Question

Can someone please explain how you get the calculations for the shear and moment diagrams with an example and a step by step process? They are so confusing to me. It would be much appreciated if someone showed me.

Answer 1

When a force is applied on beam two actions take place

1. Shearing of beam

2. Bending of beam

Breaking of beam is called shearing of beam,where the beam splits into parts.

Bending of beam under force action is called bending of beam.

Shear force:- It is defined as the algebric sum of forces acting either on left hand side or right hand side of the section

It's unit is 'N' or 'KN'

Bending moment:- it is defined as the algebric sum of moment of forces acting either on left hand side or right hand side of the section.

It's unit is 'N-mm' or 'KN-mm'

Take a section on beam than if to the left of the section there is upward force and to the right of the section there is downward force, both these forces we have to take them positive .

Similarly if to the left of the section there is downward force and to the right of the section there is upward force ,both these forces we have to take them positive.

Take a section on beam ,if there is a clockwise moment to the left of the section and anti clockwise moment to the right of section both these moments are taken as positive and these kind of moments are called as sagging bending moment.

Similarly, if there is a anti clockwise moment to the left of the section and clockwise moment to the right of the section ,both these moments are taken as negative and these kind of moments are called as hogging bending moment.

Important points to be noted while drawing SFD and BMD are:-

1. Length of SFD and BMD must be equal to the span of the beam.

2. SFD is drawn below the loaded beam and BMD is drawn below SFD.

3. For simply supported beam , bending moments is zero at the support.

4. For cantilever beam, bending moment will be zero at the free end.

5. Calcate shear force and bending moment at all critical points .

6 If there is no load present between two points then shear force will be constant.

SHEARING FORCE DAIGRAM

BENDING MOMENT DAIGRAM