Steel Buildings in Europe

Part 6: Fire Engineering 6 - 33 tension members and for restrained beams, but not for unrestrained beams and columns when buckling is a potential failure mode. The critical temperature of a non-composite steel member can be calculated using the simple model given in §4.2.4 of EN 1993-1-2. The critical temperature decreases with the degree of utilisation of the member, μ 0 , which is defined as the ratio of the design effect of actions at elevated temperature, E fi,d , to the member resistance at normal temperature but using partial factors for fire design R fi,d,0 such that: μ 0 = E fi,d / R fi,d,0 For Class 4 sections, a critical temperature of 350°C is recommended. The critical temperature of composite members is given by §4.3.4.2.3 of EN 1994-1-2. 4.4.3 Load bearing capacity approach The basis for this approach is to calculate the resistance of the member after the required period of fire resistance and to compare it to the design effect of the actions at elevated temperature, E d,fi . The steps that must be followed to apply the load bearing capacity approach are shown in Figure 4.7. Section classification As for normal temperature design, cross-sections are classified according to Table 5.2 from EN 1993-1-1. The  coefficient is modified by applying a 0,85 factor, in order to account for the reduction in yield strength and elastic modulus of the steel at elevated temperatures, as specified in §4.2.2 of EN 1993-1-2. This modification lowers the c / t limits for the various section classes, so some sections may have a more onerous classification than for normal design. Tension, shear and bending resistance of steel members in fire The simple models to calculate the design tension, shear and bending resistances of steel members under standard fire are given in §4.2.3.1, §4.2.3.3 and §4.2.3.4 of EN 1993-1-2. These models are based on the assumption that the members have a uniform temperature throughout, and make use of a reduced yield strength and the relevant partial factors for fire design. However, in reality the temperature across and along a member is hardly ever uniform, which affects its mechanical behaviour. For example, if a steel beam supports a concrete slab on its upper flange, the temperature in the top flange is lower than in the bottom flange and therefore its ultimate moment resistance is higher than for a uniform temperature equal to that of the bottom flange.