The Chartership Forum

Computational Engineering / module 6

From physical structure to analytical model

Abstraction as an engineering decision — what each level keeps, what it throws away, and what it may then be asked.

What you will understand

  • Describe abstraction as a choice about which behaviours to discard
  • Compute what a line model discards, rather than asserting it is small
  • Use Saint-Venant's principle as the licence to simplify locally
  • Match a question to the cheapest level of model that answers it honestly
  • Estimate the cost of each level before building it
  • Write a model-purpose statement that makes the model reviewable

Lessons

  1. What abstraction discards

    Comparing bending and shear deflection in a cantilever model.

  2. Choosing the level

    Matching the question to the cheapest model that can answer it — and pricing the alternatives first.

  3. The model-purpose statement

    Five lines that make a model reviewable — and the order they matter in.

Module summary
  • Abstraction is not simplification for convenience; it is a decision about what to discard
  • A line model discards shear deformation — 15 % of the answer at L/d = 2 and 0.3 % at L/d = 16
  • The span/depth ratio at which that matters is computable, not a rule of thumb
  • Saint-Venant's principle is the licence to simplify locally: the disturbance is gone within about a depth
  • Every level answers some questions and cannot answer others — and will still return a number
  • The cost between a line model and a shell model is a factor of over a hundred
  • A model without a stated question will eventually be asked the wrong one
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

A frame model of a building is asked for the stress at a beam cope. What happens?

Practice

A member has a bending deflection of 4.20 mm and a shear deflection of 0.55 mm. What percentage of the true deflection does an Euler–Bernoulli line model omit?

Practice

A shell model has 111 432 degrees of freedom at a 0.5 m mesh. Approximately how many would it have at a 0.25 m mesh, assuming the frame nodes are negligible?