Steel Buildings in Europe

Part 6: Fire Engineering 6 - 27 The temperature rise of a member in fire is predominantly governed by radiation and convection mechanisms, via a complex diffusion process. It depends on the thermal properties of the materials and the thickness of the protection layer, if it is fire-protected. Due to the rapid increase of the gas temperature, the heat energy from the fire (i.e . thermal action) flows into the member through the exposed surfaces, and heats up the member. As a result, the temperature of the member rises, typically following the curves shown in Figure 4.1 (b) for different scenarios of protected and unprotected members. 4.1.2 Modelling fire behaviour As a thermal action on building structures, a fire can be classified into localised and fully-developed fires. Localised fire A localised fire is in the pre-flashover phase and occurs only in some part of a compartment. A localised fire is unlikely to spread to the whole compartment and to cause a flashover, due to its slow propagation and the low temperature developed. A localised fire is generally modelled using plume, zones and computational fluid dynamics (CFD) models. Plume model Annex C of EN 1991-1-2 gives a so-called plume model to determine the thermal action of a localised fire. If the flame remains below the ceiling, as shown in Figure 4.2 (a), the model is used to calculate the corresponding temperature along its vertical axis. However, if the flame impacts the ceiling, as shown in Figure 4.2 (b) then the model determines the heat flux at the level of the ceiling together with the flame length. a) Flame remains below the ceiling (b) Flame impacts the ceiling Figure 4.2 Plume model for a localised fire in EN 1991-1-2 4.1.3 Fully developed fire A fire is fully developed when all the available fuels within a compartment are burning simultaneously and the maximum heat is released. A fully developed fire is commonly modelled using either standard or parametric fires, as shown