The big idea: A beam carries load ACROSS a span. A column carries load ALONG its own axis.
What separates one beam from another is the support: a pin lets the beam rotate there, a fixed end does not, and a cantilever has only one support at all.
Each beam with its supports and its deflected shape. Step to the last panel for where each one fails.
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| Type | Supports | How it behaves | Where you see it |
|---|---|---|---|
| Simply supported | One at each end, both free to rotate | Sags into a single curve; largest bending at mid-span, none at the ends | A shelf on two brackets, a plank on trestles, a joist on two walls |
| Fixed (encastré) | Both ends rigidly built in, neither free to rotate | Deflects far less for the same load, but the ends now carry bending too | A beam cast into concrete, a bolted steel flange connection |
| Cantilever | Held rigidly at ONE end only | Free end deflects most; bending is largest at the fixed root | A balcony, a diving board, a bracket, an aircraft wing |
| Continuously supported | Three or more supports along its length | Each span deflects less than it would alone; bending reverses over the middle supports | A bridge deck on several piers, a long shelf with a centre bracket |
| Column | Vertical, load along its own axis | Fails by BUCKLING rather than crushing once slender; length matters more than material | A table leg, a scaffold standard, a lamp post, a chair leg |
Fixing the ends is free stiffness: The same beam, same material, same section, deflects several times less when both ends are built in rather than simply supported.
That is stiffness for no extra material at all — but the supports must now resist a bending moment, so the fixing has to be designed for it.
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The bending is largest where the beam breaks, and that point is different for each type. Knowing where it is tells you where to put material, where to look for a crack, and why brackets are the shape they are.
Failure points
Simply supported — the middle
Bending peaks at mid-span and is zero at the ends, which is why a joist can be notched near a wall but never in the centre.
Cantilever — the ROOT
Bending is largest where it meets the support, which is why a bracket is always thickest at the wall and tapers towards the tip.
Fixed — the ends AND the middle
Bending appears at both ends as well as the centre, so a fixed connection has to be as strong as the beam it holds.
Column — sideways, by buckling
A slender column does not crush. It bows sideways, and doubling its length makes it about four times easier to buckle — so length and end fixity matter far more than material strength.
Buckling is not a strength problem: A long thin column buckles at a load far below the one that would crush it.
Swapping to a stronger alloy barely helps; making it shorter, fatter, hollow or fixed at both ends helps enormously. That is why table legs are braced near the floor and scaffold standards are tied every two metres.
How this is tested — identifying beam and column types and explaining their function. It comes up two ways:
Paper 1 — multiple choice
- Identify a beam type from a drawing of its supports.
- Pick where the largest bending occurs in a stated beam.
Paper 2 — analysing a product
- Identify the structural members of a named product and explain each.
- Explain why a bracket is thicker at one end.
The trap: Treating a column as a beam stood upright. A column fails by buckling sideways, which depends on its length and its end fixity far more than on its material.
A wall-mounted shelf carries books on two brackets. Identify the structural members and explain the function of each.
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