Bending Moment

Whenever a body or a structure is subjected to external forces, some internal forces will be created within the body. These internal forces are called as a reaction in civil engineering. These reactions can be either in the form of shear force or in the form of bending moments.

Here, our topic is to understand what is meant by Bending moment. The bending moment can be understood in two ways. One way is the way through which it is calculated and the second way is how it is created.

How is Bending Moment Calculated

We will consider the example of a roller support as shown in the figure -1 below.

Fig.1.A beam End with Roller Support
Now, we know that a roller support have only one reaction, that is the vertical shear force as shown in above figure. Why is it so???

The answer is simple, the roller support is a support that restricts vertical movement. It will allow the movement in both the horizontal and roatational directions. So there will only exist a reaction in the vertical direction.

Hence we can conclude, a reaction is a internal force that is created in the direction that is restrained. As shown in the figure -2 below, its a fixed support.

Fig.2.A Beam with Fixed End Support

As shown in the figure-2 above, there will be three reactions. They are :
  • 1 Horizontal
  • 1 Vertical reaction
  • 1 Bending Moment
Here all the movement in restrained, hence it will have all reactions. This is the first way of understanding what is bending moment.

Bending Moment Calculation

Bending moment at a point A  is calculated as the sum of the product of force and the distance from the action of the force to the point.

Based on the force and the distance calculated is either from left or right of the point A the sign convention is followed. 

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