What is the difference between CFT and SRC composite columns?

Two types per AISC 360 Chapter I: Concrete-Filled Tubes (CFT) — a round or rectangular HSS filled with structural concrete. The steel tube serves as permanent formwork during construction. After curing, the tube confines the concrete core, increasing compressive strength 10-30% in round CFTs. Advantages: fast construction (tube erected first provides immediate capacity), no formwork, high axial efficiency, excellent seismic performance, clean appearance. Limitations: difficult concrete inspection, more complex beam connections. Concrete-Encased Steel (SRC) — a W-shape fully encased in reinforced concrete with longitudinal bars and ties. Advantages: built-in fire protection (2-4 hr without SFRM), familiar connection details, concrete cover protects steel, high stiffness controls drift. Limitations: formwork required, larger cross-section, slower construction. CFT preferred for speed and efficiency; SRC preferred for fire ratings and connection simplicity.

How is composite column squash load calculated per AISC 360?

Per AISC 360-22 Section I2.1b, the nominal squash load for a composite column is: Pno = Fy x As + Fy,sr x Asr + 0.85 x f'c x Ac. Where: As = steel shape area, Fy = steel yield strength. Asr = longitudinal reinforcing bar area, Fy,sr = rebar yield strength (max 60 ksi for calculation per I2.1b). Ac = concrete area, f'c = concrete compressive strength (max 10 ksi for normal weight). The steel contribution ratio must be >= 0.25: Fy x As / Pno >= 0.25. This ensures the steel provides minimum ductility. If the ratio is less than 0.25, the member is designed as a reinforced concrete column per ACI 318, not as a composite column. Effective stiffness for buckling: EI* = Es x Is + 0.5 x Es x Isr + C1 x Ec x Ic, where C1 = 0.25 for CFT (accounts for concrete cracking) and C1 = 0.25 + 0.7 x (As/Ac + Asr) <= 0.7 for encased sections.

How does concrete confinement work in CFT columns?

In round CFT columns, the steel tube provides triaxial confinement to the concrete core. When the concrete attempts to expand laterally under compression (Poisson effect), the steel tube restrains it, creating radial confining pressure. This confinement increases the concrete's effective compressive strength and ductility. The strength enhancement is significant at lower D/t ratios: D/t 100 — minimal confinement, use unconfined f'c. Rectangular CFTs provide less confinement because the flat walls bulge outward rather than providing uniform radial restraint. AISC 360 conservatively limits the maximum concrete strength to 10 ksi and does not explicitly increase f'c for confinement in the squash load formula — the confinement benefit is considered a reserve beyond the design equations. For round CFTs with D/t <= 0.15E/Fy, AISC permits using full composite action without local buckling concerns.

What are the fire resistance advantages of composite columns?

Composite columns have significant fire resistance advantages over bare steel: SRC columns — the concrete encasement provides inherent fire protection. Typical 2-3 in concrete cover gives 2-4 hour fire ratings without additional SFRM. This is a major cost advantage in high-rise construction. CFT columns — the concrete fill acts as a heat sink. The steel tube heats quickly but heat is conducted into the concrete core, slowing the temperature rise. Externally applied SFRM or intumescent paint may still be needed for ratings > 1 hour, but less thickness than bare steel. For both types, the concrete core continues to carry load even after the steel loses strength at high temperature. A CFT column at 600C steel temperature may still carry 60-70% of its ambient capacity because the concrete core remains relatively cool. This redundancy is a key safety feature. EN 1994-1-2 provides detailed fire design methods for composite columns including tabulated minimum dimensions for given fire ratings.

How does composite column design differ between AISC 360, EN 1994, and AS 4100?

