Section 5.1 Notes

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

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

 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

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

      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 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:

·        high levels of paint in children’s toys

·        choking hazards in products for children

·        overheating batteries, poor safety features

·        cars with faulty parts or design