Concept of Pure Bending

• As we are aware of the fact internal reactions developed on any cross-section of a beam may consists of a resultant normal force, a resultant shear force and a resultant couple. In order to ensure that the bending effects alone are investigated, we shall put a constraint on the loading such that the resultant normal and the resultant shear forces are zero on any cross-section perpendicular to the longitudinal axis of the member,

That means F = 0

since or M = constant.

• Thus, the zero shear force means that the bending moment is constant or the bending is the same at every cross-section of the beam. Such a situation may be visualized or envisaged when the beam or some portion of the beam, has been loaded only by pure couples at its ends. It must be recalled that the couples are assumed to be loaded in the plane of symmetry.
• When a member is loaded in such a fashion it is said to be in pure bending. Example of pure bending is shown below.