EN 1993-1-8 HSS Connection Design — Welded CHS and RHS Joints

Complete reference for welded hollow section connections per EN 1993-1-8:2005 Clause 7. Covers circular hollow sections (CHS) and rectangular/square hollow sections (RHS/SHS) in T, Y, X, and K-joint configurations. Includes chord face plastification, punching shear, chord side wall bearing, and brace failure for European steel design per EN 1993.

Connection types include T, Y, X, K-gap, and K-overlap joints. Each has specific validity limits and capacity functions. Local yielding, punching shear, chord face plastification, and chord side wall bearing are the critical failure modes.

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Code Reference: EN 1993-1-8:2005 Clause 7

EN 1993-1-8 Clause 7 provides design rules for welded joints between hollow sections. The clauses cover:

Joint Types and Failure Modes

Validity Limits for CHS Joints (EN 1993-1-8 Table 7.1)

Before applying capacity formulas, the joint geometry must satisfy:

CHS T/Y-Joint Capacity — Chord Face Plastification

For CHS T and Y joints (EN 1993-1-8 Table 7.2):

[ N*{1,Rd} = \frac{k_p \times k_g \times f*{y0} \times t0^2}{\gamma{M5} \times \sin \theta_1} \times (2.8 + 14.2 \times \beta^2) ]

Where:

CHS K-Gap Joint Capacity — Chord Face Plastification

For CHS K-gap joints:

[ N*{1,Rd} = \frac{k_p \times k_g \times f*{y0} \times t0^2}{\gamma{M5} \times \sin \theta_1} \times \left( \frac{1.8 + 10.2 \times \frac{d_1}{d_0}}{\sqrt{1 + \frac{2 \times g}{d_0}}} \right) ]

The gap factor (k_g = 1 + \frac{50 \times t_0}{d_0} \times \frac{0.02}{0.02 + g/d_0}) applies for K-gap joints.

RHS T/Y-Joint Capacity — Chord Face Plastification

For RHS T and Y joints (EN 1993-1-8 Table 7.11):

[ N*{1,Rd} = \frac{k_n \times f*{y0} \times t0^2}{\gamma{M5} \times \sin \theta_1 \times (1 - \beta)} \times \left( \frac{2 \times \eta}{\sin \theta_1} + 4 \times \sqrt{1 - \beta} \right) ]

Where (\eta = h_1 / b_0) (brace depth to chord width ratio) and (\beta = b_1 / b_0) (brace width to chord width ratio).

For (\beta \leq 0.85): chord face plastification governs. For (\beta > 0.85): chord side wall bearing may govern.

Worked Example — CHS K-Gap Brace Connection

Problem: Design a CHS K-gap connection for a roof truss. Chord CHS 168.3 (\times) 8.0 (S355J2), braces CHS 88.9 (\times) 5.0 (S355J2). Brace angle θ = 45°. Gap g = 20 mm. Chord axial compression: Np,Ed = 400 kN. Brace design force: N1,Ed = 280 kN (tension), N2,Ed = 250 kN (compression).

Step 1 — Validity checks: Chord: d0/t0 = 168.3/8.0 = 21.0 < 50 — OK. Brace: d1/t1 = 88.9/5.0 = 17.8 < 50 — OK. Diameter ratio: d1/d0 = 88.9/168.3 = 0.528 — within 0.2 to 1.0 — OK. Brace angle: 45° ≥ 30° — OK. Gap: g = 20 mm > t1 + t2 = 5.0 + 5.0 = 10 mm — OK.

Step 2 — Chord stress factor kp: Chord area A0 = π (\times) (168.3² - 152.3²) / 4 = 4,030 mm² Chord stress: σp,Ed = 400,000 / 4,030 = 99.3 MPa fy0 = 355 MPa np = 99.3 / 355 = 0.280 (compression, np > 0) (k_p = 1 - 0.3 \times 0.280 \times (1 + 0.280) = 1 - 0.108 = 0.892)

Step 3 — Gap factor kg: (k_g = 1 + \frac{50 \times 8.0}{168.3} \times \frac{0.02}{0.02 + 20/168.3} = 1 + 2.376 \times \frac{0.02}{0.139} = 1 + 0.342 = 1.342)

Step 4 — Capacity (chord face plastification): (N_{1,Rd} = \frac{0.892 \times 1.342 \times 355 \times 8.0^2}{1.0 \times \sin 45°} \times \left( \frac{1.8 + 10.2 \times 0.528}{\sqrt{1 + 2 \times 20 / 168.3}} \right)) (= \frac{0.892 \times 1.342 \times 355 \times 64}{0.707} \times \left( \frac{1.8 + 5.386}{\sqrt{1 + 0.238}} \right)) (= \frac{27,210}{0.707} \times \frac{7.186}{1.112} = 38,488 \times 6.462 = 248.7) kN

Utilisation (tension brace): 280 / 248.7 = 1.126 — capacity exceeded. Increase chord wall thickness or brace diameter.

