How to set effective thin-walled section for torsion check

Although, we are able to design reinforcement in IDEA StatiCa in a very detailed way for output drawings and bill of material, some bars of shear reinforcement (mostly the opened ones) are not effective for shear or torsion. The consideration of the effectiveness of a stirrup is governed by a checkbox right next to the item in the list of stirrups. 

Let´s show it on an example of the reinforced I section.

If you set the definition of equivalent thin-walled section from real stirrups, you will probably obtain the expected shape and dimensions.

If you select the second option (Calculate from area and perimeter), you will obtain an unrealistic section. This usually happens when you define a wrong selection of effective stirrups.

The last option, the most general one, is to define the section by a Manual input from the center of gravity.

The influence of the equivalent thin-walled section on torsion has two consequences:

1) Area of longitudinal reinforcement effective in torsion is determined from the rebars that lie inside the equivalent thin-walled section

2) Area of longitudinal reinforcement effective in torsion is also considered in interaction i.e. not just in torsion checks