Key differences: (1) AISC 360 Ch I uses a single interaction equation with P-M curves. EN 1994-1-1 uses the European buckling curve approach with separate reduction factors for different column curves. (2) Concrete contribution: AISC uses 0.85f'cAc with max f'c = 10 ksi. EN 1994 uses fcd = fck/gamma_c with factor alpha_cc = 0.85. AS 4100/AS 3600 uses phi = 0.6 for concrete. (3) Effective stiffness: AISC uses C1 = 0.25 for CFT. EN 1994 uses Ke = 0.6. AS uses C1 = 0.25. (4) Steel contribution ratio: AISC minimum 0.25. EN 1994 minimum delta = 0.2 (steel contribution to axial resistance). (5) Confinement: AISC does not explicitly account for confinement in squash load. EN 1994 increases f'c for circular CFT with D/t < 50. AS 3600 allows enhanced concrete strength for confined cores. The most conservative approach is generally AISC for confinement and AS 3600 for concrete capacity factors.

Composite Column Design — AISC 360 Chapter I Reference

Q: How does concrete confinement work in CFT columns?

In round CFT columns, the steel tube provides triaxial confinement to the concrete core. When the concrete attempts to expand laterally under compression (Poisson effect), the steel tube restrains it, creating radial confining pressure. This confinement increases the concrete's effective compressive strength and ductility. The strength enhancement is significant at lower D/t ratios: D/t 100 — minimal confinement, use unconfined f'c. Rectangular CFTs provide less confinement because the flat walls bulge outward rather than providing uniform radial restraint. AISC 360 conservatively limits the maximum concrete strength to 10 ksi and does not explicitly increase f'c for confinement in the squash load formula — the confinement benefit is considered a reserve beyond the design equations. For round CFTs with D/t <= 0.15E/Fy, AISC permits using full composite action without local buckling concerns.

Q: What are the fire resistance advantages of composite columns?

Composite columns have significant fire resistance advantages over bare steel: SRC columns — the concrete encasement provides inherent fire protection. Typical 2-3 in concrete cover gives 2-4 hour fire ratings without additional SFRM. This is a major cost advantage in high-rise construction. CFT columns — the concrete fill acts as a heat sink. The steel tube heats quickly but heat is conducted into the concrete core, slowing the temperature rise. Externally applied SFRM or intumescent paint may still be needed for ratings > 1 hour, but less thickness than bare steel. For both types, the concrete core continues to carry load even after the steel loses strength at high temperature. A CFT column at 600C steel temperature may still carry 60-70% of its ambient capacity because the concrete core remains relatively cool. This redundancy is a key safety feature. EN 1994-1-2 provides detailed fire design methods for composite columns including tabulated minimum dimensions for given fire ratings.

Composite columns combine structural steel shapes with concrete to create compression members that exceed the capacity, stiffness, and fire resistance of either material alone. AISC 360-22 Chapter I governs two fundamental types: concrete-encased steel shapes (SRC) and concrete-filled hollow structural sections (CFT). Both types leverage the high compressive strength of concrete and the ductility and tensile capacity of steel, producing members that are particularly efficient in high-rise buildings, heavy industrial structures, and seismic force-resisting systems.

This reference covers the complete design workflow: section classification, squash load calculation, effective stiffness, slenderness and buckling, P-M interaction, steel contribution ratio limits, fire resistance benefits, and detailed worked examples. Comparisons with AS 4100/AS 3600 and EN 1994-1-1 are provided for engineers working across multiple design codes.

CFT vs. encased composite columns

Concrete-filled tubes (CFT)

A round or rectangular HSS filled with structural concrete. The steel tube serves as permanent formwork during construction -- no additional forming or shoring is needed. After the concrete cures, the tube confines the concrete core, increasing its effective compressive strength and ductility. In round CFT columns, triaxial confinement can raise concrete strength by 10-30% depending on the D/t ratio.

Advantages of CFT columns:

Fast construction: the steel tube is erected first, providing immediate load-carrying capacity before concrete is placed
No formwork or shoring required
High axial capacity per unit cross-section area
Excellent seismic performance due to confinement
Aesthetically clean appearance (exposed steel finish)

Limitations:

Inspection of concrete fill is difficult (voids may go undetected)
Connection design to beams is more complex than for encased sections
Maximum concrete strength is limited by AISC to 10 ksi (normal weight)
Narrow D/t or b/t ratios may trigger local buckling before full composite action develops

Concrete-encased steel (SRC)

A steel W-shape (or built-up section) fully encased in reinforced concrete with longitudinal bars and ties. The concrete provides fire protection, increases member stiffness for buckling resistance, and adds significant compressive capacity. The steel core contributes ductility and energy dissipation capacity, making SRC columns popular in seismic regions.

