Steel Buildings in Europe

Part 7: Fire Engineering 7 - 16 5.2 Thermal Models Considering the high thermal conductivity of steel and the small thickness of steel profiles commonly used in the construction, it is sufficiently accurate to ignore thermal gradients within the cross-section of structural members and assume a uniform temperature when uniformly heated. Consequently, simple design equations can be used to predict the temperatures of steel members that are fully exposed to fire or steel members that support a concrete slab and are exposed on three sides. Similar rules exist for fire- protected steel sections, although the thermal properties of the proposed protection material are needed, which can be difficult to obtain. For the composite steel-concrete members, strictly speaking, there are no simplified models to estimate the evolution, as a function of time, of temperature distribution through members. To simplify the design, information on temperature distribution at current time of standard fire exposure (i.e. 30, 60, 90 and 120 minutes) is given in EN 1994-1-2. 5.2.1 Unprotected steel member Heating of the unprotected steel members can be determined by means of the simple analytical approach given in EN 1993-1-2. In this method, the temperature rise depends on the thermal actions (expressed in terms of net heat fluxes), the thermal properties of the steel and the section factor of the element A m / V defined as the ratio between the surface area exposed to the heat flux A m [m²/m] and the volume of the element by unit length V [m 3 /m]. The section factors for some unprotected steel members are shown in Figure 5.3. b h t t t A m /V=Perimeter exposed to fire /Cross-section area A m /V=1 / t A m /V=2 / t Figure 5.3 Example of section factor for unprotected steel members Assuming an equivalent uniform temperature distribution in a cross-section, the increase of temperature   a,t in an unprotected steel member during a time interval  t may be determined from: t h c k A /V     net,d a a m sh a,t   with  t  5 s (4) where: sh k is the correction factor for the shadow effect caused by local shielding of radiant heat transfer due to shape of steel profile a C is the specific heat of steel [J/kgK]