Steel Buildings in Europe

Part 7: Fire Engineering 7 - 7 Available design methods to evaluate the fire performance of structure are briefly described below. These methods range from simple hand calculations to the use of sophisticated computer models. The overall complexity of the fire safety design will depend on the assumptions and methods adopted to predict each of the three design steps. 3.2 Fire analysis The main objective of the fire modelling is the simulation of the fire development and the prediction of thermal actions (gas temperature, heat flux) on the structural members (in order to determinate, in a following step, the temperature in the structural members). Although common practice is to represent a fire by a standard fire curve, structural fire design may be based on a design fire that provides more realistic conditions in fire compartment. In this way, parameters such as the magnitude of the fire load, the rate of heat release and the ventilation factor, which play an important role in fire severity, are taken into account. Moreover, the identification of relevant and realistic design fire scenarios is a crucial aspect of the fire safety design. The design fire scenarios used for the analysis of a building fire have to be deduced from all the possible fire scenarios. In most buildings, the number of possible fire scenarios is infinite and need to be reduced. Only ‘credible worst case’ fire scenarios will need to be studied. When the design fire scenarios are chosen, a number of fire models are available to assess the fire severity and calculate the corresponding thermal actions Different levels of fire models are relevant to the various stages of fire development. When a fire is initiated, it is localised within a compartment and, according to the characteristics of the compartment and of the fire load, it can remain localised or becomes generalised to the whole compartment. In the case of small compartments or compartments with small ventilation openings relative to the size of the compartment, the fire develops into to a fully engulfed fire. Three levels of modelling are available to describe both localised and fully generalised fires, as shown in Table 3.1. Table 3.1 Levels of fire models Levels of the model Localised fire Generalised fire Simplified model Hasemi model Heskestad model Parametrical fires Zone models 2 zone model 1-zone model Field model CFD CFD The simplified models are generally empirical models based on conventional assumptions. The zone models take into account the main parameters controlling the fire, but introduce simplified assumptions that limit the domain of application. They would be used in simple easily defined compartment geometries. The field models are more accurate but are rather complex for use