Advantages of SRC columns:

Robust fire resistance without additional spray-applied fireproofing (typically 2-4 hour ratings achievable)
Familiar connection details (beam flanges attach to the steel core)
Concrete cover protects steel from corrosion and impact
High stiffness controls drift in tall buildings
Rebar cage provides additional moment resistance

Limitations:

Formwork required during concrete placement
Larger cross-section than equivalent CFT for the same axial capacity
Heavier total weight (concrete encasement adds significant dead load)
Construction sequence is slower (form, place, cure)

Selection guidance

In practice, CFT columns are preferred when fast erection and high axial efficiency are priorities -- parking structures, industrial buildings, and architecturally exposed columns. SRC columns are preferred when fire ratings, connection simplicity, or drift control drive the design -- high-rise office towers, hospitals, and seismic moment frames.

AISC 360-22 Chapter I provisions

Scope and limitations (Section I1.3)

AISC imposes strict material and geometric limits to ensure that the composite design equations produce safe, reliable results:

Steel contribution ratio: 0.01 <= As*Fy/Pno <= 0.9. This ensures truly composite behavior -- if the ratio is below 0.01, the member behaves as a reinforced concrete column (design per ACI 318); if above 0.9, it behaves as a bare steel column (design per AISC Chapter E).
Concrete compressive strength: 3 ksi <= f'c <= 10 ksi for normal-weight concrete, 3 ksi <= f'c <= 6 ksi for lightweight concrete. Higher strengths are outside the calibrated range of the AISC provisions.
Minimum reinforcement ratio for encased sections: 0.4% of the concrete area (longitudinal bars). This minimum controls shrinkage cracking and provides ductility.
Maximum width-to-thickness ratios for filled HSS: Per Table I1.1a, the steel tube must satisfy compactness limits for the full composite capacity equations to apply. Noncompact and slender filled sections use reduced capacity equations.

Section classification for filled HSS (Table I1.1a)

Classification	Round HSS (D/t)	Rectangular HSS (b/t or h/t)
Compact	<= 0.15*E/Fy	<= 2.26*sqrt(E/Fy)
Noncompact	> compact but <= 0.19*E/Fy	> compact but <= 3.00*sqrt(E/Fy)
Slender	> 0.19*E/Fy	> 3.00*sqrt(E/Fy)

For Fy = 50 ksi and E = 29,000 ksi, the compact limit for round HSS is D/t <= 87.0, and for rectangular HSS, b/t <= 54.4. Exceeding these limits requires the use of reduced capacity equations that account for local buckling of the steel tube.

Squash load calculation (Section I2.1)

The squash load Pno represents the theoretical maximum axial compressive strength of the composite cross-section when all materials reach their design stresses simultaneously. It is the starting point for all column capacity calculations.

Encased sections

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

The 0.85 factor on concrete accounts for the difference between cylinder test strength and actual in-place strength, as well as the variability of concrete in a composite cross-section.

Filled compact sections

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

Where C2 depends on section geometry:

C2 = 0.85 for rectangular HSS (no confinement benefit)
C2 = 0.95 for round HSS (reflecting triaxial confinement from the circular steel tube)

For noncompact filled sections, a linear reduction is applied between the compact and noncompact limits. For slender filled sections, the concrete contribution is further reduced and only the steel area outside the local buckling wave is effective.

