Design factors
Before drawing a structure, a designer weighs these design factors:
- Function: what job must it do (carry cars, shelter people)?
- Strength: it must carry its own weight and the load without breaking.
- Stability: it must not tip, slide or sink.
- Cost: materials, labour and upkeep.
- Looks: shape, colour and how it fits the place.
- Environment: use of local, recycled or low-energy materials, and effect on nature.
These can conflict: a stronger beam costs more. The designer finds the best balance.
Design steps
The usual steps are:
- Need: find the problem and write what the structure must do.
- Sketch several ideas and choose one.
- Build a model (card, straws, clay or a computer model).
- Test it: load it and watch where it bends or fails.
- Improve: add a brace, change a shape, then test again.
The steps form a loop. Models are cheap and failures are cheap, so test early.
Natural structures
Nature has had millions of years to improve its structures.
- Honeycomb: six-sided cells use the least wax and leave no gaps.
- Egg shell: a thin curved shell spreads a push over its whole surface, so it is strong against pressure from outside.
- Tree: wide at the base where the load is largest and thin at the top; roots anchor it.
- Bone and bamboo: hollow tubes are strong for their weight.
Designers copy these ideas; this is called learning from nature.
Architectural structures
Builders use a few proven shapes:
- Arch: turns a downward load into a push along the curve to the supports. Works well with stone and brick that resist squeezing.
- Dome: an arch turned all the way round; covers a large space with no pillars.
- Truss: bars joined in triangles. A triangle cannot change shape, so thin bars carry big loads.
- Beam: a flat piece on two supports; the middle bends, with squeezing on top and stretching below.
Look at the free-play step to compare beam, arch and truss bridges.
Appreciating a structure
To appreciate a structure means to look at it carefully and judge it fairly. Ask:
- Function: what job does it do and does it do it well?
- Structure: what shape holds it up, and where are the loads and forces?
- Material: what is it made of and why is that a good choice?
- Setting: does it suit the place, the people and the climate, and does it look good?
Write a short note giving one good point and one idea to improve.
Try it
Build a bridge from one sheet of paper that spans 15 cm between two books. Test it with coins. Then fold it into a zigzag or a tube and test again. Write which factor you improved. Look for an arch, a truss or a dome on your way to school and answer the four appreciation questions.
Key formulas and definitions
- Six factors: Function, Strength, Stability, Cost, Looks, Environment
- Design loop: Need → Sketch → Model → Test → Improve
- Triangle = rigid; four-sided frame = can change shape
- Arch and dome: load becomes compression along the curve
- Beam: top in compression, bottom in tension
Worked examples
1. A village wants a footbridge over a 6 m stream with a small budget. Which factors matter most?
Cost and function (safe crossing) matter most, then strength and stability. Local timber or bamboo may suit the environment. A simple beam or small truss is likely.
2. Why is a honeycomb made of six-sided cells and not round ones?
Hexagons fit together with no gaps and use less wall material than separate circles. Round cells would leave empty spaces between them.
3. A student tests a model bridge and it bends in the middle. Name two ways to improve it.
Add triangles (a truss) or deepen the beam, or shape it as an arch. Then test again with the same load.
Common mistakes
- Thinking only strength matters. A structure can be strong but too costly, ugly or unsafe for the environment.
- Skipping the model and test steps.
- Believing a four-sided frame is rigid. Only a triangle is.
- Describing a structure without saying what job it does.