Steel connection design
A steel frame is only as good as its connections, where the forces pass from one member to the next. We design bolted and welded steel connections to BS EN 1993-1-8 for clients across the UK.
Why is connection design a separate job from sizing the beams?
Because the members and the joints between them are checked differently. A beam can be the right size while the connection at its end cannot pass the load, which makes the joint the weakest link. Connection design checks the bolts, plates, welds and local member zones so the force transfers safely.
Why connections decide the frame
Members are sized for the loads they carry, but the connections are where those loads actually transfer, and a joint that cannot pass the force is the weakest link no matter how large the members are. Connection design to BS EN 1993-1-8 is a series of checks on every element in the load path through the joint: the bolts in shear and tension, the plates in bearing and net section, the welds, and the local zones of the members being joined, the beam web, the column flange and web, where the force arrives. The joint is only as strong as the first of these to reach its limit, so all of them are checked, not just the bolts.
The other decision a connection makes is how the frame behaves. A simple, or pinned, connection carries the vertical reaction but is detailed to have little rotational stiffness, so it does not transfer meaningful moment and the frame relies on bracing or walls for stability. A moment, or rigid, connection is stiff enough to carry bending across the joint, which lets the frame stand up by frame action without bracing. Choosing between them shapes the whole structure, so we design the connection to match the analysis assumption, a pinned joint where the model assumed a pin, a moment joint where the model relied on continuity.
Bolted connections, welds and their checks
Most site connections are bolted, because bolting is quick and needs no site welding. Fin plates, end plates and web cleats are the common simple connections, transferring a beam's shear into a column or another beam through a plate and a line of bolts. We check the bolt group in shear, the plate in bearing and in its net and gross sections, the beam web, and the weld or bolts back to the supporting member, then confirm the connection can also take the tying force that ties the frame together for resistance to disproportionate collapse. Bolt grade, usually 8.8, diameter, edge and end distances and plate thickness all come out of these checks.
Welds are usually made in the fabricator's shop, where conditions are controlled, and used for cap plates, stiffeners and moment end plates. A fillet weld is designed for the force per unit length it carries, and we size the leg length and specify continuous or intermittent runs accordingly; a full penetration butt weld develops the full strength of the plate where that is needed. Where a moment connection puts high local forces into a column, we check whether the column web needs stiffeners or the flange needs strengthening, because the connection can fail in the column rather than in the joint plates if those local zones are overlooked.
Base plates, splices and coordination
At the foot of a column the base plate connection spreads the load into the foundation and holds the column down. We size the plate so the bearing pressure on the concrete or grout stays within limits, and design the holding down bolts for uplift and any moment at the base, a pinned base still needing bolts to position the column and resist tension. Where a column or beam has to be joined along its length, a splice connection carries the axial, shear and moment across the join with cover plates or an end plate, and we design it to transmit the member forces so the splice is not a weak point.
Connection design has to be buildable, so we detail joints that can actually be assembled in the sequence the steel goes up, with access for a spanner and bolts that can be reached. We coordinate with the fabricator's preferences where there is one, and issue clear connection details, bolt and weld specifications and any stiffener requirements, ready for fabrication and for Building Control. We deliver these designs across the UK, with site involvement arranged by region where an existing frame or connection needs assessing.
What we check.
The points our calculations resolve for a project like this.
- Every element in the load path checked: bolts, plates, welds and local member zones
- Connection stiffness matched to the analysis, pinned where a pin was assumed
- Bolt group in shear and bearing with correct grade, edge and end distances
- Tying force for resistance to disproportionate collapse
- Column web stiffeners and flange strengthening where moment connections demand
- Base plates, holding down bolts and splices designed for the member forces
Issued drawings, redacted.
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
Why is connection design a separate job from sizing the beams?
Because the members and the joints between them are checked differently. A beam can be the right size while the connection at its end cannot pass the load, which makes the joint the weakest link. Connection design checks the bolts, plates, welds and local member zones so the force transfers safely.
What is the difference between a pinned and a moment connection?
A pinned, or simple, connection carries the vertical reaction but transfers little moment, so the frame needs bracing or walls for stability. A moment connection is stiff enough to carry bending across the joint, letting the frame stand by frame action. We design the connection to match how the frame was analysed.
Are the connections bolted or welded?
Usually bolted on site, because bolting is quick and needs no site welding, using fin plates, end plates or cleats. Welding is normally done in the fabricator's shop for cap plates, stiffeners and moment end plates. We specify which, with the bolt grade, weld size and plate thickness.
Do you design base plates and holding down bolts?
Yes. The base plate spreads the column load into the foundation and the holding down bolts hold the column down and resist uplift and moment. We size the plate so the concrete is not overstressed and design the bolts, coordinating the base with the pad foundation beneath.
Can you design connections for a fabricator?
Yes. We produce connection calculations and details to BS EN 1993-1-8, coordinated with the fabricator's preferences where there is one, covering bolted and welded joints, splices and base plates. The details are issued ready for fabrication and Building Control, anywhere in the UK or Europe.
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