Steel Design · 6 min read

How to choose a steel section quickly (without opening the Blue Book)

Practical rules of thumb for picking a UB or UC trial section in seconds — based on span, load type, and whether bending or buckling will govern.

Picking a trial section is the fastest part of steel design — if you know the rules of thumb. The point isn't to do the final design in your head, it's to land within one or two sizes of the answer so the formal check passes first time.

Rule 1 — Span ÷ depth ratio

For a simply supported steel beam carrying typical building loads, beam depth ≈ span ÷ 20 to span ÷ 25. So a 6 m beam wants ~250–300 mm deep; a 9 m beam wants ~400 mm deep. Cantilevers are stiffer per metre — divide by 10 to 12 instead.

  • Simply supported, normal loads: L/20 to L/25
  • Continuous beam: L/25 to L/30 (continuity helps)
  • Cantilever: L/10 to L/12 (deflection governs)
  • Heavily loaded transfer beam: L/15 (shear and bending both tight)

Rule 2 — UB for bending, UC for compression

UBs are deep and slim — efficient in major-axis bending. UCs are square — efficient in axial load because they buckle similarly about both axes. Use a UB as a beam, a UC as a column. Mixing them up is the most common preliminary design mistake.

Rule 3 — Plastic modulus rough estimate

Wₚₗ,required ≈ Mₑd ÷ fy     (Mₑd in kNm, fy in N/mm², Wpl in cm³)
Rough first pass — bending only, no LTB or deflection

For S275, divide kNm by 0.275 to get cm³. So Mₑd = 100 kNm needs about 360 cm³. Look up a UB with Wpl,y just above that — usually a 254×146×31 or 305×127×42 UB does it.

Rule 4 — Self-weight check

Steel beam self-weight is usually 1–3% of the load it carries. For preliminary sizing, ignore it. For final design, add it as an extra Gₖ — but it almost never changes the chosen section.

Rule 5 — Deflection often governs long spans

For spans over ~8 m, deflection (SLS) usually governs over bending strength (ULS). If your bending check has lots of spare capacity, don't celebrate — check deflection before reducing the section.

Quick decision flowchart

  1. Is it a beam or column? → UB for beams, UC for columns
  2. Estimate depth from L/20 (beam) or chosen section list (column)
  3. Estimate Wpl from Mₑd ÷ fy and pick a section just above
  4. If long span (>8 m) — sanity check deflection at L/360
  5. If unrestrained — sanity check LTB by reducing capacity ~30%
  6. Open the proper calculator and run the full check

Don't memorise the Blue Book — The section properties lookup gives you A, Iy, Iz, Wpl and more for every UB and UC in seconds. (Look up section properties)

Frequently asked questions

Why not just always pick the lightest section that passes?

In theory yes — that's the most economical for the steel cost alone. In practice, fabrication cost, connection cost, and standardisation across a project often favour grouping members into a smaller number of sizes. A slightly heavier section reused 20 times can be cheaper overall than 20 different optimum sections.

When should I consider a CHS or RHS instead?

Hollow sections shine in compression (closed shape — high torsional stiffness, no LTB), in exposed architectural settings, and where the surface area matters (less paint, less corrosion). They cost more per kg but often save weight and look cleaner.

Is S355 always better than S275?

S355 is roughly 30% stronger but only ~5% more expensive — so for strength-governed design, it's usually more economical. But for stiffness-governed design (deflection, buckling), S355 gives no advantage over S275 because E is the same. Don't default to S355 without checking which limit governs.

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