Australian Composite Column Design — AS 4100 and AS/NZS 2327

Comprehensive reference for composite steel-concrete column design in Australian practice. Covers concrete-filled steel tubes (CFST), partially encased and fully encased sections, axial compression capacity per AS 4100 and AS/NZS 2327:2017, confinement, local buckling, fire rating, and construction sequence effects.

Related pages: AS 4100 Column Design | Australian Steel Grades | Fire Rating Guide | Column Capacity Calculator

Composite Column Types

CFST columns are the dominant type in Australian multi-storey construction because they combine the erection speed of steel (the tube acts as permanent formwork) with the compressive efficiency of concrete, and provide inherent fire resistance.

Axial Compression Capacity — AS 4100 Clause 8.5 + AS/NZS 2327

The nominal section capacity of a composite column in axial compression is:

Ns = As x fy + 0.85 x Ac x f'c (CFST and partially encased, Clause 8.5.2)

For fully encased sections, the coefficient on concrete may be reduced to 0.65 to account for lower confinement.

Where As is the steel area, Ac the concrete area, fy the steel minimum yield strength, and f'c the concrete characteristic compressive strength at 28 days.

Member capacity (buckling): phi_Nc = phi x alpha_c x Ns

Where alpha_c uses the buckling curve appropriate for the composite section type. Per AS/NZS 2327, the slenderness is computed using a modified radius of gyration that accounts for the different elastic moduli of steel and concrete:

Effective flexural stiffness: (EI)_eff = Es x Is + 0.6 x Ec x Ic

The 0.6 factor on concrete stiffness accounts for concrete cracking under sustained load. The effective radius of gyration is r_eff = sqrt((EI)_eff / (Es x As + 0.6 x Ec x Ac)).

Confinement Effects in CFST

Concrete confined within a steel tube experiences a triaxial stress state that enhances its compressive strength beyond the unconfined cylinder strength. AS/NZS 2327 allows an enhanced concrete contribution for CFST when certain geometric limits are met:

Enhanced concrete capacity factor: For CHS with D/t <= 90 x (235/fy), the concrete contribution factor may be increased from 0.85 to 1.0, recognising the effective confinement provided by the circular tube.

For SHS/RHS, the confinement is less effective (only near the corners) and the 0.85 factor is retained unless specific testing demonstrates otherwise.

Local Buckling of Steel Tube — AS/NZS 2327 Clause 5.3

The steel tube in a CFST is restrained from inward buckling by the concrete core but can buckle outward. The limiting diameter-to-thickness ratios are more generous than for hollow tubes:

For Grade C350L0 (fy = 350 MPa) CHS: filled D/t limit = 0.15 x 200,000/350 = 86. A CHS 323.9x6.4 has D/t = 50.6, well within the compact limit.

For practical CFST columns, the steel tube contributes 25-40% of the axial capacity, with the concrete providing 60-75%. The steel tube simultaneously functions as reinforcement, formwork, and confinement.

Minimum Steel Contribution Ratio — AS/NZS 2327 Clause 6.2

delta = As x fy / Ns >= 0.2 (minimum steel contribution to total squash load)

This ensures that the steel tube is not merely a thin permanent form (which would be uneconomical and potentially unsafe if the concrete core fails). If delta < 0.2, the section is treated as a reinforced concrete column per AS 3600.

Maximum steel contribution: No explicit upper limit, but sections with delta > 0.6 should be checked as bare steel columns (ignoring the concrete contribution) to ensure adequate fire resistance and robustness.

Construction Sequence Effects

A unique consideration for CFST columns is the construction sequence. During erection, the hollow steel tube carries the construction dead loads (wet concrete, deck formwork, temporary loads) BEFORE the concrete has hardened. These pre-loads induce stresses in the steel tube that are locked in when the concrete sets.

Design approach per AS/NZS 2327 Clause 8.2:

• Steel tube alone: Check for construction loads (self-weight + wet concrete + construction live load of 1.0 kPa minimum)
• Composite section: Check for permanent and imposed loads after concrete hardening, with the steel tube stress from the pre-load phase included

For typical multi-storey construction (3-5 day floor cycle), the pre-load in the steel tube from construction is approximately 15-25% of the bare steel capacity and can usually be accommodated within the composite design. For high-rise construction (40+ storeys), a more refined construction sequence analysis is warranted.

Fire Resistance

CFST columns have inherently good fire resistance because the concrete core acts as a heat sink. The external steel tube loses strength rapidly above 500 degrees C, but the concrete core retains significant capacity, and the tube continues to provide confinement even after yielding.

Fire resistance periods (unprotected CFST, 32 MPa concrete, 60 minute FRL):

Load ratio = factored load at fire limit state / ambient temperature capacity.

For 90-120 minute FRL or for high load ratios, external fire protection (intumescent coating, vermiculite spray, or board encasement) is required. The Australian tunnel fire standard (AS 4825) may require 120-180 minute FRL for critical infrastructure.

Worked Example — CHS 323.9x9.5 CFST Column

Problem: Design a 5.0 m CFST column for an office building. N* = 2,400 kN (dead + live from 4 floors). CHS 323.9x9.5 cold-formed to AS/NZS 1163, Grade C350L0 (fy = 350 MPa). Concrete fill: f'c = 40 MPa. Pin-ended, Le = 5.0 m.

Step 1 — Section properties: D = 323.9 mm, t = 9.5 mm, D/t = 34.1 As = pi x (323.9^2 - 304.9^2) / 4 = pi x (104,911 - 92,964) / 4 = pi x 11,947 / 4 = 9,383 mm^2 Ac = pi x 304.9^2 / 4 = 73,049 mm^2

Step 2 — D/t limit check: D/t limit filled = 0.15 x E/fy = 0.15 x 200,000/350 = 86 > 34.1. Compact — full confinement applies.