Typical cross-section properties

The following table shows representative properties for common composite column configurations:

Section	Steel Shape	Overall Size	f'c (ksi)	As (in^2)	Ac (in^2)	Pno (kips)	EIeff (kip-in^2 x 10^6)
Encased SRC	W14x90	24 x 24 in	5.0	26.5	543.2	4,013	80.7
Encased SRC	W12x65	20 x 20 in	4.0	19.1	358.9	2,415	33.2
Round CFT	HSS 16x0.500	16 in dia	6.0	24.35	176.7	2,259	35.8
Round CFT	HSS 20x0.625	20 in dia	5.0	38.07	275.0	3,254	70.4
Rectangular CFT	HSS 14x10x0.625	14 x 10 in	5.0	26.80	113.2	1,907	25.1
Encased SRC	W10x49	16 x 16 in	5.0	14.4	237.6	1,747	17.9
Round CFT	HSS 12x0.375	12 in dia	4.0	13.71	99.4	1,086	11.2

Note: Values computed per AISC 360-22 Chapter I with Fy = 50 ksi and Es = 29,000 ksi. All sections assume no additional longitudinal rebar (Asr = 0) for filled tubes, and 0.4% minimum rebar for encased sections.

Steel contribution ratio

The steel contribution ratio delta = As*Fy/Pno is a fundamental parameter that determines whether composite design provisions apply:

delta < 0.01: The steel contribution is negligible. Design as a reinforced concrete column per ACI 318. The steel shape is essentially just reinforcement.
0.01 <= delta <= 0.9: The member is a true composite column. Use AISC Chapter I provisions.
delta > 0.9: The concrete contribution is negligible. Design as a bare steel column per AISC Chapter E. The concrete fill provides only architectural enclosure or fire protection.

Most practical composite columns have delta between 0.2 and 0.6, indicating significant contributions from both materials. A lower delta means the column behaves more like a concrete member (more ductile post-peak), while a higher delta means it behaves more like a steel member (higher strength-to-weight ratio).

The steel contribution ratio also affects the C1 stiffness reduction factor. Columns with a higher steel ratio (more steel relative to total area) get a higher C1 value, meaning the concrete stiffness is penalized less -- because the steel tube or encasement provides confinement that helps the concrete maintain its stiffness.

Effective stiffness EI* (Section I2.1b)

The effective flexural stiffness of a composite column is not simply the sum of individual stiffnesses. AISC reduces the concrete contribution to account for cracking, creep under sustained load, and the inherent variability of concrete properties:

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

Where:

Es = 29,000 ksi (steel modulus)
Is = moment of inertia of the steel section
Isr = moment of inertia of the reinforcing bars about the centroidal axis
Ec = 57sqrt(f'c1000) ksi (concrete modulus, with f'c in ksi)
Ic = moment of inertia of the concrete (total section minus steel minus rebar)
C1 = 0.25 + 3*(As + Asr)/(Ag) <= 0.7

The C1 factor ranges from 0.25 (minimum, when steel area is negligible) to 0.70 (maximum, when steel is a large proportion of the cross-section). This means the concrete contribution to stiffness is effectively reduced to between 25% and 70% of its theoretical elastic value. The reduction accounts for:

Cracking: Concrete in tension cracks at low strain, reducing the effective section
Creep: Sustained axial loads cause long-term shortening that reduces effective stiffness
Material variability: Concrete properties are more variable than steel

For creep under high sustained loads (common in columns supporting large gravity loads), some engineers apply an additional long-term stiffness reduction of 0.70 to the C1EcIc term, though this is not explicitly required by AISC.

Slenderness and column buckling (Section I2.1b)

Once Pno and EIeff are established, the elastic critical buckling load Pe is computed using the Euler formula:

Pe = pi^2 * EIeff / (K*L)^2

The nominal compressive strength Pn then follows the same formulation as AISC Chapter E for bare steel columns, using the tangent modulus theory:

If Pno/Pe <= 2.25 (inelastic buckling):

Pn = Pno * 0.658^(Pno/Pe)

If Pno/Pe > 2.25 (elastic buckling):

Pn = 0.877 * Pe

The transition point at Pno/Pe = 2.25 corresponds to a slenderness ratio lambda_c of approximately 1.5. Stocky columns (low KL, high EIeff) fall in the inelastic range and develop nearly their full squash load. Slender columns (high KL, low EIeff) fall in the elastic range and are limited by buckling.

Design strength: phi*Pn = 0.75 * Pn, where phi = 0.75 for composite columns. This is lower than the phi = 0.90 used for bare steel columns, reflecting the additional variability introduced by the concrete component.

