What is the difference between the basic internal forces, the principal internal forces and the design internal forces in RFEM?
Basic internal forces:
These are the surface internal forces which are related to the local coordinate system (x,y,z) of the surface. The x- and y-axes lie in the surface, the z-axis is set perpendicular to the surface. For example: The bending moment m-x represents the bending moment creating bending stresses in x-direction (not about axis x!)
Principal internal forces:
These are the internal forces that are related to the principal axes (1,2,3) of the surface. The internal forces in direction of axis 1 represent the extreme values (maxima and minima). The angle between the surface axis x and axis 1 is displayed as alpha_b for bending moments.
Design internal forces:
These internal forces are required for the design of reinforced concrete structures. The plus and minus signs ( + and - ) indicate the positive or negative side of the surface.
The internal forces in surfaces are described in detail in the RFEM manual:
Structural engineering software for finite element analysis (FEA) of planar and spatial structural systems consisting of plates, walls, shells, members (beams), solids and contact elements