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3.1.1 Ultimate Limit State
Ultimate Limit State
If stresses from compression and bending occur together in a cross-section, you can determine the stress-strain ratio ψ taking into account the compression zone factor α in two ways (factor ψ is required for the determination of the c/t-ratio according to  Table 5.2):
- Fixed NEd, increase MEd to reach fyd
Only the stress component from bending is increased to reach the yield strength.
- Increase NEd and MEd uniformly
The stress components from axial force and bending are increased uniformly until the yield strength fyd is reached.
You can access the check box For limit c/t of Class 3, increase material factor ε acc. to 5.5.2(9) if the stability analysis in the Stability tab is deactivated. This is based on the specifications for classification in  clause 5.5.2 (10). With the stability analysis being deactivated, it is possible to handle cross-sections classified as class 4 like cross-sections of class 3 by increasing the factor ε.
If you select the option Use SHAPE-THIN for classification of all supported cross-section types, the effective cross-section values of class 4 sections will be calculated by the method used in the cross-section program SHAPE-THIN. For cross-sections classified as 'General' (belonging neither to a rolled nor a parametrized cross-section table), the classification will generally be performed with SHAPE-THIN. You can design these cross-sections only elastically as class 3 or class 4 sections.
Optionally, you can Determine effective widths according to EN 1993-1-5, Annex E. Annex E of  describes alternative methods for determining the effective cross-section areas for stresses below the yield strength (see also the article in the Knowledge Base on our website).
The width/thickness ratios relevant for the classification can cause problems for cross-sections with curved elements from SHAPE-THIN. The check box Ignore classification of curved parts allows you to exclude short fillet arcs from the classification as soon as the user-defined c/t-ratio is below the limit (see the article in the Knowledge Base). Then, longitudinal ribs or folds and grooves of sheeting have no influence on the design.
Cross-sections assigned to class 1 or 2 are designed plastically by RF-/STEEL EC3. If this is not desired, you can activate the Elastic design also for those cross-section classes.
If the stability analyses are not performed with the equivalent member method according to  clause 6.3 but with the internal forces calculated according to the second order theory, you can use this check box to decide if factor γM1 (instead of γM0) is used for the cross-section designs.
The partial safety factor γM1 is relevant for the resistance determination in case of instability (structural component check). You can check and, if necessary, modify it in the National Annex Settings dialog box (see Chapter 2.10).
Find more information on stability analyses in an article of the Knowledge Base at our website.
The check box Use linear interaction acc. to 6.2.1(7) determines whether to use a linear addition of the utilization ratios for the moments and axial forces according to , Eq. (6.2), or Eq. (6.44) as conservative approximation for the resistance verification of the cross-section.
In the input field, you can define the shear stress component from torsion up to which the torsional stresses are neglected in the cross-section design. So it is possible to suppress warnings of oversized torsional stresses for cross-sections of class 4.