Effective length factor K

The effective length factor K is determined using the same alignment chart or analytical methods as for bare steel columns (AISC Commentary Chapter C). For composite columns in braced frames, K typically ranges from 0.7 to 1.0. For unbraced frames (moment frames), K ranges from 1.0 to 2.0+. The higher stiffness of composite columns relative to bare steel often results in lower K factors because the relative stiffness of the column to the beams (G-factor) is more favorable.

P-M interaction for composite columns

Composite columns in building frames almost always resist combined axial load and bending moment. AISC provides interaction diagrams based on the plastic stress distribution method (Section I1.2). The P-M interaction curve is constructed from four anchor points:

Point A (pure axial): P = Pno, M = 0. The full cross-section is in uniform compression.
Point B (pure flexure): P = 0, M = Mn. The section resists bending only, with the plastic neutral axis positioned so that compression forces above equal tension forces below.
Point C (maximum moment): At an intermediate axial load level, the moment capacity actually exceeds the pure flexure capacity. This occurs because the axial load precompresses the concrete, delaying cracking and allowing a larger effective section.
Point D (decompression): Pd = 0.85f'cAc for encased sections; higher for filled sections with confinement. This represents the point where the entire concrete area is in compression.

Linear interpolation between these four points produces a conservative interaction surface. For more accurate results, the strain compatibility method can be used to generate a continuous P-M curve with as many points as needed. This method assumes plane sections remain plane and uses the material stress-strain relationships for steel and concrete.

Biaxial bending

When moments act about both principal axes simultaneously, AISC permits the use of the simplified reciprocal load method:

1/Pn_xy = 1/Pn_x + 1/Pn_y - 1/Pno

where Pn_x and Pn_y are the nominal axial capacities at the respective uniaxial moments, and Pno is the pure axial capacity. For square or circular sections with symmetric reinforcement, the biaxial reduction is small. For rectangular sections, the reduction can be significant and should be carefully evaluated.

Fire resistance benefits

One of the most compelling practical advantages of composite columns is inherent fire resistance:

Encased columns (SRC)

The concrete encasement provides direct thermal insulation to the steel core. Typical fire resistance ratings without any additional fireproofing:

Concrete Cover (in)	Fire Rating	Notes
1.5	1 hour	Minimum cover for rebar
2.0	1.5 hours	Common for office buildings
2.5	2 hours	Typical for high-rise columns
3.0	3 hours	Required for some institutional buildings
4.0+	4 hours	Special occupancy / petrochemical

The rebar continues to carry load even after the steel core has lost significant strength at elevated temperatures, providing redundancy that bare steel columns lack.

Filled columns (CFT)

Concrete-filled HSS columns also exhibit significant fire resistance. The concrete core absorbs heat and provides thermal mass, while the steel tube eventually yields and sheds load to the concrete core. Research has shown that CFT columns can achieve 1-2 hour fire ratings without external protection, depending on the load ratio and section size. For higher ratings, thin intumescent coatings or board encasement can be applied to the exterior.

The fire resistance mechanism is different from encased columns: in CFT, the steel tube fails first, transferring load to the concrete core which continues to resist the axial load. The concrete is partially insulated by the steel tube during the early stages of the fire, delaying the onset of spalling.

Economic impact

Eliminating spray-applied fireproofing (SFRM) on steel columns can save $5-15 per linear foot of column, depending on the required rating and the local market. For a 30-story building with 20 columns per floor at 12 ft story heights, this translates to $36,000-$108,000 in fireproofing savings alone -- often enough to offset the additional cost of the concrete fill.

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

Given: W14x90 steel section encased in a 24x24 inch concrete column. Concrete compressive strength f'c = 5 ksi. Steel yield stress Fy = 50 ksi. 8-No. 8 longitudinal bars (Asr = 6.32 in^2) with Fysr = 60 ksi. Effective length KL = 20 ft = 240 in.

