When the program displays a slenderness error message, it may be because of the following reasons:

• The global slenderness check fails. This means the slenderness of the bar is greater than that recommended by the selected code.
• Due to one of its components. This means that one of the plates making up the section of the bar has not got enough thickness.

The program carries out a slenderness check when the section in question is in compression, regardless of the magnitude of the force. If this case arises, the stress for the section of that series is not analysed. Hence, that section is not valid.

In the case of horizontal sections that are designed to fundamentally resist bending forces, a buckling coefficient of 0 may be applied to them so the program does not check the slenderness of the section.