Transcript Structural Strength and Stability

Grade 7 Science: Form and Function
Learning Goals/Success Criteria
I can explain centre of gravity
 I can identify factors that help keep
structures stable
 I can identify and explain some specific
building construction techniques that
help make structures strong and stable
 I can identify some reasons that cause
structural failure

Stability of Structures

How do you keep from falling when you are
standing on a moving bus?
Discuss

Stability: the ability of a structure to remain
in or return to a stable, balanced position
when forces act on it
Stability of Structures

Centre of gravity: the point around which an
object’s mass is equally balanced in all
directions – where the mass seems to be
concentrated

All structures have a centre
of gravity. On a human who is
standing, this is located deep
inside your body just below
your belly button.
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Stability of Structures

Centre of gravity changes every time
you move or bend your body in different
shapes (Try it: what happens when you
stand on one leg? Lean over?).

The centre of gravity depends on the
shape of the object and how its mass is
distributed.
Stability of Structures
Which vehicle is more stable?
Why?
Stability of Structures

The car has greater stability because of
two features: it has a low centre of
gravity (the mass is concentrated lower
to the ground) and it has a wide support
base (when compared to its height).
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Conditions for greatest stability:
Objects with low centre of gravity and a
wide support base tend to be stable.
Stability of Structures

Stability decreases as the centre of
gravity rises. If the centre of gravity
rises so high that it is no longer over it’s
support base, the object will fall over.
This is why transport
trucks must be careful
going around corners!
Stability of Structures

The Leaning Tower
of Pisa in Italy was
closed for many
years due to the
tower’s centre of
gravity having
shifted to a
dangerous point of
instability. This has
since been restored
(more on this later!).
Stability of Structures

Did you ever wonder why a tightrope
walker uses a long pole? The pole
lowers the centre of gravity, thereby
contributing to stability.
Making Structures Strong

Many structures have common features
that have been proven to strengthen
structures:

Beam: any level structure designed to
support a load.
Making Structures Strong Beams
There are several ways to make a beam
stronger for its purpose:
 1. material: steel is stronger than wood
but heavier – engineer must consider
the purpose of the beam
 2. I-Beams: a beam that is in the shape
of a letter I is much stronger and lighter

Making Structures Strong Beams

3. Corrugation: multiple folds in a
material that provide additional strength
(think cardboard boxes!)
Making Structures Strong Beams

4. Rebar: steel
reinforcing rods are
placed in concrete
as they can
withstand tension
much better than
concrete on its own.
“Reinforced
concrete” is able to
resist both
compression and
tension.
Making Structures Strong Beams

Cantilever: a beam supported at only
one end.
This is a
cantilever draw
bridge.
Making Structures Strong Beams
Cantilevers are useful in spanning great
distances without the use of a central
support.
 This cantilever is on the Observation
Tower over the American Falls at NF:

Making Structures Strong Beams

Supporting the beam:
 How a beam is supported is also important.
 Tie:

 Gusset:
Strut:
Making Structures Strong – Other
Shapes

Designers sometimes want to use
shapes other than beams to make
structures stronger and more interesting
looking. They can do this by adding
triangles (trusses) or curves (arches and
domes).
Making Structures Strong – Other
Shapes

Truss: a network of beams that form
triangles. It can be used as a bridge or
a cantilever and for many other things.
Look at the next
few slides. What
shape is used is
used to
strengthen the
structure?
Making Structures Strong – Other
Shapes
The Arch: a curved structure used to
span a space while supporting a load
 The curved design transfers
compression force downward so they
are strong as well as aesthetically
pleasing.

Making Structures Strong – Other
Shapes

The Dome: a shell structure that looks
like the top half of a sphere. It directs
compression downwards in many
planes. This is better than an arch
which directs compression downwards
in one plane.
The Superdome in New Orleans
Does this dome look familiar?
Structural Failure

Despite all of the scientific knowledge
available, structural failure sometimes
still occur.

Documentary on how the NYC twin
towers came down structurally:
http://www.youtube.com/watch?v=mcaz
6N75mjM (44 min)
Structural Failure
Destroyed in Seconds:
 Portugal: (2:30min)
http://www.youtube.com/watch?v=GXS_
IkhgStk
 In Manilla, Philippines: (2:06)
http://www.youtube.com/watch?v=MMC
4MpoGIhU

Structural Failure

Of course the famous Tacoma Narrows
Bridge Collapse: (4:02)
http://www.youtube.com/watch?v=3mclp
9QmCGs
This is the new bridge:
 (Located in the state of
 Washington, USA)
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Structural Failure
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Of course, structural
failure is not just
limited to buildings
and bridges....
Collapsed dome in Malaysia.
Structural Failure

Definition of structural failure: the
failure of a structure as a result of the
structure, or part of the structure, losing
the ability to support a load.

There are many reasons why a structure
might fail. Read through your text
pages 306 – 309 and take notes about
the top 4 reasons for structural failure.