The Chartership Forum
Lesson review: expert verification remains open

This lesson has received a full-context editorial review and a check of its technical explanations. Selected arithmetic has been checked separately. This is not independent engineering sign-off.

Checks needed for project use

  • An independent structural reviewer must check the illustrated models, prediction feedback, geometry and full interactive input ranges.
  • Check sign conventions, extraction completeness and load case before diagnosing an equilibrium mismatch. Symmetry requires a unique stable response; bifurcation/nonlinear branches can break it. Solver verification is separate from model validation.
  • Some feedback still diagnoses omitted hinges/releases too categorically. Confirm diagrams and solver output locations; fixed-base columns can bend with pinned knees, and symmetry checks require a unique stable solution.

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

Module 17 · Course notes

Modelling and checking: is the result even possible?

An analysis is only as good as the model behind it. A program analyses the model entered, subject to its formulation, tolerances and numerical limits. The skill this module builds is the audit: a short list of independent checks that catch an impossible result before it becomes a design, no matter how cleanly it converged.

A model is a set of choices: which supports, which releases, how members connect, what element types. Each choice can be wrong, and a solver cannot tell you it solved the wrong structure. So every result is a claim to be tested, and the tests are the ones this whole course has built — read independently, not from the plotted numbers.

Start with the cheap, powerful checks: do the reactions balance the loads? Is the moment zero at every hinge? Does a symmetric structure under symmetric load give a symmetric result? Is there frame action where you expect it? Is the deflected shape credible, at a sensible scale?

bending moment (tension side)

a model check: the moment must be zero at the hinge
A genuine internal hinge cannot carry moment. If a result shows moment there, the release was omitted from the model — a topology error no amount of convergence can fix.

The common modelling errors each leave a signature you can learn to read:

- A missing restraint or a wrong release — moment at a hinge, or a base moment where the detail is a pin. - A frame modelled as a truss (all joints pinned) — no moment transfer and excessive sway; the members show axial force only where you expected bending. - Broken symmetry — a symmetric model giving asymmetric reactions points to a stray release, a local-axis slip, or an uneven mesh. - A misleading deformation scale — auto-scaling makes a hairline movement look like a collapse, or hides a real one; always check the magnitude, not just the picture.

Deflected shape

frame action, or a frame modelled as a truss?
A rigid frame under horizontal load sways a controlled amount and its columns bend. If a model shows excessive sway and axial-only members, inspect the element formulation and releases. In this fixed-base example, releasing only the knees would still leave bending in the columns.

Worked example

Worked example — auditing a suspicious result

A plotted result looks tidy. Run it past the audit before trusting it.

  1. Step 1 — totals and hinges

    First check the reactions sum to the applied load and that the moment is zero at every hinge. A failure here is decisive: the model is wrong, whatever the plot looks like.

    bending moment (tension side)

  2. Step 2 — symmetry and behaviour

    Then check behaviour: a symmetric frame under symmetric gravity should not sway, and its columns should bend as rigid joints transfer moment. If the result sways, or shows axial-only columns, suspect a release or a frame-as-truss error, and explain it before accepting the numbers.

    Bending moment

In the exercises, run the audit: reactions, hinges, symmetry, frame action and scale — and decide whether each result can be believed.

How to read these problems

The three-step method

  1. 1Points of certainty. Match the displacement and rotation that each support prescribes, including any settlement. Infer the displacement direction from the whole structure and load case; it need not be downward at every loaded point.
  2. 2Deflected shape and reaction directions. Sketch the compatible deflected shape. To find a reaction's direction, imagine removing that support: the direction that pushes the structure back to its place is the reaction's sense (it may be a hold-down).
  3. 3Bending moment and contraflexure. Draw the bending-moment diagram on the tension side and check it against the shape: hogging where the curve is convex-up, sagging where convex-down, zero at pins and at every contraflexure.

Rules that must always hold

  • 1.Moment is zero at a genuine moment release and at an unloaded simple end. A continuous member can carry moment over a translational support.
  • 2.A bending-moment diagram crosses the baseline exactly at a point of contraflexure.
  • 3.Under a distributed load the bending-moment diagram is curved; under point loads alone it is straight lines.
  • 4.At a fully fixed support the deflected shape leaves the support with zero rotation (tangent along the member).
  • 5.For an initially straight elastic beam without imposed curvature, zero curvature implies zero bending moment.
  • 6.The moment is drawn on the tension side: sagging below the member, hogging above it.

Now predict for yourself

Modelling and software checking exercises

5 exercises on reactions that do not balance, moment at a hinge, a frame modelled as a truss, broken symmetry and misleading deformation scales. Predict, then reveal an explanation.

Start the exercises →

This lesson is educational material. It uses simplified examples to teach principles, and must not be relied on for real design or safety-critical decisions.