Steel beam (RSJ) design
When a wall comes out or an opening is formed, a steel beam has to pick up everything that wall used to carry. We size and check that beam to the Eurocodes and issue calculations Building Control will accept, for clients across the UK.
What is the difference between an RSJ and a UB?
RSJ refers to the older rolled steel joist section. Most beams we specify today are universal beams or universal columns, which are more efficient shapes for the same weight of steel. We size whichever section suits the span and loads, and it is still commonly called an RSJ on site.
What sizing a steel beam actually involves
The term RSJ has stuck from the old rolled steel joist sections, but most modern beams are universal beams (UB) or universal columns (UC) rolled to BS EN 10025. Sizing one is not a matter of picking a section from a chart. We first gather the load: the roof, the floors, any masonry above and any point loads from beams that frame into it, converted into a uniformly distributed load or a set of point loads with the correct load factors from BS EN 1990. Dead and imposed actions are combined at the ultimate limit state to check strength, and at the serviceability limit state to check deflection.
With the design loads known, we run three checks that govern almost every domestic beam. Bending: the applied moment must stay below the section's moment capacity, which for a compact section is the plastic modulus times the design yield strength. Shear: the applied shear must stay below the shear capacity of the web. Deflection: under unfactored imposed and dead load the beam must not sag more than the limit that protects the finishes above it. On most residential jobs deflection, not strength, decides the final section, because a beam strong enough to stand up can still crack the plaster if it flexes too much.
Deflection, lateral stability and web checks
Deflection limits follow the UK National Annex to BS EN 1993-1-1 and the finishes being carried. We commonly work to span/360 for beams supporting plastered ceilings or brittle finishes, and tighten that where a beam carries a masonry wall above, because brickwork tolerates very little movement before it cracks. A slender beam that satisfies bending can easily fail deflection, so we iterate the section until both the strength and the movement sit inside the allowances, rather than leaving the builder to discover a bouncy floor later.
The other check people forget is lateral torsional buckling. A steel beam that is not restrained along its compression flange can buckle sideways before it reaches its full bending capacity. Where the beam is built into masonry or has a floor bearing on the top flange the restraint is usually adequate, but a bare beam in a large opening may need the effective length reducing with restraints, or a heavier section chosen. We also check web bearing and buckling at the supports, which is what drives the size of the padstone underneath each end.
Connections, fire and getting it built
Where beams meet, the connection has to carry the reaction from one member into the next. A single beam over an opening usually bears directly on padstones, but where a secondary beam frames into a main beam we design a bolted or welded cleat, fin plate or end plate to BS EN 1993-1-8, sized for the vertical reaction and any tying force. We note bolt grade, weld size and the padstone required at each bearing so nothing is left to guesswork on site.
Steel in a house normally needs fire protection to achieve the required period of resistance under Approved Document B, whether that is intumescent paint, boarding or being encased behind plasterboard. We flag the protection needed so it is priced from the start. The finished output is a set of calculations, a beam schedule and connection details your builder can order steel from and Building Control can approve, delivered anywhere in the UK or Europe, with site visits arranged by region where a survey of the existing structure is needed.
What we check.
The points our calculations resolve for a project like this.
- Load take-down from roof, floors and masonry above into a single design load case
- Bending, shear and deflection checks to BS EN 1993-1-1 and the UK National Annex
- Deflection limited to span/360 or tighter where brittle finishes or masonry are carried
- Lateral torsional buckling and web bearing checks at each support
- Padstone and bearing length set by the end reaction
- Fire protection to the period required by Approved Document B
From enquiry to sign-off.
Enquiry
Send drawings or describe the problem. We confirm the scope, the deliverables and a target timescale.
Information
We agree the survey, drawings or data we need and any site access required.
Engineering
Design, calculation or assessment to the relevant Eurocodes and UK National Annex.
Issue
A clear, defensible report or set of calculations, with assumptions and limitations stated.
Common questions
What is the difference between an RSJ and a UB?
RSJ refers to the older rolled steel joist section. Most beams we specify today are universal beams or universal columns, which are more efficient shapes for the same weight of steel. We size whichever section suits the span and loads, and it is still commonly called an RSJ on site.
How much does steel beam design cost?
For a single beam over an opening our fee starts for the calculations and a beam schedule, fixed before we begin. More beams, longer spans or connection design between members will affect the fee, which we agree up front.
What size steel beam do I need?
That depends on the span and everything the beam has to carry. We take down the load from the roof, floors and any wall above, then check bending, shear and deflection to arrive at the lightest section that works. We will not quote a size without the loads, because guessing risks a bouncy or overloaded beam.
Does the steel beam need fire protection?
Usually yes. Exposed structural steel in a dwelling normally needs protecting to meet the fire resistance period in Approved Document B, often with intumescent paint, boarding or plasterboard encasement. We tell you what is required so it can be priced from the outset.
Can you design the padstones and connections too?
Yes. The calculations include the padstone or bearing needed at each end, sized from the beam reaction, and any bolted or welded connection where beams meet. That way the whole load path is covered in one package.
Related in structural design & elements.
Tell us what you are building.
Send the drawings or describe the project. We confirm scope, deliverables and a realistic timescale.
Start a project