Answer:
When modeling stiffeners (ribs), you should pay attention to a few things. Under Downloads below, you can find a test model.
Generally, the objects lying in the surface (that is, lines and nodes) are automatically integrated into the surface for planar surfaces. For curved surfaces, this should be done manually. Therefore, the line has been removed from the surface manually in the left model (see the image). As a result, the line is only connected to the surface at the nodal points.
For the following Models 2 and 3, the lines are already integrated automatically into the surfaces. This means that the member is rigidly connected to the surface over its entire length.
Model 4 is modeled using surfaces and provides largely the same results as with members. I would not define the stiffeners as the "rib" member type, as they are primarily reserved for reinforced concrete design (design as floor beams).