Step 5 — Revised section: Try CHS 168.3 (\times) 10.0 chord: t0 = 10.0 mm, d0/t0 = 16.8. A0 = 4,970 mm², np = 400/4,970/355 (\times) 1000 = 0.227 (revised). kp = 1 - 0.3 (\times) 0.227 (\times) 1.227 = 0.916 kg = 1 + (50 (\times) 10.0/168.3) (\times) 0.02/(0.02 + 0.119) = 1 + 2.971 (\times) 0.144 = 1.428

(N_{1,Rd} = \frac{0.916 \times 1.428 \times 355 \times 100}{0.707} \times 6.462 = \frac{46,466}{0.707} \times 6.462 = 65,724 \times 6.462 = 424.7) kN

424.7 kN > 280 kN — OK. Utilisation = 0.66.

Step 6 — Punching shear check: (N*{1,Rd,ps} = \frac{f*{y0} \times t0}{\sqrt{3} \times \gamma{M5}} \times \pi \times d_1 \times \frac{1 + \sin \theta_1}{2 \sin^2 \theta_1}) (= \frac{355 \times 10.0}{1.732} \times \pi \times 88.9 \times \frac{1 + 0.707}{2 \times 0.500}) (= 2,049 \times 279.3 \times 1.707 = 977) kN >> 280 kN — OK.

Step 7 — Weld design: Fillet weld throat: a = 5.0 mm (full strength for CHS 88.9 (\times) 5.0 S355). (F*{w,Rd} = f_u / (\sqrt{3} \times \beta_w \times \gamma*{M2}) \times a = 490 / (1.732 \times 0.90 \times 1.25) \times 5.0 = 1,257) N/mm weld length. Weld length ≈ π (\times) 88.9 = 279 mm. Total weld capacity: 1,257 (\times) 279 / 1,000 = 351 kN > 280 kN — OK with 25% reserve.

Weld Detailing for HSS Joints

Design Resources

• EN 1993 Steel Grades
• European Steel Properties
• EN 1993 Bolt Capacity
• EN 1993 CHS Connection
• EN 1993 Weld Sizes
• All European References

Frequently Asked Questions

How does EN 1993-1-8 address HSS connection design? EN 1993-1-8 Clause 7 provides nominal capacity formulas for welded connections between hollow sections (CHS and RHS). The four primary failure modes are: chord face plastification (most common for β ≤ 0.85), punching shear (thin chords with small braces), chord side wall bearing (β > 0.85 for RHS), and chord shear (K-gap joints). The capacity formulas in Tables 7.2-7.14 include chord stress effects (kp or kn factors) that account for the axial stress in the chord member. Each joint type has specific validity limits in Tables 7.1 and 7.10.

What CHS connection types are covered by EN 1993-1-8? EN 1993-1-8 Clause 7 covers welded CHS T, Y, X, K-gap, and K-overlap joints. Each has specific validity limits for diameter ratio (d_i/d_0 = 0.2 to 1.0), wall slenderness (d/t ≤ 50), and brace angle (θ ≥ 30°). N-joints (one brace perpendicular, one diagonal) follow the K-joint rules. For joints outside these validity limits, the capacity must be determined by testing per EN 1993-1-8 Clause 7.1.2 or by finite element analysis validated against test data.

What is the chord stress effect in HSS joint capacity? The chord stress factor kp (CHS) or kn (RHS) reduces the joint capacity when the chord is in compression. For tension chords, kp = kn = 1.0 (no reduction). For compression chords, kp = 1 - 0.3 × np × (1 + np) where np = σp,Ed / fy0. A chord at 50% utilisation (np = 0.5) gives kp = 0.775 — a 22.5% reduction in joint capacity. For highly loaded chords (np > 0.6), the reduction is severe and may govern the connection design. This makes chord stress the most influential parameter in HSS connection design.

When should K-overlap joints be preferred over K-gap joints? K-overlap joints (25% ≤ λov ≤ 100%) offer higher capacity per brace than K-gap joints because the overlapping brace transfers force directly to the overlapped brace rather than through the chord face. Overlap joints avoid chord face plastification as the governing failure mode and are preferred when: (1) the chord is highly loaded (np > 0.5), (2) the gap between braces would be very small (< t1 + t2), or (3) higher capacity is needed without increasing chord thickness. The disadvantage is that detailing is more complex — the hidden part of the overlapping brace is not welded, and inspection is difficult.

What is the minimum brace angle for welded HSS joints? EN 1993-1-8 Clause 7 requires θ ≥ 30° for all CHS and RHS welded connections. Below 30°, the eccentricity between brace and chord axes becomes excessive, and the weld length on the chord face is insufficient to develop the brace capacity. For θ < 30°, use a gusset plate connection or a thickened chord section. In truss design, brace angles of 35-55° are typical — 45° is the most efficient for balancing truss depth and brace length.

Reference only. Verify all values against the current edition of EN 1993-1-8:2005 Clause 7. Educational reference only.