------------------------ | ----------------------- | ---------------- | | CHS (concrete-filled) | Reinforced (rho_s > 3%) | a (alpha = 0.21) | | CHS (concrete-filled) | Unreinforced | a (alpha = 0.21) | | RHS/SHS | Reinforced | a (alpha = 0.21) | | Partially encased I-section | -- | b (alpha = 0.34) |

Load Introduction (Clause 6.7.4)

When the column load is applied to the concrete surface (e.g., from an upper floor slab bear), the load must be transferred from the concrete to the steel tube through bond and friction at the interface. EN 1994-1-1 Clause 6.7.4 requires that:

  1. The shear stress at the steel-concrete interface does not exceed the design bond strength tau_Rd.
  2. For CFST with the internal surface in its as-rolled condition: tau_Rd = 0.25 MPa.
  3. For CFST with internal shear connectors or surface profiling: tau_Rd increases proportionally.

The load introduction length over which the bond can be mobilised is limited. If the required transfer length exceeds the available column length between load introduction points, mechanical shear connectors (headed studs welded to the internal tube surface) must be provided.

For a CHS 273 x 10 column with internal perimeter 795 mm and tau_Rd = 0.25 MPa, the bond transfer capacity per metre of column length is: F_transfer = 0.25 x 795 x 1.0 = 199 N/mm = 199 kN/m.

If the load to be transferred from concrete to steel in a 1 m column segment is 100 kN, the bond is adequate. If the load is 300 kN, shear studs are required.

Worked Example -- CHS 273 x 10 CFST Column

Given:

Step 1 -- Axial resistance (confined): N_pl,Rd = 8,260 x 355/1.0 + 50,265 x 30/1.5 x [1 + 10 x (10/273) x (355/30)] = 2,932,300 + 1,005,300 x [1 + 4.334] = 2,932,300 + 1,005,300 x 5.334 = 2,932,300 + 5,362,000 = 8,294 kN

Step 2 -- Effective flexural stiffness: I_a = pi x (273^4 - 253^4)/64 = 71.66 x 10^6 mm^4 I_c = pi x 253^4/64 = 201.0 x 10^6 mm^4

(EI)_eff = 210,000 x 71.66 x 10^6 + 0.6 x 33,000 x 201.0 x 10^6 = 15.05 x 10^12 + 3.98 x 10^12 = 19.03 x 10^12 N.mm^2

Step 3 -- Critical load and slenderness: N_cr = pi^2 x 19.03 x 10^12 / 4,000^2 = 11.74 x 10^6 N = 11,740 kN

lambda_bar = sqrt(8,294/11,740) = sqrt(0.706) = 0.840

Step 4 -- Buckling resistance: Curve a (alpha = 0.21): Phi = 0.5 x [1 + 0.21 x (0.840 - 0.2) + 0.840^2] = 0.5 x [1 + 0.134 + 0.706] = 0.920 chi = 1/[0.920 + sqrt(0.920^2 - 0.840^2)] = 1/[0.920 + 0.375] = 0.772

N_b,Rd = 0.772 x 8,294 = 6,402 kN

Commentary: The CHS 273 x 10 CFST column achieves a buckling resistance of 6,402 kN with a column weight of only 65 kg/m. An equivalent steel-only UC column would require approximately 356 x 406 x 235 UC (235 kg/m) for the same capacity. The composite column achieves a 72% weight saving for the same load capacity, at the cost of concrete placement operations on site.

UK National Annex Provisions

The UK NA to BS EN 1994-1-1 confirms:

  1. gamma_Ma = 1.00 for structural steel in composite columns.
  2. gamma_c = 1.50 for concrete (referenced through the UK NA to BS EN 1992-1-1).
  3. The confinement formula of Clause 6.7.3.2(6) is adopted without modification, with eta_c = 10 for CHS.
  4. For fire design of composite columns, the UK NA to BS EN 1994-1-2 provides specific tabulated data for UK fire resistance periods (30, 60, 90, 120 minutes).
  5. The simplified design method of Clause 6.7.3.4 is permitted, with its limits on eccentricity (e/D <= 0.1) and slenderness (lambda_bar <= 2.0).

Design Resources

Frequently Asked Questions

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

EN 1994-1-1 Clause 6.7.3.2(6) specifies an enhancement factor of [1 + 10 x (t/d) x (fy/f_ck)] for CHS sections. For a CHS 273 x 10 in S355 with C30/37 concrete, the enhancement factor = 5.33, meaning the concrete contributes over 5 times its unconfined compressive strength. This confinement is only available for CHS (not RHS), and only when lambda_bar <= 0.5 and e/D <= 0.1.

What concrete grade is used for UK composite columns?

C30/37 is the standard concrete grade for composite columns in UK building structures. Higher grades (C40/50, C50/60) are used for heavily loaded columns or to reduce column dimensions. Self-compacting concrete (SCC) is recommended for CHS columns to ensure complete filling without vibration, as the steel tube prevents access for compaction. The maximum aggregate size should be limited to 10-14 mm for CHS diametres <= 300 mm to prevent arching and ensure flow.

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

The UK National Annex to BS EN 1994-1-1 adopts the recommended values: gamma_Ma = 1.00, eta_c = 10 for CHS, and the buckling curve selection. The UK NA confirms gamma_c = 1.50 for concrete. The simplified method is permitted. The UK NA also provides supplementary guidance on the fire design of composite columns, reinforcing that CHS columns with diametre >= 200 mm and L/D <= 30 typically achieve 60-minute fire resistance without additional protection.

Educational reference only. All design values are per BS EN 1994-1-1:2004 + UK National Annex, BS EN 1993-1-1:2005, and BS EN 1992-1-1:2004. 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.