Design sections have a local coordinate system, with r, s and z axes:

• R-axis is collinear with the design section and is positive in the direction from end 1 to end 2. This direction is also referred to as "Lateral".
• S-axis is 90 degrees counter-clockwise to the r-axis (still in the x-y plane) and goes through the "design centroid" (see below). This direction is also referred to as "Axial"
• Z-axis is parallel to the global z-axis and goes through the "design centroid". This direction is also referred to as "Vertical"

Forces for the design section are considered as the forces that the structure in the positive-s region applies to the structure in the negative-s region.

A design section drawn from point B to point A will have the same forces as a design section drawn from point A to point B except that the following two forces will have opposite signs:

• V_z (vertical shear on the s-face)
• M_z (moment about the z axis)