EN 1993-1-8 Moment-Resisting Connections — Extended End Plate, Haunch & Stiffener Design

Complete reference for EN 1993-1-8:2005 moment-resisting steel connections. Covers extended end plate moment connections (Clause 6.2.7), haunched connections (Clause 6.2.8), column web panel shear resistance (Clause 6.2.6), transverse stiffener design (Clause 6.2.6.2), and backing plates. Includes T-stub model analysis, bolt row force distribution, and interaction of moment with axial force in the tension and compression zones.

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Moment-Resisting Connections — Overview

Moment-resisting connections in steel frames transfer bending moments between beams and columns. Under EN 1993-1-8, connections are classified by stiffness (rigid, semi-rigid, nominally pinned) and by strength (full-strength, partial-strength, nominally pinned). For rigid, full-strength moment connections in multi-storey frames, the extended end plate bolted connection is the most common solution in UK and European practice.

The design philosophy under EN 1993-1-8 Clause 6.2 follows a component method: the connection is decomposed into basic components (column web in shear, column web in transverse compression, column web in transverse tension, column flange in bending, end plate in bending, bolts in tension, beam flange and web in compression), and each component is checked individually. The weakest component governs.

Extended End Plate Connection — Component Design

The extended end plate (EEP) connection is the workhorse moment connection in European steel design. It consists of a welded end plate on the beam that is bolted to the column flange, with bolt rows above the top flange and below the bottom flange (the "extension") providing the moment lever arm.

Key Components per EN 1993-1-8 Clause 6.2

Each bolt row contributes to the moment resistance based on the minimum of the component resistances at that row. The design moment resistance Mj,Rd is the sum of the bolt row forces times their respective lever arms.

Column Web Panel Shear (Clause 6.2.6.1)

The column web panel must resist the shear transmitted by the beam flanges:

Vwp,Ed = sum_Ft,Ed (from top rows) - sum_Fc,Ed (bottom rows)

For a single-sided connection: Vwp,Ed = Mb1,Ed / z - Vc1,Ed where z is the lever arm.

For double-sided connections, the shear from both beams is superimposed. The UK NA Annex considers the effect of axial compression in the column, which enhances shear resistance.

Design shear resistance (unstiffened):

Vwp,Rd = 0.9 * fy,wc * Avc / (sqrt(3) * gamma_M0)

Where Avc is the shear area of the column (EN 1993-1-1 Clause 6.2.6).

Worked Example — UKC 203x203x86, S355, single-sided connection:

• Avc (rolled I-section, load parallel to web) = A - 2btf + (tw + 2r)_tf but not less than eta _ hw * tw
• For UKC 203x203x86: h = 222.2 mm, b = 209.1 mm, tw = 12.7 mm, tf = 20.5 mm, r = 10.2 mm
• Avc = 11,000 - 2209.120.5 + (12.7 + 20.4)*20.5 = 11,000 - 8,573 + 679 = 3,106 mm^2
• eta _ hw _ tw = 1.0 _ 181.2 _ 12.7 = 2,301 mm^2 — OK
• Vwp,Rd = 0.9 _ 355 _ 3,106 / (1.732 * 1.00) = 573 kN

If Vwp,Ed > Vwp,Rd, supplementary web panels (doubler plates) or diagonal stiffeners are required.

Calculation of beta Parameter (Clause 5.3, Table 5.4)

The transformation parameter beta accounts for the distribution of panel shear moments:

• Single-sided connection, M1 only: beta = 1
• Double-sided, M1 = M2, equal and opposite: beta = 0 (zero panel shear)
• Double-sided, M1 = M2, same sign (double curvature): beta = 2
• General case: beta = abs(Mb1,Ed - Mb2,Ed) / max(abs(Mb1,Ed), abs(Mb2,Ed))

For a typical braced frame interior column: M1 = 300 kN.m, M2 = 180 kN.m (opposite sign): beta = abs(300 - (-180)) / max(300, 180) = 480/300 = 1.6.

beta = 2.0 is the conservative assumption for design (maximum panel zone shear).

Column Web Transverse Compression (Clause 6.2.6.2)

The column web must resist the compression force from the beam bottom flange. The design resistance:

Fc,wc,Rd = omega * kwc * beff,c,wc * twc * fy,wc / gamma_M0

But should be reduced by rho if the column web slenderness exceeds limits:

lambda_p_bar <= 0.72: Fc,wc,Rd = omega * kwc * beff,c,wc * twc * fy,wc / gamma_M0 (no reduction)
lambda_p_bar > 0.72: Fc,wc,Rd = omega * kwc * rho * beff,c,wc * twc * fy,wc / gamma_M1

Where:

