Joint Configurations — EN 1993-1-8 Chapter 7

Hollow section joints are classified by the load path geometry. The brace members transfer axial forces (tension or compression) to the chord, which distributes the load through chord wall mechanisms.

Key Geometric Parameters

Per EN 1993-1-8 Table 7.1, the joint capacity depends on dimensionless geometric ratios:

Joints outside these validity ranges require special justification or are outside the scope of EN 1993-1-8 Chapter 7.

Failure Modes for RHS Joints

Mode A — Chord Face Plastification (Clause 7.4.1)

The most common failure mode for RHS joints with moderate β ratios (0.35–0.85). The chord face deforms plastically under the brace axial force.

For T and Y joints: N_1,Rd = (k_n × f_y0 × t_0² / sinθ_1) × (2β / (1 − β) + 4 / √(1 − β)) / γ_M5

Where k_n = 1.0 for chord face in tension, or k_n = 1.3 − 0.4n/β (≤ 1.0) for compression chord with stress ratio n = σ_0,Ed / f_y0.

For K and N gap joints: N_1,Rd = (k_n × f_y0 × t_0² / sinθ_1) × (14βγ^0.5) / γ_M5

Mode B — Chord Side Wall Buckling (Clause 7.4.2)

Governs for RHS joints with β close to 1.0, where the brace is nearly as wide as the chord and loads the chord side walls directly.

For T, Y, and X joints: N_1,Rd = (f_b × t_0 / sinθ_1) × (2t_1 + 10t_0) / γ_M5

Where f_b = χ × f_y0 with χ from buckling curve a (EN 1993-1-1 Clause 6.3.1.1 for the chord side wall as a strut). Effective length = 3.46 × (h_0/sinθ_1 − 2t_0).

Mode C — Brace Punching Shear (Clause 7.4.3)

Critical for joints with thin chord faces and small brace angles. The brace perimeter punches through the chord face.

N_1,Rd = (f_y0 / √3) × t_0 × (2h_1/sinθ_1 + 2b_e,p) / (sinθ_1 × γ_M5)

Where b_e,p is the effective punching shear perimeter width per Clause 7.4.3(2).

Mode D — Chord Shear Failure (Clause 7.4.4)

Governs in K-gap joints with high brace forces and small gap. The chord cross-section shears between the brace toes.

N_0,Rd = 0.9 × f_y0 × A_v,0 / (√3 × γ_M5)

For RHS chords: A_v,0 = (2h_0 + α × b_0) × t_0 with α = 1 / (1 + 4g²/(3t_0²)).

CHS Joint Design — CIDECT Design Guide No. 8

CHS T- and Y-Joints

Chord face plastification controls for most CHS joints: N_1,Rd = (f_y0 × t_0² / sinθ_1) × (2.8β + 14.2β²) × k_p × k_a / γ_M5

Where k_p accounts for chord prestress and k_a accounts for the gap between chord and brace welds.

CHS K-Gap Joints

The standard truss joint configuration: N_1,Rd = (f_y0 × t_0² / sinθ_1) × (1.8 + 10.2 × d_1/d_0) × k_g × k_p / γ_M5

Where k_g = γ^0.2 × (1 + 0.024γ^1.2 / (1 + e^(0.5g/t_0 − 1.33))), accounting for the gap effect on chord plastification.

Worked Example — RHS K-Gap Joint

Step 1 — Validity Check

Step 2 — Chord Face Plastification (Mode A)

k_n = 1.3 − 0.4 × |240|/(355 × 4,660 × 8/1000) / 0.60 (simplified: stress ratio n < 0, tension chord face → k_n = 1.0 at the tension brace side)

N_1,Rd = (1.0 × 355 × 8² / sin50°) × (14 × 0.60 × 25^0.5) / 1.00 = (355 × 64 / 0.7660) × (14 × 0.60 × 5.0) / 1.00 = 29,660 × (42.0) / 1.00 = 1,245,700 N = 1,245.7 kN

Brace utilisation: 180 / 1,245.7 = 0.144 — OK (14.4%)

Step 3 — Chord Shear (Mode D)

For the K-gap joint, chord shear between the braces: A_v,0 = (2h_0 + α × b_0) × t_0 with α = 1 / (1 + 4 × 35² / (3 × 8²)) = 1 / (1 + 4,900/192) = 1 / 26.5 = 0.038

