Bending Moment

Summary

Bending Moment - A bending moment is the reaction in a structural element when external forces cause the element to bend. It can be measured in kNm.

Technical Information

For a structure to remain in static equilibrium, the bending moments induced by external forces must not exceed the moment capacity of the elements upon which they act.
A bending moment will cause both compressive and tensile stress in a cross section. Because plain concrete is much weaker in tension than it is in compression, it will tend to fail on its tension face when subjected to a bending moment. This is why tensile reinforcement is used in a lot of concrete structural elements.

A bending moment diagram is a useful way to visualise the distribution of bending moments in a structural element. It gives the magnitude of the bending moment at any given point in the element.

In a horizontal member, such as a beam or floor, the following statements are true:

Bending moments which cause tension on the top of the member are known as hogging moments.
Bending moments which cause tension on the bottom of the member are known as sagging moments.
Points at which there is no prevailing bending moment are known as points of contraflexure.