1. Describing Structures
Grade 7 Unit 4 Topic 2
Describing Structures

2. The Structure’s Function
Function is what the object or structure is supposed to do.
Most structures have several functions.
One very important function of any structure is to support its own weight.

3. The Structure’s Function-cont.
Structures do more than just support loads. They also:
Contain
Transport
Shelter
Lift
Fasten
Separate
Communicate
Break
hold

4. The Structure’s Function-cont.
Designers have a hard time creating structures that perform all of their functions equally well.
In order to guarantee that a structure can perform its functions, designers work to a set of specifications that give precise, measurable standards that their structure must meet.

5. The Structure’s Aesthetics
Aesthetics: A branch of philosophy that studies the principles of beauty; the properties of an object that make it pleasing to the senses.
The structure’s aesthetic quality is very important, since the best designs look good.

6. The Structure’s Aesthetics-cont.
The structure’s aesthetic appeal does not only require the designers to carefully choose
materials
methods used to make a structure
but also the structures:
shapes and arrangement
textures
colours

7. The Structure’s Aesthetics-cont.
Above all engineers and architects try to keep their designs simple.
More often then not, clean designs look better than over-complicated, busy ones.
Designers therefore adhere to the KISS principle.

8. Designing for Safety
Margin of safety: the need for something built or manufactured to perform as expected for a long time, so that people’s safety and health are not at risk. In a structure, a margin of safety would ensure that the structure has extra strength to support more load than normal.

9. Designing for Safety-cont.
Almost all structures are built with a margin of safety.
An example of margin of safety would be how roofs in Canada are designed to support enormous weights, so that large amounts of accumulated snow will not collapse the roof.

10. Balancing Safety with Cost
Making structures stronger usually makes them more expensive.
Adding material, using stronger materials, and / or using skilled craftsmen are all factors that will increase the strength of a structure, but will also increase the cost.

11. Balancing Safety with Cost-cont.
While designers strive to allow a comfortable safety margin for any conditions they can imagine, there are occasions where things may happen to a structure that are totally unexpected.
Even well-designed structures can fail due to unexpected conditions.

12. Materials
Choosing which building materials to use in a structure is another important design decision.
The properties or characteristics of a material must match the purpose of the structure.

13. Materials-cont.
Properties: the characteristics of materials; every material has it own unique set of properties; examples of properties include:
Colour
Odour
Density

14. Materials-cont.
Combinations of different materials are often required to give a structure the properties it needs.
Three common methods of creating combination materials are:
Composite
Layering
Weaving or Knitting

15. Materials-cont. 1. Composite Materials
-Composite: of materials, made up of several different materials, with different properties, to fulfill a specific purpose.
-Different composites have different properties based on what they are made of and how they are made.

16. Materials-cont. 1. Composite Materials-cont.
-Reinforced concrete is a composite material made up of steel rods and concrete. Where the steel rods support strong tension (pulling) forces, and the concrete supports strong compression (pushing) forces.
-Other composites include fibreglass and plastic molded into boat hulls, and plastic with nylon mesh used in garden hoses.

17. Materials-cont. 2. Layered Materials:
-Layers of different materials, pressed and glued together, often produce useful combinations of properties.
-Lamination: a process in which a layer of material is pressed or glued onto other layers. Examples include car windshields, drywall, linoleum, and plywood.

18. Materials-cont. 3. Woven and Knit Materials:
-Hair-like fibres are spun (twisted together) into long, thin strings called yarn are then looped and knotted together to make knit materials, or woven together in a crisscross pattern using a loom.
-Knit materials stretch in all directions so they fit well over complex shapes.

19. Materials-cont. 3. Woven and Knit Materials-cont.:
-Weaving and knitting are not the only ways to make flexible materials.
Paper and felt are pressed and matted together.
Aluminum foil and plastic wrap are melted and dissolved.
-No matter how they are made, materials that can be folded or rolled are extremely useful, especially for lightweight structures that must be moved and / or stored.

20. Choosing Materials
Choosing one material over another means balancing the advantages and disadvantages of each possible choice.
Stronger materials are often more expensive.

21. Choosing Materials-cont.
To pick the most suitable materials for a structure, architects, engineers, and designers will usually consider the following 4 factors:
Cost:
Appearance
Environmental Impact
Energy Efficiency

22. Choosing Materials-cont.
Cost:
The lowest cost materials may not always be best.
They can be poor quality, or hard to work with
They could wear out quicker or require more maintenance.
There are times; however, where the least expensive material could do an acceptable job.

