Shear, Moment Equations & Diagrams | Additional Study Material for Mechanical Engineering PDF Download

Shear Force and Bending Moment Diagrams

You probably can tell from the examples previously that the shear force SF and bending moment BMvaries along the beam, due to the varying loads.

From an engineer’s point of view, you would want to find out where the maximum SF or BM is – i.e. the weakest part of the beam. This is so that you can design to ensure that it’s safe!

Likewise you would want to know where the minimum SF or BM is, so that you will not overdesign that portion of the beam

So how do we conveniently see the SF and BM along the beam? Well that’s what the SF and BMdiagrams are for!

Shear, Moment Equations & Diagrams | Additional Study Material for Mechanical Engineering

There are 2 methods to construct the SF and BM diagrams:

Method 1: Equation approach

In this method, you basically obtain the expression for SF (or V) and BM (or M) as a function of the distance x from the left end of the beam. The equations are obtained using the equations of equilibrium such that the internal forces ensure equilibrium of the section cut:

Shear, Moment Equations & Diagrams | Additional Study Material for Mechanical EngineeringShear, Moment Equations & Diagrams | Additional Study Material for Mechanical Engineering

The equations obtained are then used to construct the SF and BM diagrams. Note that you will need one equation for every single change in loading (i.e. when a new force comes in, as you move from the left to right of the beam):

Shear, Moment Equations & Diagrams | Additional Study Material for Mechanical Engineering

It might seem vague at the moment, but it will make more sense once you work through an example.

Method 2: Direct method

This is the recommended method but it really takes practice to master. Basically this method works by directly constructing the SF diagram using the FBD, and BM diagram using both the SF diagram and FBD.

Two key relationships for this method are as follows:

dV/dx = -w  dM/dx = v

The relationships basically say that the gradient of the SF diagram is equal to the –ve of the distributed load, while the gradient of the BM diagram is equal to the SF:

Shear, Moment Equations & Diagrams | Additional Study Material for Mechanical EngineeringShear, Moment Equations & Diagrams | Additional Study Material for Mechanical Engineering

We also present to you a few examples of how different forces acting on a beam are represented in SFand BM diagrams:

Shear, Moment Equations & Diagrams | Additional Study Material for Mechanical EngineeringShear, Moment Equations & Diagrams | Additional Study Material for Mechanical Engineering

Shear, Moment Equations & Diagrams | Additional Study Material for Mechanical Engineering

So how do we actually construct the SF and BM diagrams directly?

The document Shear, Moment Equations & Diagrams | Additional Study Material for Mechanical Engineering is a part of the Mechanical Engineering Course Additional Study Material for Mechanical Engineering.
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FAQs on Shear, Moment Equations & Diagrams - Additional Study Material for Mechanical Engineering

1. What are shear and moment equations in mechanical engineering?
Ans. Shear and moment equations are mathematical representations used in mechanical engineering to analyze the internal forces and bending moments within a structure. These equations help engineers determine the distribution of shearing forces and bending moments along a beam or any other structural element.
2. How are shear and moment diagrams used in mechanical engineering?
Ans. Shear and moment diagrams are graphical representations that show the variation of shear forces and bending moments along a structural element. These diagrams provide engineers with a visual understanding of how internal forces and moments change along the length of a beam. They are essential for designing and analyzing structures, as they help identify critical points where the forces and moments are maximum.
3. What is the significance of shear and moment equations in mechanical engineering?
Ans. Shear and moment equations are crucial in mechanical engineering as they allow engineers to determine the internal forces and moments acting on a structure. By calculating these forces and moments accurately, engineers can ensure the structural integrity and safety of a design. Additionally, shear and moment equations aid in selecting appropriate materials and dimensions for a structure, optimizing its performance.
4. How do engineers derive shear and moment equations for different loading conditions?
Ans. Engineers derive shear and moment equations by applying the principles of equilibrium and compatibility. They consider the external loads acting on a structure, such as point loads, distributed loads, or moments, and then apply these principles to determine the internal forces and moments. Different loading conditions require different approaches, such as integration or superposition, to obtain the shear and moment equations for a given structure.
5. Can shear and moment equations be used in real-life applications in mechanical engineering?
Ans. Yes, shear and moment equations are widely used in real-life applications in mechanical engineering. These equations are essential in designing bridges, buildings, and other structures subjected to various loads. By using shear and moment equations, engineers can ensure that the structural members can withstand the anticipated forces and moments, leading to safe and reliable designs.
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