The short answer
A beam calculation is the engineering working that decides the correct size of beam to span an opening and safely carry the load above it. The engineer first works out the load that the beam must support — the dead weight of any wall, floor or roof over the opening, plus the imposed loads from people, furniture and snow. They then check a candidate steel section, such as a 203x133 UB or a traditional RSJ, against three things: bending (will it break under the moment), shear (will it tear at the supports) and deflection (will it sag too much). The work is done to the Eurocodes, principally BS EN 1993 for steel, and the result is a stated beam size with bearing details. For most domestic openings, it is the deflection check — keeping sag within span/360 — that ends up deciding the final size.
When a builder says "we'll get the beam calc done", this is the engineering behind it. Here is what the calculation works out and how the size is chosen.
At a glance
- Step 1Work out the load over the opening
- Step 2Choose a trial beam section
- ChecksBending, shear, deflection
- Worked toBS EN 1993 (steel) / 1995 (timber)
- OutputFinal beam size + bearing
Working out the load first
Before any beam can be sized, the engineer has to know what it is carrying. They build a load take-down — adding up everything that bears on the opening:
- Dead loads: the permanent self-weight of the masonry, floor structure, roof and finishes above. These are taken from standard material weights (for example brickwork at roughly 2 kN/m² per leaf).
- Imposed (live) loads: the variable loads — a residential floor is commonly taken at 1.5 kN/m² under BS EN 1991-1-1, plus snow on a roof and the weight of any occupants.
- Tributary width: how far either side of the beam the loads gather. A beam carrying a floor that spans 4m onto it picks up half that span from each side.
- Point loads: a concentrated load, such as another beam or a column landing on this one, which is treated separately from the spread load.
These are combined under BS EN 1990 to give the worst-case design load the beam must resist. The combination is not simply additive: the Eurocodes apply partial safety factors to dead and imposed loads so that the design allows for the loads being heavier than assumed and the material being slightly weaker than its nominal strength. This is why a beam is never sized for the bare measured load alone — it is sized for a deliberately conservative design load, which builds in the margin that keeps the structure safe over its life.
The three checks that size the beam
With the load known, the engineer tests a trial section against three failure modes. A beam must pass all three.
| Check | What it asks | Why it matters |
|---|---|---|
| Bending | Can it resist the moment? | Stops the beam snapping mid-span |
| Shear | Can it resist force at supports? | Stops it tearing at the bearings |
| Deflection | Does it sag within limits? | Stops cracked plaster, bouncy floors |
| Bearing | Can the supports take the load? | Stops crushing the wall below |
Indicative summary; full design follows BS EN 1993 and the UK National Annex. Sources: IStructE; Eurocode 3.
Why deflection usually decides the size
For a typical domestic steel beam, the section is rarely chosen because it would otherwise break — modern steel is very strong in bending. It is chosen because of deflection. Engineers limit how far a beam may sag, commonly to span/360 where brittle finishes like plaster are carried, and span/250 as a general limit. On a 4-metre opening, span/360 allows only about 11mm of sag under load.
Choosing the section and finishing the design
Steel sections come in standard sizes, each with published properties. A beam is described by its dimensions and weight per metre — for example a 152x89x16 UB is 152mm deep, 89mm wide and weighs 16kg per metre. The engineer iterates: pick a trial section, run the bending, shear and deflection checks, and if it fails any, step up to the next size and re-check until it passes all three with a sensible margin. A traditional RSJ (rolled steel joist, a UB by another name) is the workhorse of domestic openings, while a UC (universal column) is squarer and used where a beam must also resist sideways load or act as a post.
Once the section is fixed, the calculation is not finished until the ends are dealt with:
- Bearing length: how much the beam sits onto the wall at each end, typically a minimum of around 100mm, so the load is spread rather than concentrated on a thin strip of masonry.
- Padstones: a concrete or high-strength masonry block under each end spreads the beam's reaction so it does not crush the brick or block below — itself a small calculation.
- Lateral restraint: a slender beam can buckle sideways under load; the design confirms it is held against this, often by the floor or wall it supports.
- Connections: where one beam lands on another, the connection (bolts or a welded cleat) is designed so the joint is as sound as the members.
The completed beam calculation is then issued as a stamped pack stating the final size, the bearing and padstone details, and the loads assumed. The builder uses it to order the steel and form the supports, and Building Control checks it against Part A before the beam goes in. Any later change to what the beam carries — an added storey, a heavier finish — means the calculation has to be redone, because every check traces back to the original load.
Frequently asked questions
Is an RSJ the same as a UB?
Effectively yes. RSJ (rolled steel joist) is the traditional name; modern beams are universal beams (UB) to the Eurocodes. Both describe the I-shaped steel section used to span openings. The calculation chooses the exact size and weight per metre.
Can I size a beam myself from a span table?
Span tables exist for simple timber joists and standard lintels, but a structural steel beam carrying a wall or multiple floors needs a proper calculation to the Eurocodes. Building Control will normally want a designed beam, not a guessed one, for anything significant.
What is the deflection limit on a domestic beam?
Commonly span/360 where the beam carries brittle finishes such as plaster, and span/250 as a general working limit. On a 4-metre span, span/360 allows only about 11mm of sag — which is often what decides the final beam size.
Sources & further reading
- The Institution of Structural Engineers — what an engineer does
- Planning Portal — Approved Document A (structure)
- LABC — beams and Building Control
Figures on this page are typical UK ranges drawn from published sources and depend on your specific project. They are guidance, not a quotation.