Table of Contents
Bending Moment Definition
In this tutorial, we will simply answer the question: what is a bending moment? A bending moment is a force normally measured in a force x length (e.g. kNm). Bending moments occur when a force is applied at a given distance away from a point of reference; causing a bending effect. In the most simple terms, a bending moment is basically a force that causes something to bend. If the object is not well-restrained the bending force will cause the object to rotate about a certain point. It might also be worth noting you can experiment and try our Free Beam Calculator to calculate the shear and bending moment diagrams of a beam.
This is often a difficult idea to comprehend, therefore consider an example of a plastic ruler overhanging a desk. If one end of the ruler is lying on the desk and is being held down, and then a force is applied to the other end of the ruler, it will cause the ruler to bend. The ruler will experience the greatest bending moment at the end where the force is applied.
Bending Moment Example
To calculate the bending moment about a reference point, we take the magnitude of the force and multiply it by the distance of the force from the point. Note that the force must be perpendicular to the line between the point where the force is applied and the reference point. Though that sounds confusing, it can be simplified in our example:
In the above example, the bending moment at point A is simply the distance multiplied by the force. Therefore, the Bending Moment at Point A = 0.2(10) = 2 Nm.
It is important to note that to use the above formula, the force (in this case a 10 N downward force) must NOT pass through the point. This is obvious as if we applied the same formula (distance x force) the distance would equal zero and therefore there would be no bending moment force.
Bending Moment Units
The Bending Moment force in the beam typically uses the units of Force x Length, most commonly kNm or kip-ft.
The reason for this is simple, the bending moment force is simply calculated as the force by the distance from the force. So it makes sense the unit should match those two units being multiplied by each other.
SkyCiv Beam Software
SkyCiv Beam Analysis Software allows users to analyze beam structures easily and accurately. You can get an analysis of your beam member, including reactions, shear force, bending moment, deflection, stresses, and indeterminate beams in a matter of seconds.
If you want to give it a try first, Free Beam Bending Moment Calculator is a great way to start, or simply sign up for free today!