Function - What is the structure supposed to do? What was
it designed for?
Most structures have several functions, which may include:
· supporting (its own weight)
· containing (substances)
Precise, measurable standards normally are indicated in the specifications the structure must
comply with in order to perform its function/s.
Aesthetics - is the study of
beauty in nature.
· The best designs usually 'look good' - 'aesthetically pleasing'
· The aesthetics are usually accomplished by the shape, texture, color, type of material, symmetry and simplicity
of the repeated pattern used in the design.
Safety - all structures are designed and built within an
acceptable margin of safety (but usually, structures are designed with a built-in large margin of safety).
Cost - adding extra strength to a structure costs money,
as well as using more highly skilled workers and better materials does.
· Designers plan their structures to withstand conditions they hypothesize will
occur. Good design is a compromise between a reasonable margin of safety and reasonable cost.
· Usually, totally unexpected events will cause even the best (well-designed) structures to fail (example:
the World Trade Centre Towers).
Materials - the properties or characteristics of different materials must match the purpose
of the structure.
There are different kinds of strength
- tension (pulling) ?. steel rods
- compression (pushing) ?. concrete
To enable the structure to withstand both types of forces acting on it, a composite material is used - reinforced concrete (concrete poured over steel rebar (rods).
Layers of different materials (Tetra Pak) are pressed and glued together, combining the properties of the different
materials. The layers are often called laminations.
Woven or Knit Materials
Spinning or twisting, looping or knotting fibres together gives material added strength. A loom is used to weave
two or more pieces of yarn together in a criss-cross pattern to make cloth. Pressing, gluing, melting and dissolving
are also ways to combine materials to gain strength.
When choosing materials involves weighing advantages and disadvantages of the different materials ( higher quality,
stronger materials are usually more expensive)
Factors to consider:
- will inexpensive material you use allow the structure to perform its function over a reasonable time?
- is the appeal of the structure 'pleasing' over time?
- does the structure harm the environment?
- does the structure conserve energy?
Tough Tissue Test (p. 289)
Which Tissue would you buy?
How do Advertisers promote and sell the least effective Tissue to the consumer?
Joints - how do you fasten the structure together?
Mobile Joints are joints that allow movement
Rigid Joints do not allow movement
- Fasteners (nails, staples, bolts, screws, rivets
and dowels). Unfortunately, the holes made in the structure, by the fastener, actually weaken the structure. One
fastener allows movement when the parts are pushed or pulled, whereas, more than one will make a more rigid joint
- but, will also weaken it more.
- Interlocking shapes (like Lego) fit together because of their
shape. Dovetail joints in drawers, dental fillings and folded seams are some examples.
- Ties, like thread, string and rope, fasten things together.
- Adhesives, or sticky substances can also hold things together.
Thermosetting glues (hot glue) and solvent-based
glues (drying glue) strengthen the joint because of the bonds between the particles (like
epoxy resins). Even the strongest adhesives can fail under extreme
conditions and if the joint is stronger than the material it is joining, the material next to the joint can fail.
Adhesives can also be a health hazard (like Super Glue - which
dries very quickly when you use it - possibly bonding your skin if you touch it, or they can release harmful chemical
vapours as they harden.
- Melting - Pieces of metal or plastic can be melted together
(welding, soldering - brazing or using chemicals)
Post-It Notes - An accidental glue (that turned into a
huge success story). It did not meet the specifications, because it couldn't hold things together very well. (p.
Traditional Structures (Project - Making a Model)
Create a replica scale model of a Famous Bridge Structure or
choose from one of the Seven Wonders of the World. Directions for the accompanying research report - Use the questions
on p. 295. Add interest in the project by having them create their model as an 'Edible
Model'. (They can eat it after they have presented it.)
Topic 2 Review p. 296
WRAP-UP p. 297
>>>> A good review of Topics 1 - 2 in this Unit <<<