Step 1 -- Material properties:

As = 26.5 in^2 (from AISC manual for W14x90)
Is = 999 in^4 (strong axis, from AISC manual)
Ac = 24*24 - 26.5 - 6.32 = 543.2 in^2
Ec = 57*sqrt(5000) = 4031 ksi
Ag = 24*24 = 576 in^2

Step 2 -- Steel contribution ratio:

Pno = 5026.5 + 606.32 + 0.855543.2 = 1,325 + 379 + 2,309 = 4,013 kips

delta = AsFy/Pno = 5026.5/4013 = 0.330

0.01 <= 0.330 <= 0.9 -- composite design provisions apply.

Step 3 -- Effective stiffness:

C1 = 0.25 + 3*(26.5 + 6.32)/576 = 0.25 + 0.171 = 0.421 <= 0.7 OK

Ic = 24^4/12 - 999 - 6.32*9^2 = 27,648 - 999 - 512 = 26,137 in^4

EIeff = 29,000999 + 0.529,000512 + 0.4214,031*26,137

EIeff = 28,971,000 + 7,424,000 + 44,354,000 = 80,749,000 kip-in^2

Step 4 -- Elastic critical load:

Pe = pi^2 * 80,749,000 / (240)^2 = 13,834 kips

Step 5 -- Nominal strength:

Pno/Pe = 4,013/13,834 = 0.290 < 2.25 (inelastic buckling)

Pn = 4,013 _ 0.658^0.290 = 4,013 _ 0.888 = 3,564 kips

Step 6 -- Design strength:

phiPn = 0.75 * 3,564 = 2,673 kips

Comparison: The bare W14x90 steel column (without concrete encasement) has a design capacity of approximately 600 kips at KL = 20 ft. The composite column provides 2,673 / 600 = 4.5x the capacity -- a dramatic increase from adding concrete encasement.

Worked example -- Round CFT HSS 16x0.500

Given: HSS 16.000x0.500 (round) filled with f'c = 6 ksi concrete. Fy = 50 ksi, KL = 18 ft = 216 in.

Step 1 -- Section properties:

Outside diameter D = 16 in, wall thickness t = 0.500 in
As = pi/4 _ (16^2 - 15^2) = pi/4 _ (256 - 225) = 24.35 in^2
Ac = pi/4 * 15^2 = 176.7 in^2

Step 2 -- Compactness check:

D/t = 16/0.500 = 32.0

Compact limit = 0.15E/Fy = 0.1529,000/50 = 87.0

32.0 < 87.0 -- Compact. Use full composite equations with C2 = 0.95 for round.

Step 3 -- Squash load:

Pno = 5024.35 + 0 + 0.956*176.7 = 1,218 + 0 + 1,007 = 2,225 kips

Step 4 -- Steel contribution ratio:

delta = 50*24.35/2,225 = 0.547 -- within limits.

Step 5 -- Effective stiffness:

C1 = 0.25 + 324.35/(pi/416^2) = 0.25 + 0.363 = 0.613 <= 0.7 OK

Ec = 57*sqrt(6000) = 4,415 ksi

Is = pi/64*(16^4 - 15^4) = pi/64*(65,536 - 50,625) = 730.9 in^4

Ic = pi/6415^4 = pi/6450,625 = 2,485.1 in^4

EIeff = 29,000730.9 + 0 + 0.6134,415*2,485.1 = 21,196,000 + 6,728,000 = 27,924,000 kip-in^2

Step 6 -- Elastic critical load and design strength:

Pe = pi^2 * 27,924,000 / (216)^2 = 5,917 kips

Pno/Pe = 2,225/5,917 = 0.376 < 2.25

Pn = 2,225 _ 0.658^0.376 = 2,225 _ 0.853 = 1,898 kips

phiPn = 0.75 * 1,898 = 1,424 kips

AS 4100 and EN 1994 comparison

AS 4100 / AS 3600 (Australia)

Australia does not have a dedicated composite column standard equivalent to AISC Chapter I. Instead, composite columns are designed under AS 3600 (Concrete Structures) with contributions from the steel section referenced through AS 4100. Key differences from AISC:

