UK End Plate Connection Design — EN 1993-1-8 + UK NA Worked Example

Design of a bolted end plate moment connection per EN 1993-1-8 Clause 6.2 with UK National Annex. Worked example for a 533UB beam to 254×254 UC column connection using M20 Class 8.8 preloaded bolts. Covers the T-stub model in tension, compression zone, bolt row distribution, prying forces, and weld design per UK practice.

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End Plate Connection Design per EN 1993-1-8 Clause 6.2

EN 1993-1-8 Clause 6.2 provides the component method for bolted end plate connections. The connection is modelled as a set of components in tension, compression, and shear:

T-Stub Model (Clause 6.2.4)

Each bolt row in tension is modelled as an equivalent T-stub flange. The design resistance is the minimum of three failure modes:

Failure Mode Resistance Characteristic
Mode 1 — Flange yielding (complete) FT,1,Rd = 4Mpl,1,Rd / m Full flange yielding, bolt fracture
Mode 2 — Bolt failure with flange yielding FT,2,Rd = (2Mpl,2,Rd + nΣFt,Rd) / (m + n) Bolt fracture after partial yielding
Mode 3 — Bolt failure FT,3,Rd = ΣFt,Rd Pure bolt tension failure

Where Mpl,Rd is the plastic moment resistance of the T-stub flange (beff × tf² × fy / 4γM0), and m, n are geometric parameters defining the bolt-to-weld and bolt-to-edge distances.

Worked Example — 533UB to 254UC Connection

Connection details:

Tension Zone — Bolt Row 1 (Top Row)

Effective length of T-stub: leff = min(4m + 1.25e, 2πm, etc.) per Table 6.4

For a bolt row next to a stiffener (beam flange position): leff = 2πm = 2 × π × 35 = 220 mm (governing)

Mpl,1,Rd = leff × tf² × fy / 4γM0 = 220 × 20² × 355 / 4 = 7.81 × 10⁶ N·mm = 7.81 kN·m

Mode 1: FT,1,Rd = 4 × 7.81 / 35 = 0.892 × 10⁶ N = 892 kN

Mode 2: FT,2,Rd = (2 × 7.81 + 35 × 2 × 141.1) / (35 + 35) = (15.62 + 9,877) / 70 = 9,893 / 70 = 141 kN

Mode 3: FT,3,Rd = 2 × 141.1 = 282 kN

Min = 141 kN (Mode 2 governs — bolt failure with flange yielding)

Connection Moment Resistance

Bolt rows from top (tension) to bottom (compression):

Total moment: Mj,Rd = Σ FT,Rd,i × hi (where hi = distance from compression flange centre)

Assume h1 = 490 mm, h2 = 420 mm:

Mj,Rd = 141 × 0.490 + 141 × 0.420 = 69.1 + 59.2 = 128.3 kN·m

Weld Design

Beam flange welds: Full-strength butt weld (complete joint penetration) — no check required. Beam web weld: 10 mm fillet each side, throat a = 7.1 mm.

Shear resistance of web weld per side: Fw,Rd = a × fu / (√3 × βw × γM2) = 7.1 × 470 / (1.732 × 0.85 × 1.25) = 3,337 / 1.84 = 1,813 N/mm = 1.81 kN/mm

For total web depth (net of flanges): 2 × 1.81 × (529 − 31.2) = 2 × 901 = 1,802 kN — far exceeds typical shear.

Conclusion

The end plate connection using 8 × M20 Class 8.8 bolts with a 20 mm S355 end plate achieves Mj,Rd ≈ 128 kN·m — adequate for the beam moment at the connection location (typically 50-60 % of the beam midspan moment for a continuous frame). For full-strength (moment capacity ≥ beam section capacity), additional bolt rows or larger bolt sizes would be required.

Design Resources

Frequently Asked Questions

What is the T-stub model for end plate connections?

The T-stub model treats each bolt row in the tension zone as an equivalent T-shaped flange welded to a rigid web. The flange represents the end plate, the web represents the beam flange, and the bolts represent the tension anchors. Three failure modes are checked: Mode 1 (complete flange yielding), Mode 2 (bolt failure with partial flange yielding), and Mode 3 (pure bolt failure). Mode 2 governs for well-designed end plate connections — the end plate yields moderately before the bolts reach their tension capacity.

How many bolt rows are typically used in UK end plate connections?

UK end plate connections typically use 2-4 bolt rows in the tension zone. For beam spans of 6-12 m, 8 bolts total (2 rows of 4) is the standard for 533UB and 610UB sections. For deeper sections or higher moments, 12 bolts (3 rows of 4) or 16 bolts (4 rows of 4) may be used. The number of effective rows is limited by the lever arm and the compression zone depth — rows near the neutral axis contribute little to moment resistance.

What is the standard UK end plate thickness for a moment connection?

The standard end plate thickness for a moment-resisting connection in UK practice is 15-25 mm depending on bolt size and moment demand. For M20 bolts: 15 mm (light moment), 20 mm (standard moment), 25 mm (heavy moment). For M24 bolts: 20-30 mm. The plate must be thick enough to avoid Mode 1 (complete flange yielding) failure — which requires FT,1,Rd ≥ FT,2,Rd. The UK Steel Construction Institute (SCI) design guides recommend end plate thicknesses between 0.7d and 1.2d (d = bolt diameter).

When are full-strength butt welds required for end plate connections?

Full-strength butt welds (complete joint penetration, CJP) are required when the weld must develop the full yield strength of the connected member — typically for moment-resisting frames in high-ductility demand situations. For end plate connections in simple frames (nominally pinned), fillet welds are adequate. The UK NA to BS EN 1993-1-8 requires CJP welds for beam flanges in moment connections designed for ductility (seismic or plastic hinge rotation). For standard braced frames, single-V butt welds with backing bars are standard for the tension flange fillet welds.

Related Pages

Educational reference only. All design values are per BS EN 1993-1-1:2005 + UK National Annex and BS EN 10025-2:2019. Verify all values against the current editions of the standards and the applicable National Annex for your project jurisdiction. Designs must be independently verified by a Chartered Structural Engineer registered with the Institution of Structural Engineers (IStructE) or the Institution of Civil Engineers (ICE). Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent professional verification.