Composite Column Types — EN 1994-1-1 Clause 6.7

Concrete-Filled Steel Tube (CFT)

Steel tube filled with concrete. The tube acts as permanent formwork and reinforcement. Highest strength-to-weight ratio. No longitudinal reinforcement required in typical applications.

Fully Encased Section

Steel I-section fully encased in concrete with longitudinal and transverse reinforcement. Enhanced fire resistance.

Partially Encased Section

Steel I-section with concrete between flanges. Reduced fire protection requirement.

Plastic Resistance to Compression — Clause 6.7.3.2

N_pl,Rd = A_a × f_yd + A_c × f_cd + A_s × f_sd

For concrete-filled tubes (typically no reinforcing steel A_s):

N_pl,Rd = A_a × f_y / γ_Ma + A_c × (0.85 × f_ck / γ_c)

Where:

• γ_Ma = 1.00 (steel partial factor)
• γ_c = 1.50 (concrete partial factor)
• 0.85 = coefficient for long-term effects

Relative Slenderness — Clause 6.7.3.4

λ̄ = √(N_pl,Rk / N_cr)

Where N_cr is the elastic critical force from:

N_cr = π² × (EI)_eff / L²

The effective flexural stiffness:

(EI)_eff = E_a × I_a + 0.6 × E_cd × I_c + E_s × I_s

The factor 0.6 accounts for concrete cracking and long-term effects.

Buckling Curves — Table 6.5

Worked Example — CHS 219.1×10 CFT in S355, C30/37

Plastic Resistance

Buckling Check

The buckling capacity is 2489 kN, 13% less than the squash load.

Comparison — CHS 219.1×10 Hollow vs Filled

Concrete filling provides 40-55% increase in axial capacity for the same steel tube.

Design Applications

Common Design Scenarios

This reference covers structural design scenarios commonly encountered in structural steel design practice:

• Strength verification: Check member or connection capacity against factored loads per the applicable design code
• Serviceability checks: Verify deflections, vibrations, and other serviceability criteria
• Code compliance: Ensure design meets all provisions of the governing standard
• Connection detailing: Verify weld sizes, bolt quantities, and edge distances

Related Design Considerations

• System behavior: consider the interaction between members and connections
• Load paths: verify that forces can be transferred through the structure to the foundations
• Constructability: check that the design can be fabricated and erected practically
• Cost optimisation: evaluate alternative sections or connection types for economy

Worked Example

Problem: Verify a typical steel member for the following conditions:

Typical span: 6.0 m | Load: service loads per applicable code | Section: common section in this category

Design Check:

1. Determine governing load combination (ULS or SLS per EN 1990)
2. Calculate maximum internal forces (moment, shear, axial)
3. Compute nominal capacity per code provisions
4. Apply resistance/safety factors
5. Verify interaction if combined forces exist

Result: Use the results from the Steel Calculator tool to verify design adequacy.

Frequently Asked Questions

What European Standard governs structural steel design?

EN 1993 (Eurocode 3: Design of Steel Structures) is the primary standard for structural steel design in Europe. EN 1993-1-1 covers general rules for buildings, EN 1993-1-8 addresses connection design, and EN 1993-1-2 covers fire design. The standard uses limit state design with partial safety factors (γM). National Annexes adapt parameters to each member state. Companion standards include EN 10025 for hot-rolled products, EN 1090 for execution, and EN 1994 for composite design.

What are the common steel grades used in European construction?

The most common steel grades for European construction are S235, S275, S355, S420, and S460 per EN 10025-2. S355 (minimum yield 355 MPa for t ≤ 16 mm) is the most widely used for structural applications. S275 is used for secondary members. S420 and S460 are quenched and tempered high-strength steels for weight-critical applications. Weathering steel (S355J2W) and fine-grain structural steels (EN 10025-3 and -4) are also available.

How does EN 1993 compare to other international steel design codes?

EN 1993, AISC 360 (US), AS 4100 (Australia), and CSA S16 (Canada) all use limit states design principles but differ in key details. EN 1993 uses partial safety factors (γM0 = 1.00, γM1 = 1.00, γM2 = 1.25) rather than resistance factors (φ). Buckling curves in EN 1993 follow the European Column Curve system (a0 to d) with 5 distinct curves, compared to AISC's single curve. EN 1993-1-8 has comprehensive connection design provisions including the component method for moment connections.

Frequently Asked Questions

What is the advantage of concrete-filled steel tube columns over steel-only columns?

Concrete-filled tubes provide 40-55% higher axial capacity for the same steel tube, improved fire resistance (bare steel fails at 550°C, filled section continues for 60-120 minutes), no external formwork required (steel tube serves as permanent form), and enhanced ductility under seismic loading. The concrete core also prevents local buckling of the steel tube wall.

What buckling curve should be used for concrete-filled CHS columns?

Per EN 1994-1-1 Table 6.5, use buckling curve a (α = 0.21) for hot-finished CHS (EN 10210) filled with concrete. For cold-formed CHS (EN 10219), use curve c (α = 0.49). The difference is due to the higher residual stresses in cold-formed sections.

Related Pages

• Shear Stud Design — Shear connectors per EN 1994-1-1
• HSS Section Properties — CHS, RHS, SHS tables
• Column Design — Steel column per EN 1993-1-1
• All European References

Educational reference only. Design per EN 1994-1-1:2004 Clause 6.7. γ_Ma = 1.00, γ_c = 1.50. Verify concrete strength and steel grade. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent verification.

Design Resources

Calculator tools

• Steel Column Buckling Calculator
• Steel Buckling Calculator
• Steel Column Base Design Calculator
• Composite Beam Design Calculator
• Composite Beam Calculator

Design guides

• Column Capacity Worked Example
• Column Buckling Guide
• Column Buckling Calculator Guide
• EN 1993-1-1 Column Buckling Worked Example

Reference pages

• EU Shear Stud