Module 1 · Lesson 1.2
Choosing the method
Nine ways to answer a structural question, and how to pick the cheapest one that actually answers it.
Why this matters
The most expensive mistake in computational engineering is not a wrong model. It is building a model at all when a two-line calculation would have answered the question — or building a linear one when the question was about stability.
This lesson is a decision, made explicitly, before any software is opened. It is also the decision least often written down, which is why the same argument gets had again three weeks later.
By the end of this lesson you should be able to
- List the nine routes to a structural answer and what each costs
- Match a question to the cheapest route that answers it
- Recognise when the answer is 'no model at all'
- Explain why the check on a model must be cheaper than the model
The nine routes
Roughly in order of cost, and cost here means your time plus the checker's time, not computer time — computer time has been the cheap part for thirty years.
1. Mental estimate. Span-over-twenty for a steel beam depth. A tonne per square metre for a building's floor load. These are not calculations; they are the sense of scale that lets you notice a wrong answer. An engineer without them is defenceless against their own software.
2. Hand calculation. One equation, one answer, on one sheet. Fast, transparent, checkable by anyone in five minutes. Limited to structures a formula exists for.
3. Spreadsheet. Repetitive arithmetic with the numbers visible. Excellent for a load take-down or a schedule of member checks. Poor at anything with a chain of dependency, for reasons Lesson 4.3 spends its length on.
4. Script. Repetition with logic, and a record of what was done. Worth writing when the task will recur, when the number of cases is large, or when a person doing it by hand would get bored — boredom being an excellent predictor of error.
5. Dedicated design software. A program that knows what a portal frame is. Fast and reliable inside its remit; misleading outside it, because it will still produce an answer.
6. General finite element analysis. Any geometry, at the price of building and checking every assumption yourself.
7. Specialist nonlinear analysis. When the behaviour is genuinely nonlinear — large displacement, yielding, contact, tension-only. Expensive in time and in the expertise needed to judge the result.
8. Optimisation. When the question is 'which of these' rather than 'is this adequate'. Modules 14 to 17.
9. No model at all. Sometimes the right answer is to change the structure so the question does not arise. A layout that avoids torsion needs no torsion analysis.
The rule
Use the cheapest route that genuinely answers the question, and make sure you can check it by a different route.
The second half is the one that gets dropped. A finite element model checked by another finite element model built by the same person from the same assumptions has been checked against nothing.
Try it
Which method answers this question?
Twelve engineering questions. Choose a route, then read what that route would and would not tell you.
Move through the twelve questions.
Your route
Choose the cheapest route you believe genuinely answers it.
Question 1. Is a 457 UB adequate for deflection on a 7.2 m simply supported span under 24 kN/m?
Choose a route to see what it would tell you. Nothing is revealed until you commit to an answer — deciding first is the skill being practised.
What this shows: The cheapest route that genuinely answers the question is the right route — and more than one answer is often defensible.
Worked example
Does this beam need a finite element model?
Given
- A simply supported steel beam, 7.2 m span
- Uniformly distributed load of 24 kN/m at the serviceability limit state
- Deflection limit of span/360
- No lateral restraint concerns: the beam is fully restrained by the slab
Find
The cheapest route that answers 'is a 457 × 191 × 67 UB adequate for deflection?'
Practice
A continuous three-span beam, spans 6 m, 8 m, 6 m, carries 18 kN/m. Estimate the hogging moment over the first internal support using the standard coefficient for a three-span beam under uniform load on all spans (take the coefficient as 0.100 wL² using the longer adjacent span). Give the answer in kN·m.
Check yourself
You need the peak stress at a re-entrant corner in a steel casting, to assess fatigue. Which route?
Check yourself
A question can be answered by a hand calculation, a frame model or a shell model. Which should be used?
Summary
- There are nine routes, and computer time is the cheapest ingredient in all of them
- State the question before choosing the route; a vague question gets an expensive answer
- The check must come by a different route from the answer
- 'No model at all' is sometimes the correct engineering choice
- An unnecessary model is a liability, not a spare
This is educational material. It uses simplified examples to teach principles, and must not be relied on for real design or safety-critical decisions. Module overview and checkpoint