UK Moment Connection -- Extended End Plate Design to EN 1993-1-8

The extended end plate moment connection is the standard moment-resisting joint in UK steel portal frame and multi-storey braced frame construction. It transfers bending moment, shear, and (where required) axial force from the beam into the column through a combination of bolt tension in the top rows and bearing compression at the bottom flange. The component method of EN 1993-1-8 provides a systematic framework for determining the moment resistance and rotational stiffness of the joint. This reference presents the component method design procedure, the key formulas, and a fully worked example for a portal frame eaves connection.

The Component Method

EN 1993-1-8 Clause 6.3.1 decomposes the joint into a set of basic components, each with its own resistance and stiffness. For an extended end plate beam-to-column connection, the active components are:

Each component is characterised by its design resistance and its stiffness coefficient. The weakest component in each zone (tension, compression, shear) governs, and the components are assembled into a spring model representing the joint.

Tension Zone -- Bolt Row Resistance

For each bolt row, the effective design tension resistance Ft,Rd is the minimum of:

The resistance of the column flange and end plate in bending is determined using the equivalent T-stub model per EN 1993-1-8 Clause 6.2.4. Three failure modes are considered:

Mode 1 -- Complete yielding of the flange: Ft,1,Rd = 4 x Mpl,1,Rd / m

Mode 2 -- Bolt failure with yielding of the flange: Ft,2,Rd = (2 x Mpl,2,Rd + n x SUM(Ft,Rd)) / (m + n)

Mode 3 -- Bolt failure only: Ft,3,Rd = SUM(Ft,Rd)

Where:

Compression Zone

The compression resistance is the minimum of:

Shear Zone -- Column Web Panel

VRd,wp = 0.9 x fy,wc x Avc / (sqrt(3) x gamma_M0)

Where Avc is the shear area of the column. If the design shear in the column web panel exceeds VRd,wp, a supplementary web panel (doubler plate) or diagonal stiffeners are required.

Moment Resistance Assembly

The moment resistance Mj,Rd is determined by summing the contributions of each bolt row:

Mj,Rd = SUM(hr x Ft,Rd,r)

Where hr is the lever arm of bolt row r from the centre of compression, and Ft,Rd,r is the design tension resistance of that bolt row.

Rows are considered sequentially from the top. If a top row exhausts the column web tension or compression capacity, lower rows may be limited or ineffective. The bolt rows are assumed to be in the tension zone if they are above the centre of rotation (typically the centre of the bottom flange).

Rotational Stiffness

The initial rotational stiffness Sj,ini is:

Sj,ini = E x z^2 / SUM(1 / ki)

Where z is the lever arm, and ki are the stiffness coefficients of each basic component. The joint is classified as rigid, semi-rigid, or nominally pinned based on Sj,ini relative to the beam stiffness.

Worked Example -- Portal Frame Eaves Connection

A single-storey portal frame with 30 m span, 6 m eaves height, and 7.5 m bay spacing has a design moment at the eaves of MEd = 520 kN.m and design shear VEd = 210 kN. The column is a 356 x 368 x 153 UC in S355, and the rafter is a 457 x 191 x 67 UB in S355. Design an extended end plate moment connection.

Step 1 -- Connection geometry:

Step 2 -- T-stub parameters for end plate (top bolt row): leff,nc = 2 x pi x m = 2 x pi x 40 = 251 mm (circular pattern) leff,nc' = 4 x m + 1.25 x e = 4 x 40 + 1.25 x 50 = 222.5 mm Effective leff = min(251, 222.5) = 222.5 mm

Mpl,Rd = 0.25 x 222.5 x 25^2 x 355 / 1.00 = 12.35 kN.m

Step 3 -- End plate in bending (Mode 1): Ft,1,Rd = 4 x 12.35 / 0.040 = 1,235 kN

Step 4 -- Bolt tension resistance (Mode 3): Ft,Rd per M24 10.9 bolt = 0.9 x 1000 x 353 / 1.25 = 254.2 kN Ft,3,Rd = 4 x 254.2 = 1,017 kN

Step 5 -- Failure Mode 2: n = min(1.25 x 40, 50) = 50 mm Ft,2,Rd = (2 x 12.35 + 0.050 x 1,017) / (0.040 + 0.050) = 840 kN

The governing mode for the top bolt row is Mode 2: Ft,Rd,row1 = 840 kN.

