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.