Forces
In this presentation you will:
 identify the forces that act on structures
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Introduction
Structures are exposed
to all physical forces.
Technology is used to
enable structures to
withstand the forces
acting on them.
Knowing how forces act
on structures can help
to ensure the structure
is adequate for its
intended purpose.
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Forces
All structures are subject
to forces. A bridge or
office block can weigh
hundreds of tons.
The structure has to
withstand this weight,
as well as carry the
loads due to weather
and the people using it.
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Question 1
Which of the following causes force to act on a structure?
A) The weight of the structure
B) The weather
C) The people using it
D) All of the above
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Question 1
Which of the following causes force to act on a structure?
A) The weight of the structure
B) The weather
C) The people using it
D) All of the above
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Forces on Structures
Imagine a brick in the base of a wall. The brick has to resist a load due to
the weight of the bricks above it.
As the brick is squashed, it exerts a force back up on the wall equal and
opposite to the force acting on it.
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Forces on Structures
If the load is too great,
the brick will fail and
be crushed.
In reality, bricks and
other masonry can
withstand these
large forces so they
rarely fail in this way.
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Compression
A force that acts to
squash part of a structure
is called compression.
Many construction materials,
such as stone, steel, and
concrete are extremely good at
resisting compression forces.
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Arches
The arch is an extremely
old design for a structure
that carries a compression
force. The shape of the
arch is such that the whole
structure is in compression.
Arches are built from
materials that are strong
in compression. Arches
are used in many
structures, including
bridges and dams.
Some arches have
survived for hundreds
of years.
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Tension
A force that acts to
stretch part of a structure
is called tension.
When an elastic band is
stretched, a tension
force is applied to it.
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Tension
Materials such as steel
and wood can be strong
under tension. Stone
and concrete are weak
under tension.
In a suspension bridge,
the deck hangs from two
towers by many steel
cables under tension.
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Bending
When a force acts to bend an
object, it is a bending force.
However, bending is really a
combination of compression
and tension forces.
Compression
Tension
If someone walks across a
plank of wood, the plank
bends. The top of the plank
is under compression and
the bottom is under tension.
Materials used to carry
bending forces must be
strong in tension and
compression. Steel is a
good example of this.
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Other Forces
A force that causes
twisting is called torsion.
Some bridges have to
resist torsion forces
created by the wind
trying to twist them.
A force that causes
tearing of the material
is called shearing.
Fabric structures such as
tents have to be resistant
to shearing forces.
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Example of Forces in Nature
The Tacoma Narrows bridge collapsed due to torsion and shearing forces
produced by the wind.
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Question 2
A load is applied to a column causing it to be squashed slightly.
What is this force called?
A) Compression
B) Tension
C) Torsion
D) Shearing
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Question 2
A load is applied to a column causing it to be squashed slightly.
What is this force called?
A) Compression
B) Tension
C) Torsion
D) Shearing
When an object is squashed the force applied is a compressive force.
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Question 3
A load is applied to a cable causing it to be stretched.
What is this force called?
A) Compression
B) Tension
C) Torsion
D) Shearing
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Question 3
A load is applied to a cable causing it to be stretched.
What is this force called?
A) Compression
B) Tension
C) Torsion
D) Shearing
A cable under load that is being stretched will be under the force of tension.
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Question 4
A load is applied to the deck of a bridge causing it to twist.
What is this force called?
A) Compression
B) Tension
C) Torsion
D) Shearing
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Question 4
A load is applied to the deck of a bridge causing it to twist.
What is this force called?
A) Compression
B) Tension
C) Torsion
D) Shearing
A twisting action is caused by torsion.
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Question 5
A structure in this diagram is subject to bending by a force.
Where is the tension force greatest?
A) Point A
B) Point B
A
C
C) Point C
D
B
D) Point D
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Question 5
A structure in this diagram is subject to bending by a force.
Where is the tension force greatest?
A) Point A
B) Point B
A
C
C) Point C
D
B
D) Point D
Tension will be greatest on the bottom surface of the beam.
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Summary
In this presentation you have:
 identified the forces that act on structures
End