UK Composite Column Design — EN 1994-1-1 Concrete-Filled Circular Hollow Sections

Design of concrete-filled steel tube (CFST) composite columns per BS EN 1994-1-1:2004 with UK National Annex. Covers the design principles for concrete-filled CHS columns, including axial compression resistance with concrete confinement, flexural stiffness for buckling, shear connection at the steel-concrete interface, and a worked example for a UK column application.

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Composite Column Design per EN 1994-1-1 Clause 6.7

Concrete-filled steel tubes (CFST) combine the strength of structural steel (confinement, tensile resistance) with the economy of concrete (compression resistance). EN 1994-1-1 Clause 6.7 provides the design rules.

Advantages of Concrete-Filled CHS Columns

• High axial capacity — concrete core resists compression, steel tube provides confinement
• Fire resistance — concrete core provides inherent fire resistance without additional protection
• Construction speed — CHS acts as permanent formwork, no propping required
• Slender sections — high buckling resistance due to favourable CHS buckling curve

Axial Compression Resistance (Clause 6.7.3)

Npl,Rd = Aa × fy / γMa + Ac × fck / γc × (1 + ηc × t/d × fy/fck)

Where:

• Aa = steel cross-section area
• Ac = concrete core area (inside the CHS)
• fy = steel yield strength (355 MPa for S355)
• fck = concrete cylinder strength (25-50 MPa typical)
• ηc = confinement factor (≈ 10 for circular sections, 0 for rectangular)
• γMa = 1.00, γc = 1.50 (UK NA values, γc confirmed as 1.5 per UK NA to EN 1992-1-1)

For concrete-filled rectangular sections (RHS/SHS), the confinement effect is negligible (ηc = 0).

Buckling Resistance (Clause 6.7.3.4)

The buckling resistance Nb,Rd follows the same χ-λ̄ approach as EN 1993-1-1 but with composite section properties:

Nb,Rd = χ × Npl,Rd (for eccentricities ≤ 10 %)

The non-dimensional slenderness for the composite section:

λ̄ = √(Npl,Rk / Ncr)

Where Ncr is the elastic critical force of the composite column using (EI)eff:

(EI)eff = Ea × Ia + 0.6 × Ecm × Ic

Shear Connection

For concrete-filled tubes, EN 1994-1-1 requires shear connection at the ends:

• Natural bond between steel and concrete is adequate for most static load cases
• Shear connectors (welded studs) are required at the load introduction zone for transfer of axial load from steel to concrete
• Bond stress τRd = 0.25 MPa (for CHS with internal surface in as-rolled condition)

Worked Example — CHS 273×10 CFST Column

Column details:

• CHS 273×10, S355: Aa = 82.6 cm², fy = 355 MPa (t = 10 mm ≤ 16 mm)
• Concrete inside: C30/37 (fck = 30 MPa), Ac = π × (273 − 2×10)²/4 = π × 253²/4 = 50,265 mm²
• Column length: L = 4.0 m, pinned ends (Lcr = 4.0 m)

Axial Resistance

Npl,Rd = 8,260 × 355/1.0 + 50,265 × 30/1.5 × (1 + 10 × 10/273 × 355/30)

= 2,932,300 + 1,005,300 × (1 + 4.33)

= 2,932,300 + 5,356,000 = 8,288 kN

Note: The confinement factor significantly enhances the concrete contribution. For a similar RHS section without confinement, the resistance would be:

Npl,Rd (RHS, no confinement) = 2,932,300 + 1,005,300 = 3,938 kN (less than half the CHS confined capacity).

Buckling Check

(EI)eff = 210,000 × 71.66×10⁶ + 0.6 × 33,000 × 50,265 × 63.25²/4

Wait — Ia for CHS 273×10 = π/64 × (273⁴ − 253⁴) = 71.66×10⁶ mm⁴ Ic = π/64 × 253⁴ = π × 4.095×10⁹/64 = 201×10⁶ mm⁴

(EI)eff = 210,000 × 71.66×10⁶ + 0.6 × 33,000 × 201×10⁶

= 15.05×10¹² + 3.98×10¹² = 19.03×10¹² N·mm²

Ncr = π² × (EI)eff / Lcr² = π² × 19.03×10¹² / 4,000² = 1.878×10¹⁴ / 16×10⁶ = 11.7×10⁶ N = 11,700 kN

λ̄ = √(8,288 / 11,700) = √0.708 = 0.842

Buckling curve for CHS (hot-finished): curve a (α = 0.21)

Φ = 0.5 × [1 + 0.21 × (0.842 − 0.2) + 0.842²] = 0.5 × [1 + 0.135 + 0.709] = 0.922

χ = 1 / [0.922 + √(0.922² − 0.842²)] = 1 / [0.922 + 0.375] = 0.771

Nb,Rd = 0.771 × 8,288 = 6,388 kN

Utilisation: NEd / Nb,Rd — likely < 30 % for typical building loads, showing the very high efficiency of concrete-filled CHS columns.

