There are two distinct approaches to fire design for steel structures in AXISVM.
If we want to ensure the fire resistance of the structure by providing data to the manufacturer supplying the fire protection coating, so that the appropriate coating ensuring fire resistance can be selected, then the procedure based on critical temperature is needed.
If we want to verify the structure’s compliance with the specified 30, 60, 90, etc., minute required time of resistance by calculation, we can do this through resistances and utilization rates calculated based on the steel temperature corresponding to the duration, considering the changes in material properties.
This article discusses the determination and documentation of critical temperature and the section factor in AXISVM.
Choosing protection based on critical temperature #
Manufacturers provide tables for different section types, which help determine the layer thickness of the fire protection coating that ensures the fire protection requirement based on the design temperature (which is at most the critical temperature) and the section factor.
AXISVM can calculate both the critical temperature and the section factor and document them in tabular form. With this data, specific price quotes can be requested for fire protection coatings based on the load-bearing requirement level.
Determining critical temperature in AXISVM #
The critical temperature of steel elements represents the point at which the load-bearing capacity limit state is reached under fire conditions. Calculating critical temperature is not simple, as the modulus of elasticity of steel begins to degrade at high temperatures even before the yield strength. As a result, the structure becomes more sensitive to loss of stability, and critical temperature cannot be determined in a closed form. Although section 4.2.4 of EN1993-1-2 provides a closed formula for critical temperature (4.22), it is only applicable to elements where stability loss can be excluded (e.g., tensioned bars).
Critical temperature can also be determined indirectly by finding the temperature at which the structural element fails, considering the reduced modulus of elasticity and yield strength as a function of temperature. AXISVM uses this as an iterative method to determine critical temperature.
AXISVM specific note for determining critical temperature #
The only criterion for determining critical temperature from a program-use perspective is to enable and perform the fire design of the steel member. Although fire design parameters (fire curve, fire resistance requirement) need to be set, they are irrelevant to determining the critical temperature.
If the fire design conditions are available (fire load case, fire parameters, fire effect placement on the member…), then to run the fire design and determine the critical temperature, the “Fire Resistance Design” must be enabled under the “Steel Design” tab in the “Design Parameters” dialog.
After this, the program performs the steel design for all standard cross-sectional and stability checks for the specific design member, while simultaneously for each check individually, iteratively determines the critical temperature.
Warning messages in the Design #
- “Warning: Critical temperature >1000°C”: The utilization is still less than 100% even when applying the reduction factor for the last 1000°C temperature step in the iteration (for example, for a simply supported beam loaded with zero axial force). calculations
- “Critical temperature Theta_Cr=20°C not passed” : The utilization of the design member exceeds 100% in the first iteration, performed on 20°C steel temperature.
- “Class 4 elements with no tension can be accepted if the following criterion is satisfied at all cross-sections Theta_a<=350°C“:
Documenting critical temperature in tabular form #
Design members for which fire design has been performed, and thus their critical temperature has been determined, can be listed in tabular form and exported to Excel or documentation. The table can be found under the “Utilization in Fire” section of the table manager.
Determining the section factor (A/V) in AXISVM #
In addition to the critical temperature, the section factor is another important parameter for determining the layer thickness of the fire protection coating. The section factor shows the massivity and installation method of the section and is calculated as the ratio of the fire-exposed perimeter to the cross-sectional area.
AXISVM automatically calculates the section factor for the most commonly used sections (supported sections: I-sections, rectangular hollow sections, circular hollow sections, angle sections) based on the exposure of the structural element to fire and the cross-section of the section, but the value can also be entered manually on the “Parameters for fire load” dialogue.
The section factors of elements in the model are available in tabular form under the “Parameters of fire load” section of the table manager and can be exported to Excel or documentation.
