EN 1993-1-8 Hollow Section Joints — CHS, SHS & RHS Welded Connection Design

Complete reference for EN 1993-1-8:2005 Clause 7 welded hollow section joint design. Covers circular hollow section (CHS), square hollow section (SHS), and rectangular hollow section (RHS) connections. Design methods for chord face failure, chord side wall buckling, chord shear, punching shear, and brace member failure. Addresses K, N, Y, T, and X joint configurations with gap and overlap connections. Includes worked examples using the SCI/BCSA design method for UK practice.

Quick access: EN 1993 Connection Design | Column Capacity Calculator | EN 1993 Beam Design | EN 1993 Steel Grades

Hollow Section Joint Design — Overview

EN 1993-1-8 Clause 7 provides the detailed design rules for welded joints between hollow sections. The scope of application in Table 7.1 sets geometric validity ranges that must be satisfied for the design formulae to apply. The design method is based on extensive CIDECT research and is validated for hot-finished and cold-formed hollow sections conforming to EN 10210 and EN 10219.

Hollow section joints are classified by the configuration of the incoming brace members relative to the chord:

Joints with a gap between braces (g >= t1 + t2) behave differently from joints with overlapping braces (lambda_ov > 25%). Gap joints are governed by chord face plastification, while overlap joints transfer load directly between braces.

Geometric Validity Range (EN 1993-1-8 Table 7.1)

The design formulae are valid only when the joint geometry falls within these limits:

Where: d0 = chord diameter (CHS), b0 = chord width (RHS), t0 = chord wall thickness, d1/b1 = brace diameter/width, t1 = brace wall thickness, theta = brace-to-chord angle.

For a typical truss: CHS 168.3 x 6.3 chord (S355, Class 1), CHS 88.9 x 5.0 braces (S355), beta = 88.9/168.3 = 0.53 (OK), 2*gamma = 168.3/6.3 = 26.7 < 50 (OK), theta = 45 degrees (OK).

Chord Face Failure — CHS Joints (Table 7.2)

For T, Y, and X joints in CHS, chord face failure (plastification) is the most common limit state:

T and Y joints (axial force in brace):

N1,Rd = (gamma^0.2 * kp * fy0 * t0^2 / sin(theta1)) *
        (2.8 + 14.2 * beta^2) / gamma_M5

X joints (axial force in brace, tension or compression):

N1,Rd = (kp * fy0 * t0^2 / sin(theta1)) *
        (5.2 / (1 - 0.81 * beta)) / gamma_M5

Where:

• kp = chord prestress factor = 1.0 for n <= 0 (tension); for compression n = N0,Ed/Npl,0,Rd + M0,Ed/Mpl,0,Rd
  • kp = 1.0 for n <= 0
  • kp = 1.0 - 0.3 _ n _ (1 + n) for n > 0 in CHS
• fy0 = chord yield strength
• gamma = d0 / (2*t0)
• beta = d1 / d0
• gamma_M5 = 1.00 (UK NA)

Worked Example -- CHS T Joint:

Chord: CHS 219.1 x 8.0, S355 (fy0 = 355 MPa, d0 = 219.1 mm, t0 = 8.0 mm) Brace: CHS 114.3 x 5.0, S355 (d1 = 114.3 mm, t1 = 5.0 mm) Angle: theta1 = 90 degrees (T joint) Chord unstressed: n = 0, kp = 1.0

beta = 114.3 / 219.1 = 0.522 2*gamma = 219.1 / 8.0 = 27.4 gamma = 13.7 gamma^0.2 = 13.7^0.2 = 1.685

N1,Rd = 1.685 _ 1.0 _ 355 _ 8.0^2 / 1.0 _ (2.8 + 14.2 _ 0.522^2) / 1.00 = 1.685 _ 355 _ 64 _ (2.8 + 14.2 _ 0.272) = 38,284 _ (2.8 + 3.86) = 38,284 * 6.66 = 254,971 N = 255.0 kN

Chord Face Failure -- RHS Joints (Table 7.11)

For T, Y, and X joints in RHS chords with RHS or CHS braces:

T and Y joints (axial force in brace):

N1,Rd = (kn * fy0 * t0^2 / ((1 - beta) * sin(theta1))) *
        ((2 * eta) / sin(theta1) + 4 * sqrt(1 - beta)) / gamma_M5

Where:

• beta = b1 / b0
• eta = h1 / b0
• kn = chord stress factor (= 1.0 for n <= 0; = 1.3 - 0.4*n/beta for n > 0 but <= 1.0)
• gamma_M5 = 1.00

Worked Example -- RHS T Joint:

Chord: RHS 200 x 100 x 8.0, S355 (b0 = 200 mm, h0 = 100 mm, t0 = 8.0 mm) Brace: RHS 120 x 80 x 6.3, S355 (b1 = 120 mm, h1 = 80 mm, t1 = 6.3 mm) Angle: theta1 = 90 degrees Chord unstressed: n = 0, kn = 1.0

beta = 120 / 200 = 0.60 (>= 0.35, OK) eta = 80 / 200 = 0.40 sqrt(1 - beta) = sqrt(1 - 0.60) = 0.632

