Understanding Shear Forces and Bending Moments Diagrams

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What are Shear Forces and Bending Moments?

Shear forces and bending moments are resultants which are used to conveniently represent the internal forces that develop within a beam when external loads are acting on it.

The internal forces that exist within the beam will depend on how the beam is loaded, and how it is supported. Beams are typically loaded by concentrated forces, distributed forces, and concentrated moments.

Typical beam supports include pinned supports, roller supports, and fully fixed supports.

How Do We Draw the Shear Force and Bending Moment Diagrams?

Determining shear forces and bending moments along the length of a beam typically involves three steps:

1. First we draw the free body diagram of our beam.
2. Next we use the equilibrium equations to calculate the reaction forces and moments.
3. Finally, we cut our beam at a single location and use the equilibrium equations to determine the shear force and bending moment at that location.

We repeat this process for each location along the beam and draw the result on our shear force and bending moment diagrams.