Capacity factor: AS 3600 uses phi = 0.60 for concrete in compression and phi = 0.90 for steel -- a split capacity factor system rather than AISC's single phi = 0.75
Slenderness: Uses a moment-magnifier approach (delta_b and delta_s) rather than the direct AISC column curve formulation. The magnified moment M* = delta * M2 accounts for second-order effects
Concrete modulus: AS 3600 defines Ec as a function of concrete density and strength, generally producing slightly different values than the AISC 57*sqrt(f'c) formula
Confinement: Triaxial confinement in CFT columns is acknowledged but treated more conservatively than in AISC
Creep: Explicit long-term stiffness reduction provisions for sustained loads are more detailed in AS 3600 than in AISC

EN 1994-1-1 (Eurocode 4)

Eurocode 4 provides the most detailed composite column design framework of the three codes. Key differences:

Design method: Uses the European column buckling curves (a, b, c, d from EN 1993-1-1) with an effective flexural stiffness (EI)eff that is calculated similarly to AISC but with different reduction coefficients
Buckling reduction factor: Chi = 1 / (Phi + sqrt(Phi^2 - lambda_bar^2)) where Phi includes an imperfection factor alpha that depends on the buckling curve
Concrete strength classes: EN 1994 allows C20/25 to C50/60 for normal-weight concrete and C20/25 to C40/50 for lightweight, a narrower range than AISC
Creep and shrinkage: Section 6.7.3.4 provides explicit provisions for long-term loading effects, including an effective elastic modulus for concrete under sustained loads: Ecd,eff = Ecm / (1 + NEd,S / NEd * phi_t), where phi_t is the creep coefficient
Interaction curve: EN 1994 uses a more detailed P-M interaction method with defined stress blocks for steel and concrete, producing a polygonal interaction curve rather than the four-point AISC method
Shear transfer: More detailed provisions for load introduction at connections, including shear studs, direct bearing, and friction interfaces

CSA S16-19 (Canada)

Canadian practice under CSA S16 Section 18 provides provisions similar to AISC but with different resistance factors:

phi_c = 0.80 for concrete (higher than ACI's 0.65 but reflected in the composite phi)
phi_s = 0.90 for steel
The combined resistance factor approach produces results within 5-10% of AISC for typical columns
CSA uses a simplified column curve approach similar to AISC but with different curve equations

Common mistakes

Using phi = 0.90 instead of 0.75. AISC 360 uses phi = 0.75 for composite columns (Section I2.1b), not the 0.90 used for bare steel columns. The lower phi reflects the higher variability of concrete strength and the interaction between the two materials. Using 0.90 overstates design capacity by 20%.
Ignoring the C1 stiffness reduction. Using full Ec*Ic without the C1 factor overstates column stiffness, which underestimates the effective length and overestimates both Pe and Pn. For a typical encased column, ignoring C1 can overstate EIeff by 30-50%.
Not checking steel contribution ratio limits. If delta = As*Fy/Pno < 0.01, the member is a reinforced concrete column -- use ACI 318, not AISC Chapter I. If delta > 0.9, the member is essentially a bare steel column -- use AISC Chapter E. Using the wrong standard for edge cases produces unconservative designs.
Neglecting long-term creep effects under high sustained loads. AISC accounts for creep through the C1 factor in EIeff, but this may be insufficient for high sustained-to-total load ratios (Nsd/Nd > 0.5). For heavily loaded columns in tall buildings, consider an additional stiffness reduction or use the sustained load provisions in the commentary.
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 (including the C2 = 0.95 confinement factor for round HSS) to apply. Noncompact sections use reduced capacity; slender sections may lose most of the composite benefit.
Assuming the concrete fill is always present and well-consolidated. Voids in the concrete fill (especially at the top of CFT columns) can significantly reduce capacity. Specify proper placement methods (pumping from the bottom, vibration for encased columns) and require fill verification when possible.
Ignoring load introduction requirements. Loads transferred into a composite column through beam connections must engage both the steel and concrete components. If loads are applied only to the steel section (e.g., through a simple beam-to-column web connection), the concrete may not be effectively engaged, and the full composite capacity is not achieved. Use shear transfer mechanisms (studs, bearing plates, or direct bond) per Section I6.
Applying composite provisions to non-standard cross-sections. AISC Chapter I is calibrated for W-shapes in rectangular concrete encasement and round/rectangular HSS filled with concrete. Built-up sections, double angles, or irregular encasement shapes may not fall within the scope of the standard provisions and may require testing or first-principles analysis.
Forgetting to check bearing at column base and cap. The concentrated forces at the base plate and cap plate of a composite column can exceed the concrete bearing capacity if not properly detailed. Use base plates sized for the full Pno, not just the steel contribution.

