Shear Force and Bending Moment – What Are They?

The algebraic total of all vertical forces acting on either side of a point on a loaded beam may be used to define shear force at every point along the beam.

The beam and the point at which it acts are both sheared off due to the shear force’s net effect.

Shear force is considered positive if it results in a clockwise moment and negative if it results in an anticlockwise moment.

What is the Bending Moment?

The bending moment at any point is the total of all moments caused by all vertical forces operating on either side of a point on a loaded beam.

The bending moment attempts to bend the beam. When loads act to the left of the section, clockwise moments are believed to be +ve and anticlockwise moments to be -ve.


Sign Convention used for Shear Force 

Shear force is defined as the difference between the force acting in the right-hand side of the section in the upward direction acting as -ve and the force acting in the right-hand side of the section acting as +ve.

Similarly, a force acting on the left-hand side of the section is considered positive if it is moving upward and negative if it is moving downward.

Sign Convention used for Bending Moment

Remove all loads and responses from any side of the section to start. Each load and reaction will now be introduced one at a time, and the section’s effect will be determined.

Sagging bending moments are defined as bending moments that result in concavity upwards and are regarded to be positive. A hogging bending moment is taken -ve and is responsible for convexity upwards.

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Use Of Bending Moment Diagram and Shear Force Diagram

  1. By constructing internal resistance (shear, moment, and/or bending moment), every structure will try to withstand deformation. However, the amount of resistance provided against loading will depend on the body’s properties. For instance,
  • A concrete beam has an extremely strong resistance to shearing. However, the tension is extremely low.
  • A steel beam is very resistant to shear and bending forces. However, compared to concrete, it is relatively pricey.
  1. So, when building a concrete beam, we find the points having the highest SFD and BMD values. These figures help us determine the type of material and geometry we should use to prevent deformation (or failure in general). Since concrete is so inexpensive, we aim to use it as much as possible. It lacks tension, though.
  1. Therefore, we give Steel reinforcements for zones of the highest Bending Moment (which we determine from BMD). We also find zones with the highest shear force (from SFD). We offer steel shear stirrups to withstand the extra shear if the shear force values exceed the concrete’s capability (resistance).


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