Steel Buildings in Europe

Part 6: Fire Engineering 6 - 26 4 SIMPLE CALCULATION MODELS In previous Sections, the prescriptive approach to fire design through use of datasheets has been shown to provide a safe solution. However, it does not necessarily offer the most efficient design. By using the simple calculation models described in the Eurocodes, the designer may be able to demonstrate that less or no protection is required in some or all of the structural elements, thus leading to a more economic solution to fire design. There are two approaches to these simple calculations: the critical temperature approach and the load bearing capacity approach. Based on these methods the designer may reasonably decide whether or not fire-protection is required. These methods, however, deal with individual members under standard fire exposure, instead of an entire structure in natural fire. Hence, unlike the performance-based analysis (see Section 6), they do not consider the actual behaviour of the structural member in a real fire. In order facilitate the understanding of the reasoning behind the simple methods given in the Eurocode and described in Sections 4.4.2 and 4.4.3 of this guide, an introduction to the thermal effects of fire is given in Section 4.1. 4.1 Fire behaviour and thermal actions 4.1.1 Fire action and standard fire Fire is a very complex phenomenon that involves different kinds of chemical reactions. Fire releases heat energy in the form of flames and smoke within the building compartment, as shown in Figure 4.1 (a). When a fire occurs, the temperature of the gas within the building compartment rises rapidly. For the purpose of fire design, the fire action is represented by a standard temperature-time curve, as defined in EN 1991-1-2. This curve is denoted ‘standard fire’ in Figure 4.1(b). (a) Fire (Cardington test) ( b) Standard fire curve and temperature rise of members Figure 4.1 Fire action