Although different sign conventions many be used, most of the engineers use the following sign conventions for shear forces and bending moment.
(i) The shear force that tends to move left portion upward relative to the right portion shall be called as positive shear force.
(ii) The bending moment that is trying to sag (Concave upward) the beam shall be taken as positive bending moment. If left portion is considered positive bending moment comes out to be clockwise moment.
To decide the sign of moment due to a force about a section, assume the beam is held tightly at that section and observe the deflected shape. Then looking at the shape sign can be assigned.
The shear force and bending moment vary along the length of the beam and this variation is represented graphically. The plots are known as shear force and bending moment diagrams. In these diagrams, the abscissa indicates the position of section along the beam, and the ordinate represents the value of SF and BM respectively. These plots help to determine the maximum value of each of these quantities.
