Answer:
Definition of a rib:
Ribs can be used to model T-beams (downstand beams). Using this member type, the eccentricities and effective plate widths are taken into account in the FEM model.
Arising axial forces:
If, for example, a downstand beam is defined using the Rib member type, an eccentric member is created that is adjacent to the bottom side of the plate axis.
Even in a hinged structural system that is subjected to pure bending, axial forces occur on the member axis.
The moment is divided into a compressive force (blue arrows) in the plate and a tensile force (red arrows) in the member.
The structural system is in equilibrium, so that the axial forces from the plate and the member counteract each other.