Why do civil engineers draw shear and bending moments in a diagram?

Every civil engineering student have drawn SFD&BMD. Atleast once in their lifetime they must have been questioned about it, what the hell this diagram is and what’s its real life use?

So We are trying to clear your doubts about SFD and BMD.

In short “A bending moment diagram is used to analyze the behavior of the structure on application of a load. The bending moment diagram shows where and how the moments will act on the structure, i.e. how a structure will bend, on application of a load or a combination of various types of loads”

Further, Bending Moment and Shear diagrams are used for dealing with forces in materials. In steel beam design, bending moment defines what is the lightest shape that can be used to carry this bending moment.Shear strength of steel beams are high and typically can be overlooked if there isn’t and special loading condition that warrants a look such as a column sitting at the middle of a beam or near the support. At the support, the shear stresses are at its highest and any load near this point would raise a red flag to check the capacity of the steel beam.

Whereas, In concrete, everything has to be checked because concrete is designed for bending and the internal couple within the beam must resist the external bending moment placed on it. In addition, the shear in concrete has to be checked to make sure it has the capacity and if it doesn’t, stirrups are used to raise the shear capacity to at or above the ultimate strength design of the section. In any case, steel stirrups are always placed in concrete beams.

Also, if a anyone is doing a moment frame in concrete or steel, they have to understand how bending moments and shear should look like in the frame.

But in today’s world, the engineer uses software to do all the calculation but if they do not know what a bending moment or shear diagram looks like in a loading condition, the analysis is null. Thus the study of SFD and BMD is given in syllabus.

Bending moment and shear diagrams are used all the time by engineers to design bridges, buildings, boats, cars, planes and any type of structure that has to resist forces.