Design Guidance

• Circular sections benefit from concrete confinement — axial resistance is up to 2× that of the steel-only section
• Rectangular sections do not develop confinement — use only for moment connections where flat faces are required
• Fire resistance — 60 minutes is typically achieved without additional protection for CHS diameters ≥ 200 mm with L/d ≤ 30
• Concrete placement — use self-compacting concrete (SCC) for CHS columns to avoid voids and ensure full filling

Design Resources

• UK Steel Grades Reference — EN 10025-2 grade selection for UK projects
• UK Steel Mechanical Properties — fy, fu, and elongation tables
• UK Universal Beam and Column Sizes — UB/UC section dimensions and properties
• UK Bolt Capacity Tables — Class 8.8 and 10.9 bolt resistance
• UK Beam Design Guide — EN 1993-1-1 flexure, shear, and LTB
• UK Connection Design Guide — EN 1993-1-8 bolted and welded joints
• All UK Steel Design References — complete library

Frequently Asked Questions

What is the confinement factor for concrete-filled CHS columns?

EN 1994-1-1 Clause 6.7.3 gives the confinement enhancement: the concrete compression resistance is multiplied by (1 + ηc × t/d × fy/fck), where ηc ≈ 10 for CHS. For CHS 273×10 with C30 concrete: enhancement = 1 + 10 × 10/273 × 355/30 = 1 + 4.33 = 5.33. This means the concrete apparent compression strength is over 5× the unconfined value. This significant enhancement is only valid for CHS with L/d < 20 and eccentricity ≤ 10 % of section depth. For RHS/SHS, ηc = 0 (no confinement).

What concrete grade is typically specified for UK composite columns?

C30/37 is the standard concrete grade for composite columns in UK building structures. C40/50 is used for higher load requirements. Self-compacting concrete (SCC) is recommended for CHS columns to ensure complete filling without vibration. The maximum aggregate size should be 10-14 mm for CHS columns with diameter ≤ 300 mm. The UK NA to EN 1994-1-1 confirms γc = 1.50 for concrete.

Does the UK NA modify EN 1994-1-1 composite column design?

The UK NA to BS EN 1994-1-1 adopts the recommended values: γMa = 1.00 for structural steel, γc = 1.50 for concrete, γMs = 1.25 for shear connectors. The confinement formula (Clause 6.7.3) and the buckling method (Clause 6.7.3.4) are unchanged. The UK NA confirms that the simplified method (Clause 6.7.3.4) may be used for columns with eccentricity ≤ 10 % of the section depth.

What fire resistance do concrete-filled CHS columns provide without additional protection?

Concrete-filled CHS columns provide inherent fire resistance of 60-120 minutes without additional fire protection, depending on the column diameter, concrete fill, and load ratio. The concrete core absorbs heat and maintains structural integrity, while the steel tube provides containment even when its strength is reduced at high temperatures. For 60-minute fire resistance: minimum CHS diameter 200 mm, L/d ≤ 30, axial load ratio ≤ 0.5. For 120-minute: minimum CHS diameter 300 mm, L/d ≤ 20, load ratio ≤ 0.3. These values are per EN 1994-1-2 guidance.

Related Pages

• EN 1993 Steel Design Overview
• European Steel Properties
• EN 1993 Beam Design Guide
• EN 1993 Column Buckling
• EN 1990 Load Combinations
• UK Steel Chemical Composition
• UK Steel Charpy Values

Educational reference only. All design values are per BS EN 1993-1-1:2005 + UK National Annex and BS EN 10025-2:2019. Verify all values against the current editions of the standards and the applicable National Annex for your project jurisdiction. Designs must be independently verified by a Chartered Structural Engineer registered with the Institution of Structural Engineers (IStructE) or the Institution of Civil Engineers (ICE). Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent professional verification.