Module 15 · Level 2 · reactions and load path
How a square slab spans A roughly square slab is supported on all four edges. In how many directions does it carry its load?
Predict first, then reveal Reason from the supports, the movement and equilibrium — not from what looks familiar.
A Two — it spans in both directions at once, sharing the load between them by stiffness (the shorter span, being stiffer, takes more). B One — a slab always spans the short way only. C None — a slab carries load in shear, not spanning.
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Module 15 · Level 3 · internal force diagram
What is a yield line? At collapse a slab develops yield lines. What is a yield line, and what does the pattern of them represent?
Predict first, then reveal Reason from the supports, the movement and equilibrium — not from what looks familiar.
A A line along which the slab hinges plastically; a valid pattern of them divides the slab into rigid regions that rotate about their supports, folding it into a collapse mechanism. B A line of zero bending moment, like a point of contraflexure. C A crack that forms randomly with no relation to collapse.
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Module 15 · Level 4 · software check
Is an assumed yield-line pattern safe? You calculate a slab's collapse load from an assumed yield-line pattern. Can you rely on that value as a safe estimate of capacity?
Predict first, then reveal Reason from the supports, the movement and equilibrium — not from what looks familiar.
A No — yield-line analysis is an upper-bound method: an assumed pattern gives a load at or above the true collapse load, so it can be unsafe. You must find the pattern giving the lowest load. B Yes — any yield-line pattern gives a safe, conservative answer. C Yes, provided the pattern looks symmetric.
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Module 15 · Level 3 · change and sensitivity
When does a slab span one way? A slab is supported on all four edges but is long and narrow — much longer in one direction than the other. How does it span?
Predict first, then reveal Reason from the supports, the movement and equilibrium — not from what looks familiar.
A Essentially one way — across the short direction, like a series of beams; the long-direction span is too flexible to attract much load. B Still equally two ways, because it has four supported edges. C Only the long way, because that is the main span.
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