• beff,c,wc = tfb + 2*sqrt(2)*af + 5*(tfc + s) + sp (sp is the length obtained by dispersion through the end plate at 45 degrees)
• omega = reduction factor for interaction with shear
• kwc = reduction factor for longitudinal compressive stress (kwc = 1.0 if n <= 0.7)
• rho = buckling reduction factor for class 4 webs
• lambda*p_bar = 0.932 * sqrt(beff,c,wc _ dwc _ fy,wc / (E _ twc^2))
• gamma_M1 = 1.00 (UK NA)

For a UKB 457x191x67 beam flange (tfb = 12.7 mm) connecting to a UKC 203x203x86, 6 mm fillet weld (a = 4.2 mm), S355:

• beff,c,wc = 12.7 + 21.4144.2 + 5*(20.5 + 10.2) = 12.7 + 11.9 + 153.5 = 178.1 mm
• lambda*p_bar = 0.932 * sqrt(178.1 _ 181.2 _ 355 / (210,000 _ 12.7^2)) = 0.932 _ sqrt(11,465,000 / 33,860,000) = 0.932 _ 0.582 = 0.542 < 0.72 — no buckling reduction
• Fc,wc,Rd = 1.0 _ 1.0 _ 178.1 _ 12.7 _ 355 / 1.00 = 803 kN

Transverse Stiffeners

When the unstiffened column web resistance is inadequate, transverse stiffeners (full-depth plates welded to both flanges and the web) are provided. Stiffeners must be:

• Full depth: from column flange to column flange (or as close as construction permits)
• Minimum width: 0.75 * bfc / 2 (half flange width minus root radius and weld access)
• Minimum thickness: ts >= tfb (beam flange thickness) and ts >= 10 mm
• Buckling check: stiffener outstand b/t <= 14 * sqrt(235/fy)

For S355: b/t <= 14 * sqrt(235/355) = 11.4. For a 200 mm wide stiffener: ts >= 200 / 11.4 = 17.5 mm — use 20 mm thick S355 stiffener plates.

T-Stub Model for End Plate Bending (Clause 6.2.6.5)

The tension zone is modelled as a T-stub per Clause 6.2.4. The effective length of the equivalent T-stub depends on the bolt row geometry and the yield line pattern:

Circular Yield Line Pattern (individual bolt row): leff,cp = 2 _ pi _ m

Non-Circular Yield Line Pattern (individual bolt row): leff,nc = 4m + 1.25e

Group Yield Line (end bolt row + inner row): leff,cp = 2pim + p (where p is the bolt row spacing) leff,nc = 3m + 1.25e + p

The minimum of the individual and group patterns is used for each failure mode.

Worked T-Stub — Extended End Plate, M20 Bolts, S355 Plate

Parameters:

• End plate thickness: tp = 20 mm
• Bolt row: M20 Grade 8.8, 2 bolts per row
• m = 40 mm (bolt centre to web/flange junction)
• e = 50 mm (edge distance)
• p = 90 mm (bolt row spacing, assumed 2 rows in tension zone)
• fy,p = 355 MPa (S355 plate)

Individual Bolt Row: leff,nc = 440 + 1.2550 = 160 + 62.5 = 222.5 mm leff,cp = 2 _ pi _ 40 = 251.3 mm

For Mode 1: leff,1 = min(222.5, 251.3) = 222.5 mm Mpl,1,Rd = 0.25 _ 222.5 _ 20^2 _ 355 / 1.00 = 0.25 _ 222.5 _ 400 _ 355 = 7.90 _ 10^6 N.mm = 7.90 kN.m FT,1,Rd = 4 _ 7.90 / 0.040 = 790 kN per bolt row (2 bolts)

For Mode 3: FT,3,Rd = 2 * 141.1 = 282.2 kN (bolts only, 2 bolts per row)

For Mode 2 (with n = min(50, 1.25*40) = 50 mm): FT,2,Rd = (2 * 7.90 + 50 * 282.2) / (40 + 50) = (15.8 + 14,110) / 90 = 14,126 / 90 = 157.0 kN

T-stub resistance per bolt row = min(790, 157.0, 282.2) = 157.0 kN (Mode 2 governs).

Note: The group yield line pattern must also be checked and may reduce the capacity.

Moment Resistance Calculation — Full EEP Connection

The design moment resistance is found by summing bolt row capacities times lever arms:

Mj,Rd = sum_i (Fr,tr,Rd,i * hi)

Where Fr,tr,Rd,i is the effective tension resistance of bolt row i (the minimum of column flange bending, end plate bending, bolts in tension, column web in tension, and beam web in tension at that row), and hi is the distance from the centre of compression to bolt row i.

Bolt rows are checked in order from the top. If the resistance of any row reduces the capacity, the summation is capped by the governing component.

