Sign Convention

When a beam is subjected to a load, it experiences internal forces that can be categorized as normal, shear, and bending moments.

Internal forces can be determined by using the method of sections. A special sign convention is applied as they occur in pairs, and their direction depends on which side of the section is to be analyzed.

The normal force is assumed to be positive if it creates tension, whereas it is considered negative if it generates compression in the object.

Suppose the forces acting tangentially to the cross-section are directed downwards and upwards on the left and right sides of the section, respectively. In that case, the shear force is regarded as positive, resulting in a clockwise rotation.

Similarly, if the direction of the forces on each side is reversed, a negative shear is produced, resulting in an anti-clockwise rotation.

When an external force acts on the beam, it experiences a bending moment, forming an upward or downward concave curvature. The bending moment is positive for upward curvature and negative for downward curvature.