Answer:
The reason is due less to the defined cross-section than to the material properties. There are yield strengths and limit stresses, depending on the component thickness. A non-designable situation results when the thickness of a cross-section part is greater than the existing thickness range. To fix this, you have the following options:
1) You can create another thickness range with the corresponding stresses within the add-on module in Window 1.2 "Materials".
2) As an alternative, you can select another material with suitable thickness ranges in the RFEM/RSTAB library.
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