A_v,0 = (2 × 100 + 0.038 × 200) × 8 = (200 + 7.6) × 8 = 1,661 mm²

V_pl,Rd = 0.9 × 355 × 1,661 / (√3 × 1.00) = 306,500 N = 306.5 kN

Brace vertical component: N_braces = 180 × sin50° + 150 × sin45° = 137.9 + 106.1 = 244.0 kN < 306.5 kN — OK (79.6%)

CIDECT Design Tables — Quick Reference

RHS K-Gap Joint — S355 Chord, β = 0.6, g = t_1 + t_2

CHS K-Gap Joint — S355 Chord, β = 0.5

Capacities from CIDECT DG3 and DG8. For exact values, use the design equations with the specific joint geometry.

Frequently Asked Questions

What is the CIDECT Design Guide and how does it relate to EN 1993? CIDECT (Comité International pour le Développement et l'Étude de la Construction Tubulaire) publishes the authoritative design guides for hollow section connections. Design Guide No. 3 (RHS joints) and Design Guide No. 8 (CHS joints) form the technical basis for EN 1993-1-8 Chapter 7. The CIDECT guides provide more extensive design tables, worked examples, and practical guidance than the code alone, including joint configurations that fall outside the standard validity ranges. They also cover rectangular hollow sections not fully addressed in the EN 1993-1-8 tables. CIDECT Design Guide No. 1 covers general design guide principles, No. 2 addresses structural stability of hollow sections, and No. 9 provides guidance on concrete-filled hollow section joints.

When should overlapping joints be used instead of gap joints? Overlapping K joints (where the braces overlap each other on the chord face) are used when: (1) the gap between braces would be negative or too small to allow welding access (g < t_1 + t_2), (2) brace forces are high and chord face plastification governs in a gap configuration, or (3) architectural requirements demand a compact joint. The overlapping brace force is transferred directly through the overlapped brace weld rather than through the chord face, which can increase joint capacity by 40-80% compared to an equivalent gap joint. However, overlapping joints are more complex to fabricate (the overlapped brace must be cut to fit over the overlapping brace) and require full-strength welds between the overlapping members. Overlap should be at least 25% and preferably 50-100% for full strength transfer.

How are hollow section joints checked for combined axial force and moment? EN 1993-1-8 Clause 7.6 provides a linear interaction for joints under combined loading. For brace members under combined axial force N and bending moment M: N_Ed/N_Rd + M_ip,Ed/M_ip,Rd + M_op,Ed/M_op,Rd ≤ 1.0, where M_ip,Rd is the in-plane moment resistance and M_op,Rd is the out-of-plane moment resistance of the joint. The moment resistances are derived from the same failure modes as the axial resistance, using different effective width parameters. For chord members under combined axial force and moment, the chord stress utilisation factor k_n is modified to account for the axial stress ratio at the joint location. CIDECT Design Guide No. 3 provides detailed worked examples for combined loading cases.

What weld types are used for hollow section connections? Fillet welds are standard for RHS/SHS connections where the brace end is cut to fit the chord profile. The weld must be continuous around the accessible perimeter of the brace. For gaps less than 60% of the brace wall thickness, a partial-penetration butt weld may be required on the heel side of the joint to develop full brace capacity. For CHS connections, the weld detail depends on the wall thickness ratio: fillet welds for t_brace ≤ 8 mm and moderate θ angles; full-penetration butt welds with backing rings for thick-walled CHS and critical fatigue-loaded joints. All welds must satisfy EN 1993-1-8 Clause 4.5.3.1 with the directional method. The effective throat must be at least the brace wall thickness for a full-strength connection, or a reduced throat with proportional strength reduction may be used if validated by the component method.

Design Resources

• EN 1993 HSS Section Properties — CHS, RHS, SHS Tables
• EN 1993 Weld Capacity — Fillet and Butt Welds
• EN 1993 Beam Design — Flexure per EN 1993-1-1
• EN 1993 Compact Section Limits — Class 1–4
• EN 1993 Steel Grade Properties — f_y and f_u Values
• EN 1993 Fatigue Design — hollow section fatigue
• All European Reference Guides

Reference only. Verify all values against the current editions of EN 1993-1-8:2005 Chapter 7, CIDECT Design Guide No. 3 (2nd Edition, 2009), and CIDECT Design Guide No. 8 (2008). Design calculations must be independently verified by a licensed Structural Engineer. This guide is for educational purposes only and does not constitute professional engineering advice.