The big idea: Every product that holds a shape against a load has a structure inside it, whether or not anybody drew one.
Analysing it means modelling the forces: what acts on it, which members carry them, what each member is doing, and where it would fail first.
The five forces, located on real parts. Naming them is only worth a mark once they are attached to something.
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Five steps, on any product
Simplify it to members and joints
Draw it as lines and points. A chair back becomes two uprights and a rail; a shelf becomes a beam on two supports.
Put the real loads on
Mass, where it acts, and whether it arrives suddenly. A user leaning back puts a horizontal load on a chair that nobody drew.
Follow the load to the ground
Member by member. Any member the path does not use is carrying nothing and is there for another reason.
Name what each member does
Tension, compression, shear, bending or torsion — and remember that a compression member can buckle long before it crushes.
Say where it fails first
Usually a joint, a fixing, or the most slender compression member — rarely the middle of a solid part.
Model it physically as well: A card, foam-board or 3D printed model loaded by hand tells you in minutes what a drawing cannot.
For the design project that model IS the evidence: a photograph of the model failing, with the load written beside it, is worth more than a paragraph asserting that the frame is strong enough.
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| What the analysis shows | The strengthening | Why it works |
|---|---|---|
| A rectangle that folds into a parallelogram | Triangulate it — a diagonal, or a fixed back panel | A triangle cannot change shape, so the mechanism disappears; it is the cheapest strengthening there is |
| A member that sags under bending | Increase the DEPTH, add a lip or a rib, or box the section | Bending stiffness rises with the cube of depth, so doubling depth is worth about eight times the thickness |
| A long span deflecting | Add a support in the middle | Deflection falls with the fourth power of span, so halving the span is worth sixteen times |
| A slender column bowing | Add a stretcher or rail part-way down | It halves the effective length, making the column about four times harder to buckle, for almost no material |
| A joint working loose or tearing out | Spread the load — a bigger washer, a backing plate, more fixings, a gusset | The material was never the problem; the stress concentration at one point was |
Reach for geometry before material: A stronger grade of the same material usually has almost the same stiffness, so it changes deflection very little.
Depth, span, section and triangulation change it enormously — and for far less mass and cost.
How this is tested — analysing and modelling forces in existing products, and suggesting strengthening. It comes up two ways:
Paper 1 — multiple choice
- Identify the force acting in a labelled member.
- Choose the strengthening that suits a described failure.
Paper 2 — analysing a product
- Analyse the structure of a named product.
- Suggest and justify strengthening for a product that has failed.
The trap: Recommending a thicker or stronger material. Name the arm, the span, the depth or the triangle you are changing, and say what that does.
A folding camping chair collapses sideways when a user leans back. Analyse the structure and recommend two changes.
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