UK HSS Connections — EN 1993-1-8 Design Guide

This reference covers welded hollow section connections for UK steel design per EN 1993-1-8:2005 Clause 7 and SCI P399. Hollow sections (SHS, RHS, CHS) offer architectural advantages and efficient structural performance but require specific connection design rules that differ from open-section connections.

Design requirements, worked examples, and practical design guidance are provided for common design office applications.

Code Reference: EN 1993-1-8:2005 Clause 7 and SCI P399

Joint Types Covered by EN 1993-1-8 Clause 7

Validity Limits (EN 1993-1-8 Table 7.1)

For rectangular hollow sections (RHS):

For circular hollow sections (CHS):

Design Resistance — Rectangular Hollow Section Joints

Chord face plastification (T, Y, X joints): [ N*{1,Rd} = \frac{f*{y0} t0^2}{\gamma{M5} (1 - \beta)} \times \frac{2\eta}{\sin\theta_1} \times k_n ]

Where:

• β = b₁/b₀, η = h₁/b₀
• kn = 1.0 (tension), kn = 1.0 - 0.3 n (1 + n) for compression (n = N₀,Ed/Npl,Rd)
• γM5 = 1.0 (EN 1993-1-8, matching UK NA recommendation)

Chord face plastification (K, N gap joints): [ N*{1,Rd} = 8.9 \frac{f*{y0} t0^2}{\gamma{M5}} \times \frac{\sqrt{b_0/t_0} \times (b_1 + b_2 + h_1 + h_2)}{4\sqrt{b_0 t_0} \sin\theta_i} \times k_n ]

HSS Joint Capacity Table — SHS Welded T-Joints

Chord: 150×150×8 SHS (S355), Brace: 100×100×6.3 SHS (S355)

HSS Joint Capacity Table — RHS Welded K-Joints (Gap)

Chord: 200×100×8 RHS (S355), Braces: 100×50×5 RHS (S355)

Failure Modes for Welded HSS Joints

Weld Design for HSS Joints

Fillet welds for HSS joints are designed per EN 1993-1-8 Clause 4 as for open sections. For CHS joints, welds are distributed around the circumference. For SHS/RHS joints, welds are on the three accessible sides of the brace.

Minimum weld throat for HSS connections: typically 3-4 mm for standard wall thicknesses. Effective weld length = 2h₁/sinθ₁ + b₁ for RHS braces (two side welds + one end weld).

Worked Example — SHS T-Joint

Given:

• Chord: 150×150×8 SHS, S355 (fy = 355 MPa)
• Brace: 100×100×6.3 SHS, S355 (θ = 90°)
• Chord compression force: N₀,Ed = 400 kN (n = -0.3)
• Brace tension force: N₁,Ed = 180 kN

Check validity limits:

• b₁/b₀ = 100/150 = 0.67 — OK (0.25 ≤ 0.67 ≤ 0.85)
• b₀/t₀ = 150/8 = 18.75 — OK (≤ 35)
• b₁/t₁ = 100/6.3 = 15.9 — OK (≤ 35)

Chord face plastification resistance: β = 0.67, η = 100/150 = 0.67, 2γ = 18.75, θ = 90° (sinθ = 1.0)

For n = -0.3 (compression): kn = 1.0 - 0.3 × (-0.3)(1 + (-0.3)) = 1.0 + 0.3 × 0.3 × 0.7 = 1.06 → limit kn = 1.0

N₁,Rd = (355 × 8² × 1.0) / (1.0 × (1 - 0.67)) × (2 × 0.67 / 1.0) × 1.0 × 10⁻³ N₁,Rd = (22,720) / (0.33) × 1.34 × 10⁻³ = 92.3 kN

UT = 180 / 92.3 = 1.95 — Not satisfactory

Options:

• Increase chord wall thickness: try 150×150×10 SHS (t₀ = 10 mm) N₁,Rd = (355 × 100) / (0.33) × 1.34 × 10⁻³ = 144 kN — Still not enough
• Increase brace width ratio: try 120×120×8 brace β = 120/150 = 0.8, η = 0.8 N₁,Rd = (355 × 64) / (0.33) × (2×0.8/1.0) × 10⁻³ = 110 kN
• Add chord reinforcement: plate stiffener on chord face
• Use stiffening ring or doubler plate: increases effective chord thickness

Design Resources

• UK Connection Design — General connection guidance
• UK Weld Capacity — Fillet weld design
• UK Steel Properties — Material data
• UK Steel Beam Sizes — Section dimensions
• UK Framing Systems — Structural systems
• All UK References

Frequently Asked Questions

How does EN 1993-1-8 address HSS connections?

EN 1993-1-8 Clause 7 gives design rules for welded hollow section joints with validity limits in Table 7.1. UK practice follows SCI P399 for additional guidance on SHS/RHS joints including chord face plastification, chord side wall buckling (β near 1.0), punching shear, and brace failure. The joint capacity depends on: β ratio (brace width / chord width), 2γ ratio (chord width / thickness), chord stress ratio n, and joint angle θ.

What validity limits apply to UK HSS joints?

EN 1993-1-8 Table 7.1 gives validity limits: the β ratio b₁/b₀ must be between 0.25 and 0.85 for rectangular sections, and d₁/d₀ between 0.2 and 1.0 for circular sections. Chord slenderness b₀/t₀ ≤ 35 for RHS and d₀/t₀ ≤ 50 for CHS. For K-joints, the gap g ≥ t₁ + t₂ (minimum). For joints outside these limits, testing per EN 1990 Annex D or finite element analysis calibrated against test data is required.

What is chord face plastification in HSS joints?

Chord face plastification is the primary failure mode for HSS T-, Y-, and X-joints with β < 0.85. It occurs when the brace member forces cause yielding of the chord face (the wide flange face of the cross-section). The resistance is proportional to f_y0 × t₀² (the chord yield strength times thickness squared). Increasing chord thickness is the most effective way to increase joint capacity. The chord stress parameter kn accounts for the effect of axial stress in the chord, reducing capacity when the chord is in compression.

When is punching shear the governing failure mode?

Punching shear of the chord wall governs for joints with thin chord walls and moderate β ratios (approximately 0.5-0.8). The check per EN 1993-1-8 Clause 7.5.4.2 compares the component of the brace force perpendicular to the chord face against the punching shear resistance of the chord wall. For SHS with t₀ ≤ 6.3 mm, punching shear often governs. Increasing chord thickness or adding a reinforcing doubler plate on the chord face addresses this mode.

What guidance does SCI P399 provide for UK practice?

SCI P399 ("Design of Welded Joints in Structural Hollow Sections") provides comprehensive UK design guidance supplementing EN 1993-1-8 Clause 7. It covers: (a) joints outside the validity limits of Table 7.1 (especially for wall slenderness beyond 35), (b) reinforced joints (doubler plates, stiffening rings), (c) concrete-filled hollow section joints (significant chord enhancement), (d) truss design detailing including eccentricity limits, and (e) fatigue design of HSS joints to EN 1993-1-9.

Reference only. Verify all values against the current edition of EN 1993-1-8:2005 Clause 7 and SCI P399. This information does not constitute professional engineering advice.