Key Idea: To analyse a structure you follow the load: what acts on it, which members carry it, and how it reaches the ground. Structures come in three kinds — frame, shell and solid — and five forces act in them: tension, compression, shear, bending, torsion. A load is static (steady) or dynamic (changing), and the dynamic peak is what you design for. The stress-strain graph carries five separate facts: gradient is stiffness, the yield point is where stretch stops recovering, the peak is strength, the area is toughness, and the length shows ductility. A structure fails in strength, in stiffness or in stability — and two of those break nothing at all. Geometry beats material. Depth, span and section change stiffness far more than a different grade of the same stuff.
Paper 1 — multiple choice
- Name the force acting in a labelled member
- Classify a structure as frame, shell or solid
- Read a feature off a stress-strain graph
- Work out a safety factor from two loads
Paper 2 — analysing a product
- Follow the load path through a named product
- Explain a failure that broke nothing
- Recommend strengthening and give the mechanism
- Justify a safety factor from the uncertainties
Carried into the design project
- Criterion D — a structural claim must be tested, not asserted
- Your prototype's failure mode is evidence, so record it
- Ribs and section changes are the cheapest improvements you have
Every Paper 2 structural question is the same four steps. Doing them in order turns a blank page into an answer.
| Step | The question you ask | What it gets you |
|---|---|---|
| 1. The loads | What acts on it, how big, and does it change with time? | Static self-weight plus a dynamic peak that is several times larger |
| 2. The path | Which member takes it next, all the way to the ground? | The members that matter — and the ones carrying almost nothing |
| 3. The forces | Is each member pulled, squashed, cut, bent or twisted? | Tension, compression, shear, bending, torsion, located on real parts |
| 4. The failure | Does it break, sag too far, buckle or tip over? | Strength, stiffness or stability — and only one of them breaks anything |
The five forces drawn as what they do to a member. Locate each one on a real part and the marks follow.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
Ask what carries the load. Discrete members meeting at joints — a frame. Cut one member and the path breaks. A curved or folded surface — a shell. Cut a hole and it loses most of its stiffness. Bulk material — a solid. It barely notices a small hole. Most products are a mixture. A bicycle is a frame with shell wheels and solid bearings, and saying so is a better answer than choosing one.
| Feature of the graph | What it is | What a designer does with it |
|---|---|---|
| Gradient of the straight part | Young's modulus — stiffness | Predicts deflection. Steep means it barely moves under load |
| Yield point | Where stretch stops recovering | The load after which the product is permanently changed and must be retired |
| Peak of the curve | Ultimate tensile strength | The breaking load on the label |
| Area under the curve | Toughness — energy absorbed | Decides whether it survives an impact, which is an energy problem |
| Length along the strain axis | Ductility | Decides whether it deforms visibly first or shatters with no warning |
Five separate facts on one graph. Exam questions ask for one of them at a time.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
Important: Strength vs stiffness — the peak is strength, the gradient is stiffness. A stronger grade of plastic is almost never stiffer. Strength vs toughness — the peak is strength, the AREA is toughness. Glass is strong and shatters; a helmet liner is weak and saves a head. Breaking vs failing — a shelf that sags 40 mm has failed, and nothing is broken. A bookcase that tips over has failed, and nothing is broken.
Every one of these adds stiffness without a different material — which is what makes them the cheap fixes.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
A school replaces its timber music-room shelving with a wall-mounted steel system. Analyse the new shelving as a structure.
🔒 Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
A designer doubles the depth of a timber shelf instead of switching to a denser hardwood. Explain why this is the better decision.
🔒 Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
A manufacturer specifies a safety factor of 5 for a rope swing sold for garden use. Justify this figure.
🔒 Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
A structure sags badly but nothing breaks. Has it failed?
Why does a slender column fail below its crushing strength?
What does the area under a stress-strain curve tell you?
What is the difference between a static and a dynamic load?
Why does a corrugated sheet beat a flat one of the same mass?
When is a safety factor of 1 acceptable?
Exam tips
- Answer structural questions by following the load: loads, path, forces, failure. In that order, every time.
- Locate each force on a named part. Listing the five force names earns nothing on its own.
- Keep strength, stiffness and toughness apart: the peak, the gradient and the area are three different facts on one graph.
- Two of the three failure modes break nothing. An answer that only discusses breaking has missed two thirds of the topic.
- Reach for geometry before material: depth, span, section and triangulation change stiffness far more, for far less.
- Treat a safety factor as a measure of uncertainty. Justify it from what is not known about the load and about the strength.