Repeating for the second bolt row (closer to the beam flange, different effective length): Ft,Rd,row2 = 620 kN

Step 6 -- Column web panel shear: Avc = 1.04 x h x twc = 1.04 x 334.6 x 15.1 = 5,256 mm^2 VRd,wp = 0.9 x 355 x 5,256 / (sqrt(3) x 1.00) = 969 kN Applied shear in panel = 520 / (0.457 - 0.015) = 1,176 kN > 969 kN. Supplementary web panel required. A 10 mm S355 doubler plate is welded to the column web: Avc,total = 5,256 + 334.6 x 10 = 8,602 mm^2. VRd,wp = 1,586 kN > 1,176 kN -- OK.

Step 7 -- Column web in transverse compression: beff,c,wc = tfb + 2 x sqrt(2) x af + 5 x (tfc + s) + sp = 12.7 + 2 x sqrt(2) x 6 + 5 x (20.5 + 27) + 25 = 12.7 + 17.0 + 237.5 + 25 = 292.2 mm omega = 0.85 (reduction for shear interaction) kwc = 1.0 (no longitudinal compression) Fc,wc,Rd = 0.85 x 1.0 x 292.2 x 15.1 x 355 / 1.00 = 1,332 kN

Step 8 -- Moment resistance: z = 457.2 - 12.7 - 60 = 384.5 mm (lever arm from top row to compression centre) Mj,Rd = 0.3845 x 840 + (0.3845 - 0.090) x 620 = 323 + 183 = 506 kN.m

The moment resistance is slightly below the design moment (506 vs 520 kN.m). Options: increase end plate thickness to 30 mm, add a third active bolt row with a stiffer column, or increase the lever arm by extending the plate further above the top flange.

Step 9 -- Increase end plate to 30 mm: Mpl,Rd increases to 0.25 x 222.5 x 30^2 x 355 / 1.00 = 17.79 kN.m Mode 2: Ft,Rd,row1 = (2 x 17.79 + 0.050 x 1,017) / 0.090 = 961 kN Mj,Rd = 0.3845 x 961 + 0.2945 x 685 = 370 + 202 = 572 kN.m > 520 kN.m -- OK.

Final specification: End plate 640 x 220 x 30 in S355, 4 x M24 Class 10.9 HV bolts per row (2 tension rows), 8 mm fillet weld all round beam-to-end plate, 10 mm doubler plate to column web at panel zone.

Design Resources

Frequently Asked Questions

When do I need an extended end plate vs a flush end plate for a moment connection?

Use an extended end plate (with the plate projecting above the top flange) when the moment requires a larger lever arm. The extension moves the top bolt row 50-80 mm further from the centre of compression, increasing the moment resistance by 15-25% for the same bolt configuration. Flush end plates (plate flush with the beam top flange) are adequate for multi-storey braced frames where moments are typically 40-60% of the beam plastic moment. Extended end plates are standard for portal frame eaves connections where the design moment approaches or exceeds the beam capacity.

Do I need column web stiffeners in a UK moment connection?

Column web stiffeners (horizontal stiffeners aligned with the beam flanges, also called continuity plates) are required when the column web in transverse compression (Fc,wc,Rd) or tension (Ft,wc,Rd) is insufficient. For UK portal frames with S355 sections, stiffeners are typically required when the beam flange force exceeds approximately 800 kN for a 254 UC column or 1,200 kN for a 356 UC column. Doubler plates address shear deficiency in the web panel; horizontal stiffeners address tension/compression deficiency in the flanges. Both may be required in the same connection.

What bolt grade should I use for a UK portal frame moment connection?

M24 Class 10.9 HV assemblies to BS EN 14399-4 are standard for UK portal frame eaves and apex moment connections. The higher strength (fub = 1000 MPa vs 800 MPa for 8.8) provides greater bolt tension resistance, reducing the number of bolts required. HV bolts with the longer thread length accommodate the thicker end plates (25-35 mm) typical of portal frame moment connections. Preloading is mandatory for Category E connections where slip resistance and stiffness are required.

How do I account for prying action in the end plate?

The equivalent T-stub model in EN 1993-1-8 inherently accounts for prying action through the three failure modes. Mode 1 (complete flange yielding) represents the fully developed prying mechanism. Mode 2 represents partial prying where bolts fail after some plastic deformation of the flange. Mode 3 (bolt failure only) represents no prying. The prying ratio (Mode 3 / Mode 2) indicates the prying severity: a low ratio means prying dominates. In UK portal frames, the typical prying ratio is 1.4-1.8, meaning bolt force is 1.4-1.8 times the applied tension per fastener after prying.

Educational reference only. All design values are per BS EN 1993-1-8:2005 + UK National Annex. 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.

Disclaimer: This content is for educational purposes only. Results must be verified by a licensed professional engineer. Steel Calculator provides preliminary design tools — NOT a substitute for professional engineering judgment.