Structural modelling and software checking exercises

Module 17Level 2software check

The reactions do not balance the loads

An analysis reports support reactions that, added up, do not equal the total applied load. What is the most likely conclusion?

Predict first, then reveal

Reason from the supports, the movement and equilibrium — not from what looks familiar.

AThe model is wrong — a load or a restraint has been mis-entered or missed. Global equilibrium must hold, so a reaction–load mismatch is a clear error.BIt is acceptable — reactions rarely balance loads exactly.CThe structure is indeterminate, so equilibrium need not hold.

Module 17Level 3which impossible

Moment at a modelled hinge

A beam is built with an internal hinge, but the analysis shows a non-zero bending moment right at the hinge. What has happened?

Predict first, then reveal

Reason from the supports, the movement and equilibrium — not from what looks familiar.

AThe release was not actually applied — the hinge exists on the drawing but the model kept the joint continuous, so it carries moment. Fix the release and re-run.BIt is fine — hinges carry a little moment in practice.CThe mesh is too coarse; refine it and the moment will appear correct.

Module 17Level 3software check

A frame that behaves like a truss

A rigid-jointed portal frame is analysed, but the result shows almost no bending in the members and a very large sway. What modelling error does this suggest?

Predict first, then reveal

Reason from the supports, the movement and equilibrium — not from what looks familiar.

AThe model may have lost the intended bending stiffness through axial-only elements or incorrect releases. Check the element types, knee connections and base restraints against the intended frame.BThe frame is simply very flexible; the result is correct.CThe load is too small to cause bending.

Module 17Level 4software check

A symmetric model with asymmetric results

A symmetric linear-elastic model under symmetric loading has a unique stable solution, but its reported reactions are clearly asymmetric. The totals balance. What should you check?

Predict first, then reveal

Reason from the supports, the movement and equilibrium — not from what looks familiar.

ADo not accept it — a symmetric model under symmetric load must give a symmetric result. Look for a stray release, a local-axis slip or an uneven mesh before trusting anything.BAccept it — the totals balance, so the result is valid.CAccept it — real structures are never perfectly symmetric.

Module 17Level 3software check

A dramatic-looking deflected shape

A results plot shows the structure deflecting dramatically, apparently on the verge of collapse. What should you check before reacting?

Predict first, then reveal

Reason from the supports, the movement and equilibrium — not from what looks familiar.

AThe deformation scale — plots auto-magnify movements, so a tiny, safe deflection can look alarming (or a real one can be hidden). Read the actual magnitude, not the picture.BNothing — if it looks like it is collapsing, it is.CIncrease the load until the plot looks reasonable.