Module 5 · Lesson 5.2
Loud and silent losses
Six things that go wrong in a handover, all applied to the same truss, all re-solved.
Why this matters
There is a strong instinct that the worst kind of exchange failure is the one that stops you working. It is exactly backwards.
A loss that stops the model dead costs an afternoon. A loss that leaves the model solving — producing a full set of results, no warnings, a sensible-looking deflected shape — costs whatever gets built on the strength of it. This lesson takes one truss, damages it six different ways, and re-solves it each time, so the difference is a set of numbers rather than an opinion.
By the end of this lesson you should be able to
- Classify a loss as loud, silent or harmless, from what the model does
- Explain why a defaulted section multiplies the deflection by exactly the area ratio
- Recognise the loss that makes a structure look safer than it is
- Say what a handover check should look for, given that half of these are silent
What you should already know
- Module 9's stiffness method — the deflection ratios here follow directly from it
- Module 12's diagnostics — the loud failures are loud because those checks fire
One truss, six handovers
The model sent is a 24 m Warren truss, 3 m deep, six panels, 4000 mm² members, carrying 60 kN at each interior bottom node. It solves cleanly: no mechanism, and an equilibrium residual at rounding level.
Six things then happen to it on the way across. Each is a real handover failure, and each is applied to the model and re-solved.
| What was lost | What the receiving model does |
|---|---|
| The pinned support arrived as a roller | Will not solve. The truss is a mechanism |
| Coincident nodes did not merge | Will not solve. The diagnostics report a disconnection |
| Sections defaulted to 1000 mm² | Solves. No warnings. Deflection × 4.00 |
| Material defaulted to 200 GPa | Solves. No warnings. Deflection × 1.05 |
| One load case did not come across | Solves. No warnings. Deflection × 0.72 |
| Member names were regenerated | Solves. Every number identical |
Two loud, three silent, one harmless.
Why the ratios are exactly what they are
The two defaults produce ratios you can predict from Module 9 without running anything, and that is the point of quoting them:
- Stiffness is proportional to EA/L. Quartering the area quarters the stiffness of every member, so every displacement is multiplied by exactly 4.00. Not approximately four — exactly, because the whole matrix is scaled by the same factor.
- Dropping E from 210 to 200 GPa multiplies displacements by 210/200 = 1.05.
A 5 % error from a defaulted material is well inside the range an engineer would accept as 'about right' from a model they trusted. That is why it survives.
The worst one
Look again at the lost load case. The deflection ratio is 0.72 — the received model is stiffer-looking than the sent one, and every member force falls with it.
Every check passes more easily. Every utilisation improves. Nothing anywhere in the output is out of range, because losing load moves everything in the direction that looks like good news.
An error that makes the structure look safer will not be found by looking at the results. It can only be found by checking the input.
This is the whole argument for checking the total applied load against the sum of reactions, by hand, on every received model. It takes a minute and it is the only check that catches this.
The harmless one is not free
Regenerating the member names changes no number at all. It is still a real loss: the member that the drawing calls B7 is now called M23, and the connection schedule, the check calculations and the fabrication drawings all refer to a naming scheme the model no longer uses.
That costs nothing in the analysis and a great deal at the next handover.
Try it
Model transfer exercises
One truss, damaged one attribute at a time, re-solved each time. The verdict is computed from what the model does.
What was lost in transfer
Both supports hold vertically; nothing holds the truss horizontally.
- Does the received model solve?
- no
- Warnings raised by the model itself
- 1
- Largest displacement, against the sent model
- —
- Largest member force
- —
- Verdict
- loud
The receiving model will not solve. Nobody can act on a wrong number, because there is no number.
What the receiving model says on its own
- error: Nothing holds the model horizontally. It does not matter that there is no horizontal load: the matrix has zero stiffness in that direction and the solve divides by zero.
- Losses that stop the model
- 2
- Losses that solve and are wrong
- 3
- Losses that change no number
- 1
What this shows: A loss that stops the model is the safe one. Three of these six leave it solving and wrong.
Worked example
Why a quarter of the area is exactly four times the deflection
Given
- Every member's area falls from 4000 mm² to 1000 mm² on import
- Nothing else changes: geometry, material, loads and supports are all correct
Find
The exact factor on every displacement, before running anything
Practice
Members arrive with an area of 2500 mm² instead of the 4000 mm² that was sent. By what factor is every displacement in the received model multiplied?
Practice
In the six-loss table, how many of the losses leave the model solving with no warning at all?
Predict first
A load case is lost in transfer. What happens to the member forces in the received model?
Worked example
Finding a silent loss with one sum
Given
- A received truss model solves cleanly and reports no warnings
- The sending engineer applied 60 kN at each of five interior nodes
- The received model's vertical reactions total 240 kN
Find
Whether anything was lost
Practice
A model is sent with E = 210 GPa and arrives with E = 195 GPa. By what factor are all displacements multiplied?
Practice
Applied load in a received model totals 300 kN; the vertical reactions total 240 kN. What percentage of the load is missing?
Practice
Members arrive with an area of 6000 mm² instead of the 4000 mm² sent. By what factor are the displacements multiplied?
Check yourself
Why do member forces stay unchanged when every section area is scaled by the same factor?
Check yourself
Which of these losses is caught by the receiving software, without anyone checking anything?
Predict first
A defaulted section is smaller than the real one. Which way does the error run?
Summary
- Two of the six losses are loud, three are silent and one is harmless
- A quartered area gives exactly four times the deflection and identical member forces
- A defaulted modulus gives a 5 % error, comfortably inside 'about right'
- A lost load makes the structure look safer, and no review of results will find it
- The solver checks that the model is solvable, not that it is the model you sent
- Only a check against something outside the model catches a silent loss
This is educational material. It uses simplified examples to teach principles, and must not be relied on for real design or safety-critical decisions. Module overview and checkpoint