Computational Engineering / module 12
Diagnosing computational models
The largest module in the course: what goes wrong, how each fault announces itself, and the discriminating check that separates it from the others.
What you will understand
- Run the four checks that should precede reading any result
- Trace a unit error to its source by comparing which outputs moved and which did not
- Distinguish a restraint problem from a release problem from a connectivity problem
- Recognise ill-conditioning and name its three usual causes
- Tell a converging mesh sequence from a singular one
- Diagnose a model by naming the discriminating check before naming the cause
Lessons
The checks that come first
Four things to do before reading any result, none of which takes longer than a minute.
Units, signs and magnitude
The most common serious error in structural modelling is a factor of a thousand — and the technique for finding which input it is in.
Restraints, releases and connectivity
Three faults that look alike in the output and need completely different fixes.
Conditioning and meshing
Where the precision goes, and how to tell a mesh that is converging from one that never will.
The Model Doctor
The method, applied: name the discriminating check before you name the cause.
Module summary
- Total loads against total reactions is the cheapest check in structural computing
- Determinate moments do not depend on stiffness — moment-versus-deflection localises an error
- A restraint joins the model to the world; a release describes what happens inside it
- The graphics show geometry, not connectivity
- Ill-conditioning comes from stiffness range, coordinate magnitude and element distortion
- A converging sequence settles; a singular one grows at a constant ratio per halving
- Nodal averaging deletes the mesh's own error estimate — turn it off to check
- Name the discriminating check before naming the diagnosis
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 notesCheck your understanding
Check yourself
A model's moments agree with hand calculation and its deflections are ten times too large. Where is the error?
Practice
A stress at a re-entrant corner reads 62, 85, 117 and 161 N/mm² over four successive halvings of the element size. What is the ratio between the last two values, to two decimal places?
Practice
A model applies 5 600 kN and reports 5 599.9994 kN of reaction. Express the residual as a fraction of the applied load, in scientific notation — give the coefficient when written as x × 10⁻⁷.