Answer:
Basic Internal Forces
These are the internal forces of a surface that are related to the local coordinate system (x, y, z) of the surface. The x- and y-axes lie on the surface, the z-axis is perpendicular to the surface. The bending moment mx, for example, represents the bending moment that generates bending stresses in the x-direction (not about the x-axis!).
Principal Internal Forces
These are the internal forces related to the principal axes (1,2,3) of the surface. The internal forces in the direction of Axis 1 represent the maximum values. The angle between surface axis x and Axis 1 is displayed as αb for bending moments.
Design Internal Forces
These internal forces are required for reinforced concrete design. + and - indicate the positive and negative sides of the surface.
The internal forces in surfaces are described in detail in the RFEM manual.
