What is Bending Moment? What are its sign conventions? What is BMD? What is the difference between moment and bending moment? Find out answers for these questions and many more in this presentation.

1. BENDING MOMENT
DHA SUFFA UNIVERSITY (MECHANICAL DEPT)
Karachi, Pakistan

2. WHAT IS BENDING MOMENT?
A bending moment is a measure of the bending effect
due to forces acting on a beam. It is measured in terms of
force and distance.

3. CONSIDERING FREE BODY DIAGRAM
¨Bending moment is an important factor to be
consider while designing any structural
component. For example we consider the
following which shows the free body diagram.

4. Considering forces on the left
of the section X-X
MX = RALA - F 1X1 – F2 X2
Considering forces to the
right section of X-X.
MX = RB LB - F 3X3 – F4 X4.
Considering forces to the
right section of X-X.
MX = RB LB - F 3X3 – F4 X4.
Considering forces to the right section
of X-X.
MX = RB LB - F 3X3 – F4 X4.

5. BENDING MOMENT AT A SECTION OF A BEAM
Bending moment at a section of a beam is defined
as the algebraic sum of the moment of all the
forces acting on one side of the section.

6. BMD
BENDING MOMENT DIAGRAM
A graphical
representation of the
bending moment
equation along the span
of the beam is known as
bending moment
diagram (BMD).

7. SIGN CONVENTION FOR BENDING MOMENT
It is very important to define the sign convention for
bending moment before calculating the values. The
possible bending of a beam is shown in the
following figure.

8. Sagging bending
moment is taken as +ve. It
results in developing
tension in the bottom
fibres and compression in
top fibres of the beam.
HAPPY BEAM IS +VE

9. Hogging bending
moment is taken as –ve.
And it develops
compression in the
bottom fibres and tension
in the top fibres.
SAD BEAM IS -VE

10. UNITS OF BENDING MOMENT
A bending moment is a measure of the bending
effect due to forces acting on a beam. It is a type of
stress and is measured in terms of force and
distance. so they have as unit Newton-metres (N-m)
or foot-pounds (ft-lb).

11. Moment is responsible for
a beam to rotate about
some axis.
Bending moment are a
pair of moments which
will not rotate the beam
but it will deflect it.
Difference between Moment & Bending Moment?

12. Shear force is maximum at
support and zero at centre.
Bending moment
maximum at centre and
zero at support for all
simply supported beams.
Difference between Shear Force & Bending Moment?

13. THANKYOU