N1,Rd = 1.0 _ 355 _ 8.0^2 / ((1 - 0.60) _ 1.0) _ (20.40 + 40.632) / 1.00 = 355 _ 64 / 0.40 _ (0.80 + 2.528) = 22,720 / 0.40 _ 3.328 = 56,800 _ 3.328 = 189,030 N = 189.0 kN

K and N Joints -- Gap Connections (Table 7.2, CHS)

For K and N joints in CHS with a gap between braces:

Chord face failure:

N1,Rd = (kg * kp * fy0 * t0^2 / sin(theta1)) *
        (1.8 + 10.2 * d1/d0) / gamma_M5

For brace 2:

N2,Rd = N1,Rd * sin(theta1) / sin(theta2)

Where kg = gamma^0.2 * (1 + (1/(1 + g/t0))*0.024*gamma^1.2)

Worked Example -- CHS K Joint (Warren Truss Node):

Chord: CHS 219.1 x 10.0, S355 (d0 = 219.1 mm, t0 = 10.0 mm) Brace 1 (tension): CHS 114.3 x 5.0 (d1 = 114.3 mm, t1 = 5.0 mm), theta1 = 45 degrees Brace 2 (compression): CHS 114.3 x 5.0 (d2 = 114.3 mm, t2 = 5.0 mm), theta2 = 45 degrees Gap: g = 25 mm Chord unstressed: n = 0, kp = 1.0

beta = 114.3 / 219.1 = 0.522 2gamma = 219.1 / 10.0 = 21.9, gamma = 10.95 gamma^0.2 = 10.95^0.2 = 1.615 kg = 1.615 * (1 + (1/(1 + 25/10.0))0.02410.95^1.2) = 1.615 * (1 + (1/3.5)0.02418.46) = 1.615 * (1 + 0.2860.443) = 1.615 * (1 + 0.127) = 1.615 * 1.127 = 1.820

N1,Rd = 1.820 _ 1.0 _ 355 _ 10.0^2 / sin(45) _ (1.8 + 10.2 _ 0.522) / 1.00 = 1.820 _ 355 _ 100 / 0.7071 _ (1.8 + 5.32) = 64,610 / 0.7071 _ 7.12 = 91,370 _ 7.12 = 650,554 N = 650.6 kN

N2,Rd = 650.6 * sin(45) / sin(45) = 650.6 kN (symmetric joint)

Chord Side Wall Buckling -- RHS (Table 7.11)

For RHS T, Y, and X joints where beta = 1.0 (brace width equals chord width), chord side wall buckling may govern:

N1,Rd = (kn * fb * t0 / sin(theta1)) *
        (2 * h1 / sin(theta1) + 10 * t0) / gamma_M5

Where fb = chi * fy0 and chi is the buckling reduction factor for the chord side wall treated as a column with buckling length h0 - 2t0 and slenderness lambdabar = 3.46 * (h0/t0 - 2) _ sqrt(fy0/E).

For a RHS 150 x 150 x 8.0 chord (S355, h0/t0 = 18.75), brace RHS 150 x 150 x 8.0 (beta = 1.0), theta1 = 90 degrees: lambda*bar = 3.46 * (18.75 - 2) _ sqrt(355/210,000) = 3.46 _ 16.75 _ 0.0411 = 2.38 From buckling curve c (alpha = 0.49): chi ~ 0.18 fb = 0.18 * 355 = 63.9 MPa

N1,Rd = 1.0 _ 63.9 _ 8.0 / 1.0 * (2150 + 108.0) / 1.00 = 511.2 * (300 + 80) = 511.2 * 380 = 194,256 N = 194.3 kN

This side wall buckling capacity is often the governing limit state for matched-width RHS joints.

Punching Shear -- CHS and RHS (Tables 7.2, 7.11)

Punching shear failure occurs when the brace pulls through the chord face:

CHS joints (K, N, KT, T, Y):

N1,Rd = (fy0 / sqrt(3)) * t0 * pi * d1 *
        (1 + sin(theta1)) / (2 * sin^2(theta1)) / gamma_M5

RHS joints (T, Y, X):

N1,Rd = (fy0 / sqrt(3)) * t0 * (2*h1/sin(theta1) + 2*be,p) / gamma_M5

Where be,p = (10 / (b0/t0)) * (fy0t0 / (fy1t1)) * b1 but be,p <= b1.

