoining compartments. As a result of the simulation the evolution of gas temperature in each of the two layers, the evolution of wall temperatures, evolution of flux through the openings and the evolution of the thickness of each layer are given as a function of time. The thickness of the lower layer, which remains at rather cold temperature and contains no combustion products, is very important to assess the tenability of the compartment for the occupants. Often, the local effect near the fire may be studied using a simple model such as Hasemi methodology with the two-zone models. The combination of both models then allows the determination of the gas temperature field near and far from the fire (see Figure 6.1). When the thickness of the lower layer is too small compared to the height of the compartment, the two-zone assumption becomes inapplicable and a one zone model becomes more appropriate. Moreover if the fire area is big compared to the floor area, the one-zone model assumption is usually better than the two-zone one. Some zone models include the possibility of a switch from a two-zone model to a one-zone model when some conditions for temperatures, fire area and smoke layer thickness corresponding to flashover) are encountered.