Classification of cross-sections
The classification of cross-sections is based on the chapter 5.5 of EN 1993-1-1. The classification is done according to the geometrical rules and the type of loading. The plastic resistance is considered for classes 1 and 2, elastic resistance for class 3. The cross-sections in class 4 are reduced due to the effects of local buckling to the effective cross-section. The elastic resistance of the effective cross-section is considered in the analysis of these cross-sections.
The classification is performed for any particular wall of the cross-section separately, the worst one is selected as a final class for the whole cross-section. The tensile parts are automatically classified as the class 1. This isn't formally in compliance, however, it doesn't effect results and it brings better continuity of the work in the software.
The classification is done for the combination of loading (axial force and bending moments). The bending moment about axis parallel to the particular wall is transferred into the increment of axial force in that way, that the final stress is identical to the stress of original bending moment. The plastic resistance is considered for classes 1 and 2, the elastic resistance for classes 3 and 4 in this recalculation. Brief example of the classification of I-profile with a positive value of bending moment My: The increment of axial force in the upper flange dN would have a negative value, as the positive value of bending moments causes tensile stress in the upper flange. The increment dN would be positive for bottom flange, as tension occurs there.
The plastic resistance may be excluded manually in the design by the user. The design according to classes 3 or 4 is used in these cases. The type of analysis (elastic or plastic resistance) has to be specified manually for cross-sections created in the program "Section".
The rectangular cross-sections can't be classified according to EN 1993-1-1. Following rules are used:
- Cross-sections with slenderness smaller than 9 are classified as class 1
- Cross-sections with slenderness smaller than 10 are classified as class 2
- Cross-sections with slenderness greater than 10 are classified as class 3
As it isn't possible to calculate an effective cross-section for rectangular cross-sections, the class 4 isn't included for these cross-sections. Therefore, local buckling of significantly slender rectangles is ignored.
The stress caused by the warping torsion is calculated with the help of elastic theory. Therefore, these cross-sections are automatically classified as classes 3 or 4.
In case of the trapezoidal sheeting profiles designed acc. to EN 1993-1-3 (cold-formed members) the classification is not performed. Class 4 is always assumed with subsequent calculation of the effective cross-section.