In a fire resistance configuration you can configure the detailed settings for the fire resistance analysis of an object. All specifications are made here for determining the steel temperature relevant for the design. The basic structure of the design configuration dialog box, including the assignment options to the individual objects, is described in the superordinate chapter Steel Design.
The fire resistance configurations are only available if you have selected EN 1993 as the standard for Steel Design. The fire resistance designs are currently not implemented for other standards.
All settings in this dialog box are required to determine the final steel temperature for the fire resistance design. With the determined steel temperature, the material properties are reduced in compliance with EN 1993-1-2 [1].
Analytical Determination of Steel Temperature
The steel temperature at a particular point in time is determined on the basis of the gas temperature according to various fire curves. In the fire resistance configuration, specify the required time of the fire resistance as well as the time interval to calculate the temperature.
A cross-section can be assumed to be exposed to fire on all sides or on three sides in the calculation. This setting affects the calculation of the component temperature and is also taken into account when determining the design factors according to [1]. When selecting a three-sided fire exposure, you can specify the length of the protected side or use the automatic determination. For this, a simplified assumption is used, that the side conforming the total width of a cross-section is not exposed to fire (a typical application case is a beam with an overlying concrete ceiling).
Various temperature curves are available for determining the gas temperature:
- Standard temperature-time curve
- external fire curve
- hydrocarbon curve
The detailed results of the steel design show the temperature curve in a temperature-time diagram.
Select the "Set fire protection parameters" check box to consider a protective coating of a cross-section with the fire protection materials. The temperature is then determined by considering the defined material parameters according to [1] 4.2.5.2. You can select between a hollow and a contour encasement. According to [1], only plate materials or plasters are treated as fire protection materials. A design check with such a scheme is not allowed for the protective coatings with insulation efficiency or intumescent, because those encasements change their properties depending on the temperature.
The coefficients used to calculate the temperature are calculated according to the recommended values of EN 1993-1-2 [1] and EN 1991-1-2 [2] is preset. You can adjust them as user-defined, if necessary.
The favorable effect of hot-dip galvanizing of the structural components can be taken into account when determining the steel temperature by adjusting the surface emissivity. To do this, select the "Galvanized surface of carbon steel member" check box. When determining the steel temperature, the lower surface emissivity of the galvanized surface εm,lim applies up to the set limit temperature tlim ; at higher temperatures, the surface emissivity of the carbon steel εm applies. This procedure corresponds to the provisions of the DASt Guideline 027 "Determination of Component Temperature of Hot-Dip Galvanized Steel Components in Case of Fire", the values of which are also preset.
Manual Entry of Steel Temperature
In addition to the analytical determination of the steel temperature in the program, you can also specify the critical steel temperature manually. To do this, select the "Manually" option in the list box for defining the final temperature. The fire resistance design applies the specified temperature to all objects with this fire resistance configuration assigned.
In order to determine the coefficient k1 according to [1] 4.3.3 for uneven temperature distribution in the bending check, it is necessary to specify the fire exposure (all-round or three-sided, if necessary with fire protection measures).. The coefficient k2 for an uneven temperature distribution along the length of the beam is assumed to be 1.0 for all cases.
Notes on Fire Resistance Design
The basic settings of the ultimate limit state design (for example, elastic or plastic design) or the stability design (for example, load application point) are also adopted for the fire resistance design checks from the ultimate configurations of the object. The limit values defined there for special cases also apply to the fire resistance design.
The fire resistance design checks are performed for all design situations with the type 'Accidental - Fire' assigned in the input table for the steel design (see also chapter Design Situations).
Stability analysis is performed using the equivalent member method according to [1] 4.2.3. The entries from the effective lengths assigned are also considered as effective lengths for the fire resistance design. The modulus of elasticity is not explicitly reduced, but the reduction is included in the stability analysis by means of the reduction factors according to [1].
According to the standard, the general method used for stability design checks according to EN 1993-1-1, clause 6.3.4, cannot be used for the fire resistance design checks.
For this, use a material with temperature-dependent properties ; this is explained in Chapter Materials of the RFEM manual. With the definition of temperature loads, you can specify the necessary steel temperature for the calculation (see also Chapter Member loads in RFEM Manual).
Local buckling of slender cross-section parts can also represent a governing failure mode in case of fire. Design checks for the cross-sections of Class 4 are performed according to EN 1993‑1‑2, Appendix E, in the Steel Design add-on. The shear buckling design of slender web plates is currently not implemented for the fire resistance design in the add-on.
