Module 13 · Lesson 13.2
Predict before you analyse
Why the estimate has to come first, and how to make one for a structure that has no formula.
Why this matters
This is the single habit that separates engineers who catch their own errors from engineers who do not, and it costs about ten minutes per model.
The reasoning is uncomfortable and worth stating plainly: once you have seen the answer, you cannot un-see it. Any subsequent 'check' is a search for reasons the number might be right, and the human mind is extremely good at finding them. An estimate made beforehand is the only genuinely independent statement you will ever have about your own model.
By the end of this lesson you should be able to
- Explain why prediction must precede analysis rather than follow it
- Reduce a complex structure to one that has a formula
- Predict a deflected shape and a bending moment diagram qualitatively
- Judge how close an estimate needs to be to count as agreement
Reduce a dimension
The technique is almost always the same: take the structure down a dimension until it has a formula.
A truss becomes a beam. Chord force is moment over depth — and for a parallel-chord truss that is exact, not approximate, because it is what panel-point equilibrium says. Diagonal force is shear over the sine of the diagonal's inclination.
A tall building becomes a cantilever. Base moment is the sum of the storey loads times their heights. Tip deflection follows from the standard cantilever formula with the building's overall second moment of area.
A two-way slab becomes two one-way strips. Crude, and it brackets the answer: the sum of the two strips' stiffnesses must carry the load, so each strip's share follows.
A frame becomes a series of subframes. One beam with the columns above and below, with appropriate end conditions. This is how frames were designed before computers and it still bounds the answer.
A shell becomes a beam or an arch. Whichever the geometry suggests. A barrel vault is an arch in one direction and a beam in the other.
None of these is the answer. Each is an independent statement about roughly what the answer should be, made from different assumptions, and that independence is the whole value.
Predict the shape too
A numerical estimate is one prediction. A qualitative one is often more powerful, because it catches faults that a magnitude check cannot:
- Where does it sag and where does it hog?
- Where are the points of contraflexure?
- Which members are in tension and which in compression?
- Is the response symmetric?
Sketch it before opening the results. A model that sags where you predicted hogging has told you something no magnitude comparison would have.
How close is close enough
Within about 10 % on a quantity a simplified model should get right — a determinate moment, a total reaction, an overall deflection.
Within a factor of two on something the simplification genuinely distorts — a local stress, a moment redistributed by relative stiffness.
And for a qualitative prediction, exactly right on sign and shape, or you have a finding.
The judgement is which category the quantity falls into, and it should be made before comparing. Deciding afterwards that a 40 % discrepancy was always going to be acceptable is not a check.
When they disagree
Three possibilities, in order of likelihood:
- 1.The model is wrong. Most likely, and cheapest to investigate.
- 2.The estimate is wrong — including the expectation itself. Sometimes the model is teaching you something about the structure.
- 3.The software is wrong. It happens, and it is not where to start.
What is not on the list is ignore it. An unexplained disagreement is a finding, and it stays open until it is closed in writing.
Worked example
Predicting a truss before modelling it
Given
- A parallel-chord truss, 24 m span, 3 m deep, 8 panels
- Point loads of 40 kN at each of the 7 interior bottom-chord nodes
- Diagonals at 63.4° to the horizontal
Find
Predictions for the peak chord force and the end diagonal force, before building anything
Practice
A 30 m truss, 3.5 m deep, carries a midspan moment of 1 400 kN·m. Predict the peak chord force in kN.
Practice
A 12-storey building carries a wind load of 180 kN at each floor, with floors at 3.6 m centres starting at 3.6 m. Predict the base overturning moment in kN·m. (The sum of 1 to 12 is 78.)
Check yourself
Why must a prediction be recorded before the analysis runs, rather than simply thought about?
Check yourself
A prediction and a model disagree by a factor of three. What is the right first response?
Summary
- Predict first — afterwards it is rationalisation, not checking
- Reduce a dimension until the structure has a formula
- Predict the shape and the signs, not only the magnitude
- Decide what counts as agreement before you compare
- An unexplained disagreement stays open until it is closed in writing
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