Shear and moment diagram

Shear and Moment Diagrams

Shear and bending moment diagrams are analytical tools used in structural analyses and structural design to determine the value of shear force and bending moment at different length of an element. Using these diagrams type and size of a member can be determined easily. Another application of shear and moment diagrams is that the deflection can be easily determined using either the moment area method or the conjugate beam method.

Convention

Normal Convention

The normal convention used in most engineering applications is to label a positive shear force one that spins an element clockwise (up on the left, and down on the right). Likewise the normal convention for a positive bending moment is to warp the element in a "u" shape manner (Clockwise on the left, and counterclockwise on the right).

Normal positive shear force convention (right) and normal bending moment convention (left).

Concrete Design Convention

An exception to using the normal convention is used when designing concrete structures. Since concrete is weak in tension the most important part of designing a member with a high bending moment is to show whether the top or bottom of the concrete member is in tension. Because of this the positive moment diagram is alway drawn such that the tension on top is defined to be positive. This is opposite of the normal convention. The shear convention remains the same.