Steel to Steel Connection Design: 7 Rules Every Structural Engineer Must Follow
Steel to steel connection design is one of the most technically demanding aspects of structural steelwork. Connections transfer forces between beams, columns, ties and foundations — and their performance determines whether the structure behaves as modelled. A beam designed as simply supported requires a connection that genuinely behaves as a pin; a moment frame requires a connection stiff enough to transmit significant bending moment. Getting steel to steel connection design right means understanding joint classification, design checks, detailing geometry, failure modes and how connections appear on structural drawings.
Steel to Steel Connection Design: Simple Connections Explained
The most common type of steel to steel connection design in residential and commercial construction is the simple connection — also known as a nominally pinned joint. A simple connection is designed to transmit shear, axial and tying forces, but not significant bending moment. This allows the connected beam to rotate freely at its end, consistent with the simply supported assumption made in the beam design.
In reality, even simple connections transmit a small amount of bending moment due to their geometry — they are nominal pins rather than true frictionless pins. This creates small tension forces in the connection components that would not exist in a true pin. Eurocode 3 Part 1-8 (BS EN 1993-1-8:2005) Clause 5.2.2.2 requires that a nominally pinned joint must be capable of transmitting internal forces without developing significant moments that could adversely affect members or the structure, and must be capable of accepting the resulting rotations under design loads.
A connection is classified as simple when its design moment resistance is less than 25% of a fully rigid connection and when it can rotate under applied loads. Both criteria must be satisfied simultaneously.
Steel to Steel Connection Design: Types of Simple Connection
Steel to Steel Connection Design: The 3 Groups of Design Checks
A rigorous steel to steel connection design requires up to 16 individual checks to BS EN 1993-1-8. These fall into three groups:
Steel to Steel Connection Design: How to Read Steelwork Drawings
Design intent general arrangement drawings for steel frames follow a specific protocol that every engineer involved in steel to steel connection design must understand:
| Label / symbol | Meaning |
|---|---|
| 457 × 191 × 89 UB | Universal Beam — 457mm deep serial size, 191mm wide, 89 kg/m. Note: actual depth varies with serial weight (89 kg/m gives 463mm actual depth, not 457mm) |
| 203 × 203 × 86 UC | Universal Column — 203mm serial size, 203mm wide, 86 kg/m. Actual depth is 222mm. The serial size is not the actual dimension. |
| 120 × 120 × 12 RSA | Rolled Steel Angle — actual dimensions stated accurately: 120mm leg × 120mm leg × 12mm thick |
| 150 × 100 × 5 RHS | Rectangular Hollow Section — actual dimensions: 150mm deep × 100mm wide × 5mm wall |
| V = 850 kN, M = 0 | Ultimate end reactions shown on design intent drawing for fabricator to design the connection — shear 850 kN, no moment (simple connection assumed) |
| TOS / SSL / FFL | Top of Steel / Structural Slab Level / Finished Floor Level — level datums shown on GA drawings |
| Beam drawn as single line with gap at column | Simply supported beam — gap indicates it is a separate element not continuous with the column |
| Bracing shown dashed on plan | Diagonal bracing in plan view — used for lateral stability |
Design intent drawings do not show bolt sizes, fin plate thicknesses or weld sizes — these fall under the fabricator's design responsibility for building structures. The design engineer provides the ultimate end reactions and the preferred connection form; the fabricator designs the components to achieve them. Steel grades must be explicitly stated — typically S275J0 or S355J0 for building structures.
Steel to Steel Connection Design: Failure Modes and Common Mistakes
The primary failure mode in steel to steel connection design for simple connections is shear failure — either the bolts shear through, or the beam web tears at the notch location. Shear failure at the bolt group occurs when the design shear per bolt exceeds the bolt shear resistance. Shear failure in the beam web at a notch occurs when the net area of web (full depth minus notch depth) cannot carry the shear force. Both modes are brittle — there is no significant ductility or warning before failure. The notch depth must be kept to the minimum needed to clear the column flange.
In end plate simple connections, the end plate tends to rotate about the line of bolt fixity under the eccentric shear load. This rotation causes the plate to deform and introduces additional tension forces — prying forces — in the bolts beyond those calculated from the applied shear alone. Prying failure occurs when these additional bolt tensions are not accounted for in the design. Keeping the end plate thickness ≤ 12 mm limits the rigidity that causes prying; thicker plates must be treated as semi-rigid connections with a full moment connection check.
In steel to steel connection design, a fin plate is classified as 'long' when tp/zp < 0.15, where tp is the fin plate thickness and zp is the distance from the face of the support to the first bolt line. A long fin plate is susceptible to lateral torsional buckling in the same way as an unrestrained beam — the plate can buckle laterally between the weld line and the bolt group. A lateral torsional buckling check must be carried out on long fins as part of the steel to steel connection design. Using a thicker plate or reducing the bolt offset distance zp converts a long fin to a short fin and avoids the check.
Building Regulations Approved Document A defines Consequence Classes for buildings. For Class 2A (most residential buildings over 4 storeys), all connections must be capable of resisting the tying forces defined in BS EN 1991-1-7. These tying forces — typically 75 kN minimum for perimeter elements — must be checked against the connection capacity independently of the vertical shear check. Many simple connections that pass the shear check will fail the tying force check if the bolt group or plate is undersized. Tying forces are often the governing check for steel to steel connection design in multi-storey residential work.
Steel to Steel Connection Design: Frequently Asked Questions
Who designs the steel to steel connections — the structural engineer or the fabricator?
For building structures in the UK, the design of steel to steel connection details — bolt sizes, fin plate thickness, weld sizes — is normally the responsibility of the steel fabricator, following the guidance in the National Structural Steelwork Specification (NSSS). The structural engineer provides the design intent: the ultimate end reactions at each connection (shear, axial, moment), the preferred connection form, and the steel grade. The fabricator designs the connection components to achieve those requirements. For civil engineering projects, all design responsibility typically lies with the design engineer. The fabrication drawings and connection calculations must be checked by the structural engineer before construction.
What grade of steel should fin plates be in steel to steel connection design?
Fin plates and end plates in simple steel to steel connection design should be Grade S275. Using S355 or higher grade steel for the plate increases its stiffness, causing the connection to behave in a more semi-rigid manner — transmitting more bending moment than was assumed in the design model. S275 plates maintain the nominally pinned character of the joint. This applies to the plate component only; the beam and column themselves may be S355.
How are bolt spacings determined in steel to steel connection design?
Bolt spacing rules in steel to steel connection design come from BS EN 1993-1-8 Table 3.3. The minimum end distance (bolt centre to plate edge in the direction of load) is 1.2 × bolt hole diameter. Minimum edge distance (perpendicular to load) is also 1.2 × hole diameter. Minimum horizontal and vertical pitch between bolts is 2.2–2.4 × hole diameter. Maximum spacing is the smaller of 14 × plate thickness or 200 mm for both exposed and unexposed conditions. These rules ensure that local failure — bearing tearout at the bolt hole, plate splitting — does not occur before the full bolt capacity is reached.
→ Need beam design as well as connection design? See our Steel Beam Design for Residential guide. Steel Column Design Residential → Base Plate Design for Steel Columns →
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