Good design, materials, and
construction make structures stable and strong
·
Stability and strength depends on a structure’s
material as well as the fasteners used (bolts, welds, wire, thread, glue, etc.)
·
Stability:
the ability of a structure to maintain or resume its position when an external
force has been applied to it.
Structural Strength
·
Some structures have stood for thousands of
years e.g., the Coliseum in Rome, the Pyramids in Egypt
Structural Shapes
·
Some structural strength comes from the shapes
used in its design
·
Triangles are stronger than squares and
rectangles
·
Triangular prisms are stronger than square and
rectangular prisms
Structural Components
·
Arches, beams and columns are common structural
components that are used often because they can add strength and are attractive
·
The components can be used alone or in
combination e.g., arches and columns
·
Please complete the structural
components worksheet
Structural Materials
·
It is important to choose appropriate materials
when designing and building structures; designers should consider strength,
attractiveness, cost, etc.
Centre of Gravity
·
Centre of gravity: the point at which a body’s
mass is concentrated – the body is equally balanced in all directions at this
point
o
For example, when you balance a ruler on your
finger, the centre of gravity is the middle of the ruler because each side of
the ruler is symmetrical
·
Every structure has a centre of gravity; the
location of the centre of gravity helps determine how stable the structure is
o
For example, a stool is a stable structure;
however when a person sits on the stool, the centre of gravity is higher so the
stool is more likely to tip over
Stability
·
Stability depends on materials, construction
techniques and centre of gravity
o
E.g., a table can have a high centre of gravity,
but it can be stable if it has four legs far apart
·
Form can also affect stability; a solid
structure with a high centre of gravity may be less stable than a frame table
is
·
Some structures are designed to be unstable;
e.g., front ends of cars are meant to collapse easily in a collision
When Things Go Wrong
Structural Stress and
Fatigue
·
Poorly built structures may not be able to
withstand forces
·
Large internal and external forces may weaken
the structure
·
This can result in structural stress
·
A bend in a shelf is an example of this stress;
the shelf may go back to its original shape when the load is removed
·
Permanent changes occur when the shelf cannot
withstand the stress; e.g., cracking. This is called structural fatigue.
Structural Failure
·
Ignoring structural fatigue can lead to structural failure; this is the
breakdown of a structure due to the internal and external forces acting on it
·
Structures often show signs of structural
fatigue by bending and cracking before finally failing and collapsing.
Product Recalls
·
Public recall of seriously flawed products sold
to consumers by manufacturers
·
Examples:
o
high levels of paint in children’s toys
o
choking hazards in products for children
o
overheating batteries, poor safety features
o
cars with faulty parts or design
