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The classification of cross-sections is executed according to EN 1993-1-1 art. 5.5.2 and Table 5.2.
For standard sections, the Classification is done according to the parts of the Initial Shape.
Internal compression elements (I) are classified according to Table 5.2 Sheet 1.
Outstand compression elements (SO & UO) are classified according to Table 5.2 Sheet 2.
CHS sections (FC 3) are classified according to Table 5.2 Sheet 3.
Angle sections (FC 4) are classified according to Table 5.2 Sheet 2 and in case of uniform compression also Sheet 3.
Cross-sections without Initial Shape are classified as elastic Class 3.
The elastic stresses are calculated in the endpoints of the parts Ref.[40].
The elastic stress distribution y for each part can then be determined as follows:
With f1 and f2 the elastic stresses at the ends of the part.
The EN 1993-1-1 sign convention is used which implies compression stresses are positive
To determine the plastic stress distribution a three algorithms are provided:
- Elastic Stresses
- Yield Surface Intersection
- Iterative Approach
In this method the plastic stress distribution is based on the elastic stresses f1 and f2 at the ends of the parts.
In case 
and 
the section is assumed to be in uniform compression. This implies that α is taken as 1,00 for all parts.
In case one stress is positive (compression) and the other negative (tension) the following calculation is used:
In all other cases α is taken as 1,00 for the given part.
Specifically for a doubly-symmetric I-section (Formcode 1) the α value of the web element is overruled by the following formula Ref.[40]:
Within this formula the NEd is taken as positive for compression and negative for tension.
For large compressive forces this formula can lead to an α > 1,00 in which α is limited to 1,00.
For large tensile forces this formula can lead to an α <= 0,00. In this case the element is seen as in full tension and thus no classification is required.
In case y > 0 for the web element this indicates that the entire web is in compression thus α = 1,00.
For this method a full plastic analysis is run as described in Ref.[41]. This plastic analysis is based on the Initial Shape and uses a stress-strain diagram with yielding plateau.
The yield surface is generated for the given section (using a predefined set of points) and the intersection of the actual forces is determined with this surface.
The actual intersection point does not always collide exactly with a predetermined point of the surface, so small deviations can occur.
From the location of the plastic neutral axis(PNA), which results of this analysis, the α value for the different parts can be determined Ref.[40].
For this method a full plastic analysis is run as described in Ref.[41]. This plastic analysis is based on the Initial Shape and uses a stress-strain diagram with yielding plateau.
The actual plane of deformation for the given internal forces is determined iteratively which provides an exact solution.
From the location of the plastic neutral axis(PNA), which results of this analysis, the α value for the different parts can be determined Ref.[40].
In case the setting for using Semi-Comp+ is activated, the classification limits are modified according to Ref.[40].
This modification is required in order to reach the specified safety level in accordance with the ESDEP (European Steel Design Education Programme) background of the Classification criterion.
The following gives an overview of the modifications defined in Ref.[40].
The c/t-limits in Table 5.2 of EN 1993-1-1 for internal parts in compression should be modified to 38 (instead of 42) at the limit 3/4 and to 34 (instead of 38) at the limit 2/3.
The limit 1/2 indicates the same discrepancy for internal parts in compression and should also be revised to 28 (instead of 33) accordingly
For each intermediary section, the classification for cross-section design is determined and the proper section check is performed. The classification can change for each intermediary point.
For each load case/combination, the classification for member buckling design is determined as the maximal class along the member. This class is used to perform the stability check since stability effects are related to the whole member and not to a single cross-section.
To determine this critical classification, all sections in the Ly and Lz system lengths of the buckling system are checked and the worst classification is used as the critical. Note that only sections on the actual member are used so in case the system length spans multiple members, only the sections of the actual member are used to determine the critical classification.
For non-prismatic sections, the stability section classification is determined for each intermediary section.
The alternative regulations given in EN 1993-1-1 art. 5.5.2(9) - (12) are not supported.