Worked Example — 2-Row Extended End Plate

• Beam: UKB 457x191x67, depth 453.6 mm, top flange to bottom flange centre distance ~ 441 mm
• Bolt rows: Row 1 at +80 mm above top flange (extension), Row 2 at top flange level, Row 3 at 100 mm below Row 2
• Column: UKC 254x254x107, S355
• Lever arm to centre of compression: Row 1: 521 mm, Row 2: 441 mm, Row 3: 341 mm

Assume each bolt row has Tr,Rd = 157.0 kN (from T-stub analysis above).

Mj,Rd = 157.0 _ (0.521 + 0.441 + 0.341) = 157.0 _ 1.303 = 204.6 kN.m

For a full-strength connection: Required Mj,Rd >= 1.2 * 522 (beam Mc,Rd) = 626 kN.m → this 2-row configuration would be partial-strength. A deeper EEP with 4 bolt rows or haunched connection is needed for full-strength.

Haunched Connections (Clause 6.2.8)

Haunches are used when the EEP cannot achieve sufficient moment resistance at the beam depth. A haunch increases the lever arm by extending the beam depth locally at the connection.

Types of haunches:

The haunch is modelled as part of the beam. The critical section for the beam check shifts to the end of the haunch. The haunch flange and web must be checked for:

• Flange force at the haunch-to-beam junction (tension and compression)
• Haunch web buckling under compression
• Weld between haunch and beam flange/web

The SCI P398 and P397 design guides provide standardised haunch configurations for portal frames.

Frequently Asked Questions

How does EN 1993-1-8 classify moment connections by stiffness and strength?

EN 1993-1-8 classifies joints by initial rotational stiffness Sj,ini (rigid if Sj,ini >= 25EI/L for braced frames and 8EI/L for unbraced, nominally pinned if Sj,ini <= 0.5EI/L, semi-rigid otherwise) and by design moment resistance (full-strength if Mj,Rd >= 1.2Mb,pl,Rd or Mconnected,Rd, partial-strength otherwise). Rigid and full-strength connections are preferred for multi-storey braced frames so the bare frame analysis without joint springs is valid.

What is column web panel shear and how is it resisted?

Column web panel shear (EN 1993-1-8 Clause 6.2.6.1) is the shear demand on the column web from the unbalanced beam flange forces across a connection. The panel shear Vwp,Ed is the difference between top bolt row tensions and bottom compression. Unstiffened resistance Vwp,Rd = 0.9 _ fy,wc _ Avc / (sqrt(3) * gamma_M0). If exceeded, supplementary web panels (doubler plates) or diagonal stiffeners are added. The transformation parameter beta accounts for double-sided connections.

When are column web stiffeners required in moment connections?

Transverse stiffeners (EN 1993-1-8 Clause 6.2.6.2) are required when the unstiffened column web in transverse compression or tension is inadequate. Stiffeners must extend the full column depth, have width at least 0.75 _ bfc/2, thickness at least the beam flange thickness, and satisfy b/t <= 14 _ sqrt(235/fy) for the outstand. Stiffeners also improve the load path for the tension zone and can convert a partial-strength connection to full-strength by preventing column flange bending.

What is the T-stub model and how is it applied to end plate design?

The T-stub model (EN 1993-1-8 Clause 6.2.4) idealises the tension zone of a bolted end plate as an equivalent T-section flange in bending. Three failure modes exist: Mode 1 (complete flange yielding, FT,1,Rd = 4*Mpl,1,Rd/m), Mode 2 (bolt failure with flange yielding, combination formula), and Mode 3 (bolt failure only, FT,3,Rd = sum_Ft,Rd). The effective length leff depends on yield line patterns (circular and non-circular) and bolt row grouping. The minimum of three modes determines the row capacity.

How are haunched connections designed under EN 1993-1-8?

Haunched connections (EN 1993-1-8 Clause 6.2.8) increase the lever arm at the connection, achieving higher moment resistance without increasing beam depth. The haunch is fabricated from plate and welded to the beam. The critical section for beam checks moves to the end of the haunch. Haunch flange forces, web buckling, and welds must be verified. SCI P397 provides standard haunch geometries and capacities for UK portal frame practice.

Related Pages

• EN 1993-1-8 Connection Design — Bolts, Welds & Fin Plates
• EN 1993-1-1 Beam Design — Flexure, Shear & LTB
• EN 1993 Column Buckling — Curves a0-d
• EN 1993 CHS & Hollow Section Joints
• EN 1993 Steel Grades — S235, S275, S355, S460
• Bolted Connections Calculator — Free EN 1993 Tool
• Column Capacity Calculator — Free Online Tool

Educational reference only. Verify all design values against the current EN 1993-1-8 and the applicable National Annex for your jurisdiction. Moment connection design is project-specific — always check the latest SCI/BCSA guidance. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent verification by a qualified structural engineer.