23. Choosing Materials-cont.
Appearance:
The lifespan of a structure is often considered when deciding on which materials to use.
Structures like bridges and buildings last a long time and so the materials they are made from need to remain attractive and strong over time.
Structures with short lifespans, such as cardboard boxes, need not be made of materials that weather well over time.

24. Choosing Materials-cont.
Environmental Impact:
Where the material comes from - is it recycled, a renewable resource, or a non-renewable resource?
How it is made - does it require processing that is damaging to the environment?
How it is put together – does it require harmful chemicals or materials to assemble?

25. Choosing Materials-cont.
Energy Efficiency:
The cost of many structures includes more than just the cost of materials and construction.
Once a structure is completed the energy it requires to operate is a function of the materials used to build it.
Insulating materials in homes, refrigerators, and freezers all impact the cost of operation.
The materials used in a furnace impact its ability to transfer heat.

26. Joints
Decisions about how to fasten structures together are critical because structures are often weakest where their parts are joined together.
There are two main types of joints:
Mobile joints
Rigid joints

27. Joints-cont. Mobile Joints:
Mobile Joint: a joint that is designed to allow movement; examples of a mobile joint include a door hinge and an elbow.
Their complicated parts are tricky to make, and they must be coated with a lubricant so that they move smoothly.

28. Joints-cont. Rigid Joints:
Rigid Joint: a device designed to fix an object into place; a joint that allows no movement; examples of a rigid joint include a nail and a screw.
Most rigid joints fit into 5 categories:
Fasteners
Ties
Interlocking shapes
Adhesives
Melted joints

29. Joints-cont. Rigid Joints-cont.: Fasteners:
Examples of fasteners include: nails, staples, bolts, screws, rivets, and dowels.
A major problem with fasteners is that the holes they make weaken the materials they fasten.
Nails and staples are usually forced in the materials, which can also cause cracking and separating of the material.
Bolt, screw, and dowel holes are often predrilled which doesn’t weaken the materials as much but is more time-consuming

30. Joints-cont. Rigid Joints-cont.: Interlocking Shapes:
Are carefully shaped parts, in rigid and flexible materials, that can hold themselves together.
Examples of interlocking shapes include: Lego, dovetail joints, folded seams in sheet metal, and hems in clothing.

31. Joints-cont. Rigid Joints-cont.: Ties:
Thread, string, and rope are all examples of materials used to tie material together.
Tying material together may, simply, be done by hand or it may take special equipment like a sewing machine where the needle and bobbin thread are intertwined.

32. Joints-cont. Rigid joints-cont.: Adhesives:
Adhesive: a sticky substance, such as glue or epoxy cement, that is used to hold objects or materials together.
Adhesives increase the bonding surface area and the strength of the bond by flowing into tiny rough areas on the surface of the pieces it joins.

33. Joints-cont. Rigid Joints-cont.: Adhesives-cont.:
When glue hardens it locks the pieces together.
Thermosetting glues: like those found in glue guns, harden when cooled.
Solvent-based glues: harden as they dry out.
The strongest glues create a special kind of force between the smallest particles of the pieces being joined.

34. Joints-cont. Rigid Joints-cont.: Adhesives-cont.:
Even the strongest glued joints fail under extreme conditions.
If the glue is stronger than the material it is bonding, the material next to the joint may break.
Some adhesives can be a health hazard because the bond as soon as they touch moisture. (Crazy gluing body parts together)
Some adhesives can be a health hazard because of the powerful chemicals they release as they harden.

35. Joints-cont. Rigid Joints-cont.: Melting:
Pieces of metal or plastic can be melted together.
Welding: a process in which pieces of metal or plastic are fused together by the application of heat.
Soldering: a process in which a melted material is applied to a different type of material; the melted material hardens when it cools, forming a rigid joint that hold the other material in place.

36. Joints-cont. Rigid Joints-cont.: Melting-cont.:
To increase the strength of the soldered joint the pieces to be joined may be twisted or folded together.
When soldering the pieces must be cleaned before joining, and the melted material must be cooled slowly and carefully to avoid brittle or weak joints.
There are many ways to weld joints, including: torches, an electric spark, strong chemicals, and even sound waves.