Science Tests and You
Volume 3 Issue 4
Structures: Form and Function
Chapter 4 - Designers consider the form and function of a structure and the forces that
act on it.
4.1 - Classification of Structures
- A structure can be classified by its structure.
- A structure can be classified by its function.
- A structure can be classified as a solid, frame, or shell structure.
Solid Structures
- Most solid structures are solid all the way through (dams, concrete barriers)
- They weigh more than a hollow structure made of the same material.
Frame Structures
- Are made of parts fastened together.
- The parts are often called structural components.
- A bicycle is an example of a frame structure, so is a spider web.
Shell Structure
- A strong hollow structure.
- They are very little material in their construction, but when constructed properly can be
very strong. (Eggs, igloos)
Combination Structures
- There are many structures that are a combination of frame, solid, and shell structures.
A house has solid parts (bricks) and frame structures (roof trusses, joists).
Strength - The ability of an object to withstand forces.
4.2 Forces that can act on a structure
- A force is any push or pull
- Forces act on structures
- Forces can be classified as external (wind, gravity), or internal.
- The magnitude of forces, their direction, and their point and plane of application
influence how they affect structures.
- Shear, tension, compression, and torsion are types of internal forces that can affect
structures.
An external force act on an object from outside the object. Gravity is an external force
that acts on all structures all the time.
Gravity is the natural force off attraction between two objects.
Other forces are caused by one part of the structure acting on another part of the
structure. This type of force is called an internal force. One example of this is the
weight of the roof that compresses down upon the walls of a building.
Three main points that describe how a force acts upon a structure: The forces
magnitude, its direction, and the points and plane of its application.
The point and plane of its application is the exact location where the force meets the
structure.
The magnitude describes how great the amount of force is.
The direction describes where the force is coming from. (wind)
The load describes the sum of the static and dynamic loads.
A static load is the effect of gravity on a structure.
A dynamic load describes the forces that move or change while acting on a structure.
So, think of a bridge - Gravity is pulling down on the bridge (this is the static load on the
bridge). However, a truck carrying concrete crosses the bridge (this is a dynamic load
because it is moving). Then there is a hurricane with 100kph winds (this is also a
dynamic load).
Types of internal forces:
Compression - A force that squeezes something together.
Shear - A force that pushes in opposite directions.
Tension - A forces that stretches apart to expand or lengthen.
Torsion - A force that twists.
5.3 Designing for safety
- The function of a structure dictates how strong it must be.
- A good design takes into account the function of the structure.
- Engineers ensure structural safety by using various design features.
No one can design a structure that can be 100% failure-proof. Bridges fail, roofs
collapse. However, engineers use techniques of risk management to reduce the risk of
failure. For example, a bridge that has supports in the water must be designed to
withstand a collision by a ship. However, one can also design a bridge that has an arch
with no supports in the water.
Designers often over-compensate for the various risks, and install backup systems such
as warning systems and sensors.
Engineers design for loads (you sit on a chair that can support your load), Safety (we
have building codes and fire codes (fire drill anyone?) and efficiency (if a bridge can
support the same amount of load, but uses less material).
A sensor is any device that can detect or measure real-world conditions. (thermostats,
heat, carbon monoxide, vibration sensors)
5.1 Stabilizing structures
- A structure is stable if forces are balanced
- Unbalanced forces can cause stress and fatigue on structures
- Proper materials can be used to stabilize structures.
- Building techniques can be used to stabilize structures.
Some of the strength of a structure lies in the shapes used in its design.
Squares and rectangles are not as strong as triangles.
Structural components add strength to a structure. They come in a variety of forms.
A beam is a flat structure supported at each end. If too much weight is put on a beam,
then it will bend in a u-shape.
An I-beam shape gives it strength. It has less weight than solid beams of the same
length.
A column is a solid structure can stand by itself.
A truss is a rigid framework of beams together. These are usually formed of
intersecting triangles.
A cantilever is a flat structure that is supported only at one end.
A girder or box beam are long beams in the shape of hollow rectangular prisms.
An arch is a curved structure that a support a lot of weight. The force of the weight is
carried along the sides down to its supports. This spreads out the effect of any load.
Corrugation is a sheet of metal or cardboard that is shaped into a series of pleats or
triangles.
An object’s centre of gravity is point at which a body’s mass is concentrated. The body
is equally balanced in all directions at this point.
Stability depends on materials and construction as well as an objects centre of gravity.
Some structures are designed to be unstable. (clown punching bag)
When a structure faces large amounts of internal and external forces, it experiences
structural stress and fatigue. If the forces continue it may fail. Signs of fatigue may be
cracks or chips in an object.
Structural failure occurs when there is a breakdown of a structure due to the internal
and external forces that have acted upon it.
When products are released to the public and they fail, manufacturers may conduct a
product recall to fix the problem and restore public confidence in the product.
5.2 Elements of design
- A good design takes in to account the function of its structure
- Good design considers the strength and stability needed by the structure.
- Symmetry us often used in good design.
- Ergonomic design of objects makes them easier to use.
Ergonomics is the science of designing equipment that people can use more efficiently
and safely. It minimizes the stress on the users body.
Symmetry is a balanced arrangement on opposite sides of a structure. This design
feature can be visually appealing. However, sometimes this feature may not appeal to
everyone.
A prototype is a model used to test and evaluate a design.
6.1 Determining Consumer Need
- Manufacturers try to determine need to make good decisions about products.
- Consumers make purchases based on needs and wants.
- Ergonomics may be a consideration in some purchases.
- Consumers can influence manufacturers to make good decisions.
A consumer is a buyer of things.
Manufacturers make things and are interested in what you think...so you’ll buy more.
Market research occurs when a manufacturer gather data about consumers and
buying habits.