My section is classified as Class 4 and non-designable in RF-/STEEL CSA. However, my manual calculation shows a different class. Why the difference?
The maximum b/t and h/w ratios are based on Table 1 or Table 2 depending on the internal forces in the members. For members subject to pure axial compression, Table 1 is used. For members subject to flexure or flexure+axial (beam-column), Table 2 is used.
In RFEM, the preset setting is to neglect the bending moment if the ratio is less than 1%. Therefore, the program shows “Class 4” since the check is based on Table 1.
To change this setting, go to Details and set the limit values to zero (as shown in Figure 1). By doing so, the program runs the check per Table 2 and no longer ignore the small moments.
Note: In the example model, where moments are zero at the support locations, the result is still shown as “Class 4 (non-designable)”.
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