Step 3 — Section capacity: Ns = 9,383 x 350 + 1.0 x 73,049 x 40 = 3,284,050 + 2,921,960 = 6,206,010 N = 6,206 kN (Using 1.0 concrete factor due to CHS confinement with D/t = 34.1 << 86)

Steel contribution ratio: delta = 3,284 / 6,206 = 0.53. OK (between 0.2 and 0.6).

phi_Ns = 0.90 x 6,206 = 5,585 kN (squash capacity)

Step 4 — Effective stiffness and slenderness: Es = 200,000 MPa, Ec = 28,500 MPa (estimated for 40 MPa concrete) Is = pi x (323.9^4 - 304.9^4) / 64 = approximately 116 x 10^6 mm^4 Ic = pi x 304.9^4 / 64 = approximately 424 x 10^6 mm^4

(EI)_eff = 200,000 x 116 x 10^6 + 0.6 x 28,500 x 424 x 10^6 = 23,200 x 10^9 + 7,250 x 10^9 = 30,450 x 10^9 N-mm^2

r_eff = sqrt(EI_eff / (Es x As + 0.6 x Ec x Ac)) = sqrt(30,450 x 10^9 / (200,000 x 9,383 + 0.6 x 28,500 x 73,049)) = sqrt(30,450 x 10^9 / (1,876,600,000 + 1,249,000,000)) = sqrt(30,450 x 10^9 / 3,126 x 10^6) = sqrt(9,740) = 98.7 mm

Le/r_eff = 5000 / 98.7 = 50.7

Step 5 — Buckling capacity: lambda_n per composite provisions ~ 50.7 x sqrt(350/250) ~ 60. alpha_c ~ 0.82 (AISC curve, similar to UC sections).

phi_Nc = 0.90 x 0.82 x 6,206 = 4,580 kN > 2,400 kN. OK.

Step 6 — Construction stage check: Steel tube alone: Is = 116 x 10^6 mm^4, ry = sqrt(Is / As) = sqrt(116 x 10^6 / 9383) = 111 mm Le/ry = 5000/111 = 45.0. lambda_n = 45.0 x sqrt(350/250) = 53.3. alpha_c ~ 0.86. phi_Nc_tube = 0.90 x 0.86 x 9,383 x 350 / 1000 = 2,538 kN.

Construction load includes: self-weight of tube + weight of wet concrete (73,049 mm^2 x 25 kN/m^3 = 1.83 kN/m) + construction live load = negligible compared to capacity. Construction stage OK.

Result: CHS 323.9x9.5, Grade C350L0, filled with 40 MPa concrete. Design capacity exceeds demand with a ratio of 0.52. The section is efficient and provides approximately 60 minutes inherent FRL.

Practical Design Notes

Concrete placement: Self-consolidating concrete (SCC) with 10-14 mm aggregate is strongly recommended to avoid honeycombing in the tube. Vibrate externally with a form vibrator clamped to the tube. Provide vent holes at the top of the tube for air escape during casting.

Drainage: For exterior columns, provide a drain hole (12 mm diameter) at the base plate level to prevent water accumulation and freeze-thaw damage if the tube is not fully sealed.

Shrinkage: Concrete shrinkage within the sealed tube is minimal (autogenous only, no drying shrinkage). For large-diameter tubes (D > 500 mm), consider the use of shrinkage-compensating concrete or allow for a small gap at the top of the column to be pressure-grouted after curing.

Frequently Asked Questions

What is the advantage of CFST over a bare steel column? Axial capacity is increased by 60-150% for the same external diameter compared to a hollow CHS. Fire resistance improves from essentially zero (bare steel) to 60-120 minutes without external protection. Fabrication is simplified because the tube acts as permanent formwork — no need for timber formwork, rebar cages, or stripping. Compared to a reinforced concrete column of the same capacity, the CFST column has a smaller footprint, is faster to erect, and does not require formwork or curing time before loading.

How does the selection of concrete strength affect CFST design? Higher concrete strength (50-65 MPa) increases axial capacity but the benefit levels off above approximately 65 MPa because the steel tube yields before the concrete reaches its full unconfined strength. For CHS with D/t < 50 (most practical sections), 40-50 MPa is the economic optimum. For SHS/RHS where confinement is less effective, 32-40 MPa is typical. Ultra-high-strength concrete (80+ MPa) requires special consideration for brittleness and may not be suitable without higher-strength steel tubes.

Does AS 4100 treat composite columns differently from bare steel columns? Yes. AS 4100 Clause 8.5 specifically addresses composite columns, supplementing the general column provisions of Clause 6. The key differences are: the section capacity includes both steel and concrete contributions, the buckling curve may be modified, and the effective stiffness accounts for the composite section. AS/NZS 2327:2017 provides the detailed provisions and is referenced by AS 4100 for composite construction.

Can composite columns be used in moment frames? Yes, but the beam-to-column connection detail must be carefully designed. The beam loads must be transferred into both the steel tube and the concrete core. This is typically achieved by welding stiffeners to the tube and providing through-bolts or headed studs that engage the concrete core. For SMF applications, the detailing is complex and the design should follow the recommendations of the ASI Composite Design Guide.

This page is for educational reference. Composite column design per AS 4100:2020 Clause 8.5 and AS/NZS 2327:2017. Verify material properties and construction requirements. All structural designs must be independently verified by a licensed Professional Engineer or Structural Engineer. Results are PRELIMINARY — NOT FOR CONSTRUCTION.