Frequently asked questions

Q: When is a composite column more economical than a bare steel column?

A: Composite columns become 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 with no fire rating requirement, bare steel W-shapes are typically more economical due to simpler fabrication and erection.

Q: Can I use high-strength concrete (f'c > 10 ksi) in a composite column per AISC?

A: No. AISC 360-22 Section I1.3 limits concrete strength to f'c <= 10 ksi for normal-weight concrete and f'c <= 6 ksi for lightweight concrete. These limits reflect the range of test data used to calibrate the AISC composite column provisions. Higher strengths may be used with appropriate testing and peer review, but the standard design equations do not apply.

Q: Do I need to design the rebar in an encased composite column for all the same requirements as ACI 318?

A: No. AISC Chapter I requires only 0.4% minimum longitudinal reinforcement for encased sections. The rebar does not need to satisfy ACI 318 spacing, tie, or development length requirements in full, because the steel core provides the primary structural function. However, ties must be provided at a maximum spacing of the least dimension of the column or 12 inches to prevent longitudinal bar buckling.

Q: How do I account for creep in composite column design?

A: AISC accounts for creep indirectly through the C1 factor in the effective stiffness calculation. C1 ranges from 0.25 to 0.70, reducing the concrete contribution to EIeff and thereby reducing Pe. For sustained loads that represent more than 50% of the total axial load, consider an additional long-term reduction per the AISC Commentary. EN 1994 provides explicit creep provisions using a reduced effective modulus Ecd,eff.

Q: Can composite columns be used in seismic force-resisting systems?

A: Yes. Composite columns are commonly used in special steel moment frames, composite moment frames, and braced frames in high-seismic regions. AISC 341 (Seismic Provisions) Chapter D covers composite columns in seismic systems. The ductility of the steel component combined with the confinement provided to the concrete makes composite columns well-suited for seismic loading. Additional requirements include minimum tie spacing, maximum D/t ratios, and capacity design principles.

Q: What is the difference between Pno and Pn?

A: Pno is the "squash load" -- the theoretical maximum axial compressive strength of the cross-section when all materials reach their design stresses. Pn is the nominal strength after accounting for column buckling effects (slenderness). Pn is always less than or equal to Pno, with the reduction depending on the column slenderness ratio. For very stocky columns (low KL), Pn approaches Pno. For slender columns (high KL), Pn may be significantly less than Pno.

Q: How do I model composite columns in structural analysis software?

A: Use the transformed section method or direct stiffness input. For the transformed section approach, replace the concrete with an equivalent steel area using the modular ratio n = Es/Ec. For direct input, use the gross cross-section dimensions and the AISC EIeff formula for the effective flexural stiffness. Note that the effective axial stiffness AE should also account for the concrete contribution: AEeff = EsAs + EcAc (some software requires separate AE and EI inputs).

Q: Do CFT columns need internal reinforcement (rebar)?

A: Not typically. AISC Chapter I does not require internal reinforcement in CFT columns. The steel tube provides confinement to the concrete and acts as external reinforcement. However, rebar may be added for: (1) connection force transfer where large moments are introduced at beam-column joints, (2) additional moment capacity, or (3) continuity across floor levels. When rebar is added, its contribution is included in the Asr term of the squash load equation.

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Disclaimer

This page is for educational and reference use only. It does not constitute professional engineering advice. All design values must be independently verified against AISC 360-22 Chapter I, AISC 341 (for seismic applications), and the governing project specification before use. Composite column design requires professional engineering judgment regarding constructability, load path continuity, and connection detailing. The site operator disclaims liability for any loss arising from the use of this information.

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