What this calculator does
A quick structural sense-check for a simply supported beam. The calculator compares the load you can actually carry against two independent limits — the bending capacity of the section and the deflection that building occupants would notice — and tells you which one governs.
How to use it
- Set the span and choose whether the load is uniformly distributed or a central point load.
- Select the section from the universal-beam table; each row carries its own I and S values.
- Enter the yield strength — 275 MPa for S275 and 355 MPa for S355 structural steel.
- Read the governing mode: if the deflection limit gives a lower allowable load than bending, the section is stiffer than it is strong.
Governing formula
M = wL²/8 (UDL) or PL/4 (point) ; M_n = 0.66 · Fy · Sx Where:
-
w= Uniform load [kN/m] -
P= Point load [kN] -
L= Span [m] -
Sx= Elastic section modulus [mm³]
Derived Equations:
w_allow = 8·Mn / L² w_allow(δ) = δ_lim·384·E·I / 5L⁴ governing = min(w_bending, w_deflection) Worked example — 8 m span, 12 kN/m, UB 406×178×60 in S355
The figures below were produced by running this page's own engine with the example inputs (Span (m), Load model, Uniform load (kN/m), Central point load (kN), Yield strength (MPa), Section). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Applied moment | 96 kN·m |
|---|---|
| Bending capacity | 278.82 kN·m |
| Demand / capacity | 0.34 – |
| Governing criterion | deflection (L/240) |
| Allowable load (governing) | 26.99 kN/m or kN |
| Deflection under the applied load | 14.82 mm |
- The applied moment is 12 × 8² / 8 = 96 kN·m.
- With Sx = 1,190 cm³ the bending capacity is about 279 kN·m, so the demand ratio is well under 1.
- Check the deflection limit as well — for short beams the strength check usually governs.
Assumptions & limitations
Engineering calculation. Simple-span allowable stress check: bending capacity 0.66·Fy·Sx and a serviceability limit of L/240. No lateral-torsional buckling, load combinations or connection checks.
- 0.66·Fy is an allowable-stress factor, not a code partial factor — it does not replace a full LRFD or limit-state design check.
- No lateral-torsional buckling, so an unbraced slender beam will appear stronger than it is.
- Assumes a simply supported span with no continuity, self-weight added by the engineer and no dynamic or wind uplift.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- EN 1993-1-1 (Eurocode 3) — design of steel structures, bending and buckling.
- AISC 360 — Specification for Structural Steel Buildings.
- SCI P358 / P364 — properties of universal beams and sections.