1. Ahsanullah University of
Science & Technology
Department of Civil Engineering
Pre-stressed Concrete Lab
Sadia Afrina Haque
ID: 10.01.03.007
4th Year; 2nd Semester
Sec : A

2. Free Body Diagram
External and Internal Force

3. What is a Free Body Diagram?
• A free body diagram is
a graphic,
dematerialized,
symbolic representation
of the body (structure,
element or segment of
an element) in which all
connecting "pieces"
have been removed.

4. Purpose
• A FBD is a convenient
method to model the
structure, structural
element, or segment that is
under scrutiny.
• It is a way to conceptualize
the structure, and its
composite elements, so that
an analysis may be
initialized.

5. Visualizing all the forces
acting on a single object
Free Body Diagram

6. Visualizing all the forces
acting on a single object

7. Support Reactions
• If the support prevents the translation of a body in
a given direction, then a force is developed on the
body in that direction
• If rotation is prevented, a couple moment is exerted
on the body
• So the three ways a horizontal member, beam can
be supported at the end –
 Roller
 Pin
 Fixed support

8. Support Reactions
Roller Support:
•
Prevent the beam from
translating in the vertical
direction
•
So roller only exerts a
force on the beam in the
vertical direction

9. Support Reactions
Pin Support:
•
•
•
The pin passes through a hold in the beam and two leaves
that are fixed to the ground
Prevents translation of the beam in any direction Φ
The pin exerts a force F on the beam in this direction

10. Support Reactions
Fixed Support:
•
•
•
This support prevents both
translation and rotation of the
beam
A couple and moment must be
developed on the beam at its
point of connection
Force is usually represented in x
and y components

11. Example
Dematerialize
the lamp
The forces and
moments
required to
maintain
equilibrium are
drawn

12. Thank You