A design explanation has three parts: the choice, the evidence and the reason. Evidence turns the choice from a guess into a conclusion.
This lesson is part of SPM subjects and learning resources. It uses unit skills from checking units in a classroom calculation.
What is the choice, evidence, reason pattern?
- Choice. State what is chosen, such as a wider footing.
- Evidence. Give a figure, property or labelled feature.
- Reason. Say why the evidence justifies the choice.
The pattern works for both calculation and diagram questions.
Worked example: choosing a footing size
An original classroom problem.
A column carries a load of 120 kN. The soil can safely carry 60 kN/m². A student proposes a square footing of 1.0 m by 1.0 m. Should it be changed?
Evidence: the current choice. Area = 1.0 × 1.0 = 1.0 m². Pressure = load ÷ area = 120 ÷ 1.0 = 120 kN/m².
That is twice the soil capacity of 60 kN/m², so the footing is too small.
Minimum area. Area needed = 120 ÷ 60 = 2.0 m².
New choice. A square footing of 1.5 m by 1.5 m has area 1.5 × 1.5 = 2.25 m². Pressure = 120 ÷ 2.25 = 53.3 kN/m², which is below 60 kN/m².
Reason. The wider footing spreads the same load over more area, so the pressure on the soil drops below its capacity. A 1.5 m square footing is adequate for this classroom problem.
The mistake that costs marks
A weak answer looks like this: “The footing should be bigger because it is safer.” It gives a choice and no evidence.
| Part | Weak | Stronger |
|---|---|---|
| Choice | Bigger footing | 1.5 m by 1.5 m |
| Evidence | (missing) | 120 ÷ 1.0 = 120 kN/m² against a capacity of 60 |
| Reason | Safer | Larger area lowers pressure below capacity |
Another slip is to compare a load in kN with a capacity in kN/m². These are different quantities, so turn the load into pressure first.
A note on scope
The calculation above is a classroom exercise with given values. Real foundation design uses site investigation, safety factors and rules set out in standards. Your syllabus and your school teacher decide how much detail an assessed answer needs.
Check yourself
A slab must carry a pressure no greater than 25 kN/m². The load is 50 kN. What is the smallest area, and does a 1.2 m by 1.5 m slab meet it?
Answer
Area needed = 50 ÷ 25 = 2.0 m².
The proposed slab has area 1.2 × 1.5 = 1.8 m², which is less than 2.0 m², so it does not meet the requirement.
Check: pressure = 50 ÷ 1.8 = 27.8 kN/m², which is above 25 kN/m². The slab is too small.
What to study next
Practise with the original mixed practice. For the reading skills behind design questions, see interpreting information in a technical diagram.
For a teacher to check your design explanations, see online one-to-one Civil Engineering Studies tuition.