Sign Convention of Cut Forces

Sign convention of cut forces

The sign convention adopted is that of the forces acting on the plane from its positive side , expressed in:

• Deformed space : the plane's local coordinate system.
• Basic space : the global model system

Moments are expressed about an axis, using the right hand screw rule. Thus Mxx is moment about the plane X-X axis in the relevant system, with a +ve moment being generated by clockwise twist looking from the origin down that axis.

This figure illustrates how the sign convention affects moments. Since the force above is "acting on the plane" from its outside it generates a +ve moment about the plane XX axis.

If the plane were rotated 180 degrees about its local X axis, reversing "behind" and "in front" sides, then the sign of the moment about XX would be negative. In addition the plane Y axis would now point downwards, and the shear force in Y would be +ve.