The big idea: The three types are three ways of carrying a load, not three shapes.
A frame carries it along slender members. A shell carries it in a thin curved surface. A solid carries it through its bulk. Everything else about them follows from that.
Each type drawn with its load, its strengths and its weaknesses — then the step showing that real products combine all three.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
Free preview
This is the free notes preview
You're reading the free notes. Aimnova Pro unlocks the full study experience — and you can try it with your first topic free to keep:
- FlashcardsLock in vocabulary and key terms with spaced repetition.
- Practice questionsAnswer exam-style questions and get instant AI marking.
- Mock exams & past-paper vaultSit full mocks and see exactly how examiners award marks.
- Personalised study planA daily plan built around your exam date and weak areas.
| Frame | Shell | Solid | |
|---|---|---|---|
| Load path | Along members in tension or compression, meeting at joints | Spread through a thin curved surface | Through the bulk, mostly in compression |
| Strengths | Very light for its stiffness; open, so services pass through; repairable member by member | Enormous stiffness for very little material, and it encloses a space at the same time | Simple, very strong in compression, durable, no joints at all |
| Weaknesses | Every joint is a weak point, and it needs bracing or it folds | A dent or a hole destroys the curvature doing the work; hard to repair | Heavy, wasteful of material, weak in tension unless reinforced |
| Examples | Bicycle frame, roof truss, scaffold tower, pylon, bird skeleton | Car body, hard hat, eggshell, fuselage, kettle body | Dam, brick wall, anvil, bridge pier, tree trunk |
How to tell them apart on a real product: Ask what would happen if you cut a hole in it.
Cut a hole in a frame and you lose one member, which bracing may cover. Cut one in a shell and it loses most of its stiffness, because the surface was the structure. Cut one in a solid and almost nothing changes.
Study smarter, not longer
Most students waste 40% of study time on topics they already know. Our AI tracks your progress and optimizes every minute.
Almost no real product is purely one type. The useful skill is naming which PART of a product is which, and saying why that type suits that part — which is exactly what a Paper 2 question asks for.
| Product | Frame part | Shell part | Solid part |
|---|---|---|---|
| A car | The subframe carrying the engine and suspension | The monocoque body, which is the main structure | Brake discs, hubs, the crankshaft |
| A bicycle | The frame itself — triangulated tubes | The helmet, and a deep-section rim | Bearing races, pedal spindles, the crank |
| A kettle | The bracket carrying the handle | The body, which is the structure and the container | The element boss and the base moulding |
| A building | The steel or timber frame | A roof shell, a curved facade panel | Foundations, piers, a masonry core |
A shell is only a shell while it is curved: A flat panel is not a shell — it carries load in bending, which is the weak way.
Curvature, a fold, a dish or a rib is what converts bending into in-plane forces, and it is why a flat car door skin is pressed with a subtle curve it does not need to look right.
How this is tested — frame, shell and solid structures and how they are used in products. It comes up two ways:
Paper 1 — multiple choice
- Classify a named part as frame, shell or solid.
- Identify a weakness of a stated structure type.
Paper 2 — analysing a product
- Analyse a named product's structure types and justify each.
- Explain why a shell suits one part and a frame another.
The trap: Classifying the whole product. Real products use all three, and the marks are for naming which part is which and why that type suits that job.
Analyse the structure of a bicycle, identifying frame, shell and solid elements and justifying each.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.