The Chartership Forum

Computational Engineering / module 14

Sensitivity and design-space exploration

Which inputs the answer actually depends on — and why a wide plausible range beats a large exponent.

What you will understand

  • Distinguish an uncertain input from a design variable
  • Compute an elasticity and check it against the exponent in the algebra
  • Rank variables by swing rather than by elasticity, and say why
  • Bracket an answer with corner combinations, and say when that is invalid
  • Recognise a non-monotonic response and what it costs you
  • Justify a plausible range rather than assuming a percentage

Lessons

  1. What the answer depends on

    Elasticity, swing, and the ranking that surprises people the first time they compute it.

  2. Bracketing and its limits

    Three runs that bound an answer, the assumption they rest on, and what happens when it fails.

Module summary
  • A sensitivity study is the cheapest insurance a computational model can buy
  • Elasticity is dimensionless, so it compares variables with nothing else in common
  • Swing = elasticity × range, and it is the swing that matters
  • A tight input with a big exponent may matter less than a loose one with a small exponent
  • Corner combinations bracket the answer only if every response is monotonic
  • One-at-a-time studies cannot find interactions, and structures are full of them
  • A range without a stated reason is a guess wearing a number
Teaching basis and review status

These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.

Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.

Read the lesson register and outstanding review notes

Check your understanding

Check yourself

Which input should a sensitivity study direct the most effort at narrowing?

Practice

A beam's deflection goes as 1/d³ where d is the depth. The depth could be 400, 457 or 533 mm. Taking 457 as the base, by what factor does the deflection change if the 400 mm section is used instead? Give the factor to two decimal places.

Practice

An output has an elasticity of −0.5 with respect to mass. Mass could be 20 % higher than assumed. By what percentage does the output change? Give the magnitude of the change.