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FREE BODY DIAGRAMS
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The Free Body Diagram
A
B
The Concurrent System
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Concept of Free Body Diagrams
Particle System
Rigid Body Systems
Free Body diagram of Rigid Bodies
Free Body Diagrams
Types of Beam Supports
Concept of Equilibrant
Applied and Reaction Forces in Beams
Graphical Determination of Equilibrant
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Free Body Diagrams

• Essential step in solving Equilibrium problems
• Complex Structural systems reduced into concise
  FORCE systems
• WHAT IS A FREE BODY DIAGRAM?
• A FBD is a simplified representation of a
  PARTICLE or RIGID BODY that is isolated from its
  surroundings and on which all applied forces and
  reactions are shown.
• All forces acting on a particle original body
  must be considered, and equally important any
  force not directly applied on the body must be
  excluded.

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Free Body Diagram
BC
C
BA
W
B
A
W
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REAL LIFE CONCURRENT SYSTEMS
Equilibrium of a Particle
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Problem
Force in Boom 4000
C
?
Q800
P?
B
A
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Change
Problem
E
4
D
C
3
A
B
BA? BC? CD? CE?
W100
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Y
3
1
Y
300N
400
12
5
P
P
X
X
3
F1
4
450N
F2
F1
Y
Y
4
2
7 kN
F
P
3
P
4.5 kN
X
X
F
2.25 kN
7.5 kN
3 kN
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CONCEPT OF THE EQUIBILIRIANT
R
E
Resultant
Equilibrant
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TIP-TO-TAIL METHOD
A
B
Y
C
Line of action of CA
200
X
Line of action of CB
W200
CA
CB
Measure CB and CA
RESULTANT
EQUILIBRIANT
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RIGID BODY SYSTEMS
ASimple Supported Beam
A Cantilever Beam
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A Propped Cantilever with Three Concentrated Load
A Simply Supported Beam with Three concentrated
Loads
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APPLIED AND REACTION FORCES IN BEAMS
In the Chapter on Force Systems, we discussed the
concept of APPLIED FORCES, REACTION FORCES and
INTERNAL FORCES Here we well discuss the
relevance and importance of APPLIED FORCES and
REACTION FORCES in the case of Beams. Before we
proceed further please study the animated visuals
on the next slide
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A Beam is an example of Rigid Body. Generally
loads are applied on the beams. And the beams
develop reactions. We named the loads hat are
applied on the beams like Dead Load, Live Load,
Wind Load. Earthquake Loads as APPLIED FORCES,
and the consequent reactions that are
simultaneously developed as REACTION FORCES.
These REACTION FORCES generally develop at the
supports. We use the same color code as described
earlier for clarity. The reactions develop as a
direct consequence of Newtons Third Law,. Which
states that for every action there is an equal
and opposite reaction. In the three examples
presented, if we separate the rigid body for its
supports we can see equal and opposite forces
acting at the supports..
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From the above we can describe the concept of the
FREE BODY DIAGRAM of a Rigid Body as folows. It
is representing the rigid body with all the
Forces- the APPLIED FORCES and REACTION FORCES
acting on it It is axiomatic that the Rigid Body
must be in equilibrium under the action of the
APPLIED FORCES and the REACTION FORCES. Hence
the FREE BODY DIAGRAMS can also be called as
EQUILIBRIUM DIAGRAMS, even though the former name
is more popular. Finding the REACTION of beams
for various types of APPLIED LOADS is a basic
requirement in STATICS
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The above diagrams, which show the complete
system of applied and reactive forces acting on a
body, are called free body diagrams. The whole
system of applied and reactive forces acting on a
body must be in a state of equilibrium. Free-body
diagrams are, consequently ,often called
equilibrium diagrams. Drawing equilibrium
diagrams and finding reactions for loaded
structural members is a common first step in a
complete structural analysis
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Roller, Hinge and Fixed Supports
Hinge supports
Roller Supports
Fixed Supports
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ROLLER SUPPORT
Applied Force
Reactive Forces
The Reactive Force must always be perpendicular
to the surface for a ROLLER
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Roller Support
Roller Support allows horizontal movement It
allows the beam to bend
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Rocker Support
A Rocker Support is similar to the Roller Support
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A variation of Roller Support
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Pin or Hinge Support
Pin support does no allow any movement It allows
the beam to bend
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Why Fixed Support is Important?
Hinge
A Cantilever has to be fixed to support a load
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REAL LIFE HINGES
Top part
Pin
Bottom part
A Steel Hinge
A Concrete Hinge
A Neoprene Pad Hinge
The close confinement of the steel rods will not
allow moment transfer, but only Vertical
Horizontal Forces
The shear deformation of the Neoprene pad mimics
the horizontal movement of a Roller
The rotation of the top part about the pin
allows a Hinge action
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Question 1. What is the difference between a
Rigid Body and a Particle Question 2 Explain
the Difference between a Roller Support, Hinge
Support and Fixed Support
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FREE BODY DIAGRAMS OF RIGID SYSTEMS
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• 5. Draw the Free Body Diagram for the following
  Dam

Water