To read a diagram, name each part, say what it does, then state the principle the whole diagram shows. Most questions ask for that principle.
This lesson is part of SPM subjects and learning resources. It prepares for supporting a design explanation with evidence.
What is the label, function, principle routine?
Go through the diagram in three passes.
- Label. Read every label and title. Note what sits above, below and beside what.
- Function. For each part, write a few words on what it does.
- Principle. Ask what the parts achieve together.
Write your answer from the third pass, and use the second pass as evidence.
Worked example: an original retaining wall diagram
Described in words so you can sketch it: a concrete wall stands upright, holding back soil on one side. Behind the wall there is a layer of gravel. Near the bottom, small pipes called weep holes pass through the wall. A wide base, called the footing, extends under the wall.
Question: Explain how the wall is designed to stay stable. (4 marks)
Pass 1 and 2.
| Label | Function |
|---|---|
| Wall | Holds back the soil |
| Gravel layer | Lets water drain down instead of building up |
| Weep holes | Let the drained water escape through the wall |
| Footing | Spreads the load over a wider area of ground |
Pass 3: principle.
The wall is stable because it resists both soil pressure and water pressure. The gravel and weep holes let water drain away, so water does not build up behind the wall and push on it. The wide footing spreads the weight, so the ground below can carry it without sinking.
The answer uses the labels as evidence and ends with the principle.
The mistake that costs marks
A common answer only repeats the labels: “The diagram shows a wall, gravel, weep holes and a footing.” The marker can already see that.
| Weak | Stronger |
|---|---|
| The diagram has weep holes | Weep holes let water escape, so pressure behind the wall stays low |
| The footing is wide | The wide footing spreads the load over more ground |
Attach a function to each label. If someone who had only read the labels could write the sentence, it is a description, not an explanation.
Turning the diagram into sentences
Use a short frame for each part: “The [label] [does what], so [effect].” Keep each sentence to one part and one effect.
The frame also keeps you inside the diagram. Do not add things the labels do not show, such as a soil type that was never named.
Check yourself
A diagram shows a simple roof truss with a horizontal bottom member, two sloping top members and diagonal members between them. Explain why the diagonal members are included.
Answer
The diagonal members divide the frame into triangles. A triangle cannot change shape without one of its sides changing length, so the frame resists being pushed out of shape by loads.
Each diagonal carries force along its length, either pulling or pushing, and passes it to the main members. The triangles keep the truss rigid while using little material.
What to study next
Try the original mixed practice, then see explaining a process in a clear sequence for questions about stages, not parts.
For a teacher to go through your own diagrams, see online one-to-one Civil Engineering Studies tuition.