Worked Example -- CHS T Joint Punching Shear:

Chord: CHS 219.1 x 8.0, S355 (fy0 = 355 MPa) Brace: CHS 114.3 x 5.0, S355, theta1 = 90 degrees

N1,Rd = (355/1.732) _ 8.0 _ pi _ 114.3 _ (1 + 1.0) / (2 _ 1.0) / 1.00 = 205.0 _ 8.0 _ 3.142 _ 114.3 _ 2.0 / 2.0 = 205.0 _ 8.0 _ 3.142 _ 114.3 = 205.0 * 2,873 = 589,000 N = 589.0 kN

Compare to chord face failure N1,Rd = 255.0 kN (from earlier). Chord face failure governs by a large margin -- punching shear rarely governs for CHS joints with practical geometry.

Overlap K and N Joints (Table 7.10)

For overlap joints (lambda_ov >= 25%), load transfer occurs directly between braces through the overlap, and the chord face is less loaded.

For 50% <= ov < 80%:

N1,Rd = fy1 * t1 * (beff + be,ov + 2*h1/sin(theta1) - 4*t1) / gamma_M5

Where be,ov is the effective width at the overlap.

For ov >= 80% (full overlap, treated as a single brace):

N1,Rd = fy1 * t1 * (beff + b1 + 2*h1/sin(theta1) - 4*t1) / gamma_M5

Overlap joints typically provide higher capacity than gap joints for the same chord size, because brace-to-brace load transfer is more direct. However, fabrication is more complex and the hidden weld between braces must be carefully detailed.

Design Tables -- Typical Resistance Values

CHS T and Y Joint Axial Capacities (S355, gamma_M5 = 1.00, unstressed chord, theta = 90 degrees)

Note: Capacities are indicative for unstressed chords. Chord prestress (axial load/moment in the chord) reduces capacity via kp/kn factors. Always check all limit states for the final design.

Frequently Asked Questions

What are the main failure modes for welded hollow section joints under EN 1993-1-8?

EN 1993-1-8 Clause 7 identifies six primary failure modes for hollow section joints: (1) chord face failure (plastification of the chord connecting face, most common for low-to-moderate beta ratios), (2) chord side wall buckling or yielding (for beta approaching 1.0), (3) chord shear (for K/N joints with high brace forces), (4) punching shear (brace pulling through the chord face), (5) brace failure with reduced effective width, and (6) local buckling of the brace or chord member. The governing mode depends on beta, gamma (chord slenderness), and the joint type.

What is the difference between gap and overlap K joints in hollow section design?

Gap K joints have a clear separation between brace members (g >= t1 + t2). Load transfers from brace to chord face (chord face plastification governs). They are easier to fabricate but may require a larger chord. Overlap K joints (lambda_ov >= 25%) have braces that overlap each other. Load transfers directly between overlapping braces, reducing demand on the chord face. Overlap joints achieve higher capacity for the same chord size but are more complex to fabricate and require a hidden weld. For ov >= 80%, the joint is treated as a single brace per EN 1993-1-8.

How does chord prestress affect hollow section joint capacity?

Chord prestress (axial stress or bending stress in the chord member at the joint location) reduces the available chord face capacity. EN 1993-1-8 accounts for this through the kp factor for CHS (kp = 1.0 - 0.3n(1+n) for compression, where n = N0,Ed/Npl,0,Rd + M0,Ed/Mpl,0,Rd) and the kn factor for RHS (kn = 1.3 - 0.4*n/beta for compression, kn >= 1.0). For chords in tension (n < 0), kp = kn = 1.0. Compressive prestress can reduce joint capacity by up to 30% for heavily loaded chords.

What geometric limits must be satisfied for the EN 1993-1-8 design formulae to apply?

EN 1993-1-8 Table 7.1 defines validity ranges: beta = d1/d0 or b1/b0 between 0.2-1.0 (CHS: 0.2-1.0; RHS: 0.35-1.0); chord slenderness d0/t0 or b0/t0 limited to Class 1 for CHS and <= 35 for RHS; brace wall thickness ratio tau = t1/t0 between 0.2 and 1.0; brace angle theta >= 30 degrees; brace slenderness d1/t1 or b1/t1 Class 1 or 2; gap g >= t1 + t2 (gap joints); overlap between 25% and 100%. Joints outside these limits require special justification, typically by validated FE analysis or physical testing.

How does the EN 1993-1-8 hollow section joint design compare to CIDECT design guides?

The EN 1993-1-8 Clause 7 design rules are directly based on CIDECT research and design guides (CIDECT Design Guide No. 1 for CHS, No. 3 for RHS). The equations are substantively identical, but EN 1993-1-8 incorporates the partial factor gamma_M5 for joint resistance (UK NA = 1.00) and limits the validity range slightly more conservatively than CIDECT guides. The CIDECT guides provide more detailed background, additional joint configurations (multi-planar joints), and extensive worked examples -- they are the essential companion to the EN 1993-1-8 code text for practising designers.

Related Pages

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

Educational reference only. Verify all design values against the current EN 1993-1-8, the applicable National Annex, and the CIDECT design guides. Hollow section joint design is complex -- always check the latest SCI/BCSA guidance and CIDECT recommendations. Results are PRELIMINARY -- NOT FOR CONSTRUCTION without independent verification by a qualified structural engineer.