Composite Column Design — AISC 360 Chapter I Reference

Composite columns combine structural steel shapes with concrete to increase axial capacity, stiffness, and fire resistance. AISC 360-22 Chapter I covers two types: concrete-encased steel shapes (SRC) and concrete-filled hollow structural sections (CFT). Both types leverage the compressive strength of concrete and the ductility of steel to create efficient compression members.

Types of composite columns

Concrete-encased (SRC): A steel W-shape surrounded by reinforced concrete. The concrete provides fire protection, increases stiffness for buckling resistance, and adds compressive capacity. The steel core provides ductility and tensile resistance. Common in high-rise buildings where fire ratings and high axial loads are required.

Concrete-filled tubes (CFT): A round or rectangular HSS filled with concrete. The steel tube acts as formwork during construction and confines the concrete after curing, increasing its effective strength. Round CFT columns benefit from triaxial confinement that can increase concrete strength by 10-30% depending on the D/t ratio.

AISC 360-22 Chapter I provisions

Limitations (Section I1.3)

Steel contribution ratio: 0.01 <= As*Fy/Pno <= 0.9 (ensures truly composite behavior)
Concrete strength: 3 ksi <= f'c <= 10 ksi (normal weight), 3 ksi <= f'c <= 6 ksi (lightweight)
Minimum reinforcement ratio for encased sections: 0.4% of concrete area
Maximum width-to-thickness for filled HSS: per Table I1.1a (compact/noncompact/slender limits)

Squash load Pno (Section I2.1)

For encased sections:

Pno = Fy*As + Fysr*Asr + 0.85*f'c*Ac

For filled compact sections (round HSS with confinement):

Pno = Fy*As + Fysr*Asr + C2*f'c*Ac

Where C2 = 0.85 for rectangular, and C2 = 0.95 for round (reflecting confinement). As = steel area, Asr = rebar area, Ac = concrete area.

Effective stiffness EI* (Section I2.1b)

EIeff = Es*Is + 0.5*Es*Isr + C1*Ec*Ic

Where C1 = 0.25 + 3*(As + Asr)/(Ag) <= 0.7. This reduced concrete stiffness accounts for cracking, creep, and the variability of concrete modulus under sustained load.

Column buckling (Section I2.1b)

Once Pno and EIeff are known, compute Pe = pi^2*EIeff/(KL)^2. Then:

If Pno/Pe <= 2.25: Pn = Pno * 0.658^(Pno/Pe) (inelastic)
If Pno/Pe > 2.25: Pn = 0.877 * Pe (elastic)

Design strength: phiPn = 0.75 * Pn (phi = 0.75 for composite columns, not 0.90 as for bare steel).

Worked example -- W14x90 encased in 24"x24" concrete

Given: W14x90 (As = 26.5 in^2, Is = 999 in^4), 24x24 in column, f'c = 5 ksi, Fy = 50 ksi, 8-#8 bars (Asr = 6.32 in^2, Fysr = 60 ksi), KL = 20 ft.

Concrete area: Ac = 24*24 - 26.5 - 6.32 = 543.2 in^2.

Squash load: Pno = 5026.5 + 606.32 + 0.855543.2 = 1325 + 379 + 2309 = 4013 kips.

Stiffness: C1 = 0.25 + 3*(26.5+6.32)/576 = 0.25 + 0.171 = 0.421. Ec = 57sqrt(5000) = 4031 ksi. Ic = 24^4/12 - 999 - 6.329^2 = 27648 - 999 - 512 = 26137 in^4. EIeff = 29000999 + 0.529000512 + 0.4214031*26137 = 28971k + 7424k + 44354k = 80749k kip-in^2.

Buckling: Pe = pi^280749000/(240)^2 = 13834 kips. Pno/Pe = 4013/13834 = 0.290 < 2.25. Pn = 40130.658^0.290 = 4013*0.888 = 3564 kips.

phiPn = 0.75 * 3564 = 2673 kips -- over 4x the bare steel column capacity of ~600 kips.

P-M interaction for composite columns

AISC provides a simplified P-M interaction diagram using the plastic stress distribution method (Section I1.2). The diagram passes through four anchor points:

Point A: Pure axial (Pno, M = 0)
Point B: Zero axial with full moment (P = 0, Mn = plastic moment of composite section)
Point C: Balanced point -- maximum moment occurs at an intermediate axial load
Point D: Axial at balanced condition (Pd = 0.85f'cAc for encased; higher for filled with confinement)

Linear interpolation between these points gives a conservative interaction surface.

Multi-code comparison

AS 4100 / AS 3600: Australia treats composite columns under AS 3600 (concrete structures) with contributions from the steel section. Uses a moment-magnifier method for slenderness effects.

EN 1994-1-1 (Eurocode 4): Provides detailed composite column design with full P-M interaction, including creep and shrinkage effects on long-term stiffness (Section 6.7). Uses buckling curves from EN 1993-1-1 with effective flexural stiffness.

CSA S16-19: Section 18 covers composite columns with provisions similar to AISC, but uses phi_c = 0.80 for concrete and phi_s = 0.90 for steel within the composite section.

Practical tip: when to use composite columns

Composite columns are most economical when: (1) fire rating requirements would otherwise need expensive spray-applied fireproofing on bare steel, (2) axial loads exceed what standard W-shapes can handle at the required story height, or (3) stiffness is needed to control drift in tall buildings. For columns below 500 kips factored load in a low-rise building, bare steel W-shapes are typically more economical.

Common mistakes

Using phi = 0.90 instead of 0.75. AISC 360 uses phi = 0.75 for composite columns (Section I2.1b), reflecting the higher variability of concrete strength.
Ignoring the C1 stiffness reduction. Using full EcIc overstates stiffness and underestimates buckling effects.
Not checking steel contribution limits. If As*Fy/Pno < 0.01, the member is a reinforced concrete column, not a composite column -- use ACI 318 instead.
Neglecting long-term creep effects. Sustained loads cause concrete creep that reduces stiffness over time. AISC accounts for this through the C1 factor, but additional reduction may be needed for high sustained-to-total load ratios.
Overlooking local buckling of filled HSS. The D/t or b/t ratio must satisfy the compactness limits in Table I1.1a for the full composite capacity equations to apply.

Run this calculation

Related references

Disclaimer

This page is for educational and reference use only. It does not constitute professional engineering advice. All design values must be verified against AISC 360-22 Chapter I and the governing project specification before use. The site operator disclaims liability for any loss arising from the use of this information.