The Chartership Forum

Structural Steel Design / module 20

Checking, review and knowing what you do not know

The question the other nineteen modules assumed an answer to: how do you find out whether a design is right — and how does a design say what it cannot vouch for?

What you will understand

  • Check an answer by order of magnitude before checking it by arithmetic
  • Say what each level of independence can find, and what it is structurally blind to
  • Rank findings by consequence rather than by the size of the numerical error
  • Ask the review questions that find the most per minute spent
  • Read this course's own register of unverified values, and say why it is generated
  • State honestly what this course is and is not fit for

Lessons

  1. How to find an error

    Not by reading the calculation again. This lesson is about the three things that find most of what is findable, and about why checking has levels rather than a single state called 'checked'.

  2. What this course does not know

    The last lesson, and the one that turns the method back on the course itself. Every value it could not verify is listed — by the code, not by hand.

Module summary
  • Serious errors are wrong by factors, and factors are visible without a calculator
  • 62 kg/m² reported as 62 000 is over a hundred times outside the band; 108 is 1.2 times and merely unusual
  • Every level of checking has a structural blind spot — including independent design
  • A check inside the same model cannot question the model, however careful the checker
  • A 5% error at 98% utilisation crosses the resistance; a 50% error at 20% does not
  • The first three review questions find more than the rest, and none checks a number
  • All 47 design values have been read from the published standards and their UK National Annexes
  • A register you can trust is one that is generated — this one emptied itself, nobody edited it
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

Practice

A beam check was at 94% of resistance. A review finds the moment was under-estimated by 9%. What is the utilisation once corrected, as a percentage?

Practice

A brace check was at 25%. A review finds its force was under-estimated by 60%. What is the utilisation once corrected, as a percentage?

Practice

A steel tonnage is reported as 62 000 kg/m² for a multi-storey office, where the usual range runs to 90 kg/m². By what factor does it exceed the top of the band?

Practice

Of the 47 design values in the profile, 33 come from BS EN 1993-1-1 alone. What proportion of the profile is that, as a percentage?

Practice

A frame is reported with αcr = 3.75. The usual range for a braced frame starts at 10. By what factor does it fall below the band?