Canadian Composite Column Design — Concrete-Filled HSS per CSA S16-19

Complete reference for concrete-filled HSS composite column design per CSA S16-19 Clause 18. Covers axial compression capacity combining steel and concrete contributions, confinement effects on concrete strength, slenderness limits, construction sequence checks, and a step-by-step worked example.

Quick access: HSS section properties → | Shear stud → | Column design →

CSA S16 Composite Column Framework

Per CSA S16-19 Clause 18, a concrete-filled HSS composite column's factored axial compression resistance Cr is:

Cr = phi × [As × Fy + A_c × 0.85 × f'c × (1 + alpha_c × t/D × Fy/f'c)]

Where:

Simplified Equation (Without Confinement)

For rectangular/square HSS (no confinement benefit):

Cr = phi × (As × Fy + A_c × 0.85 × f'c)

For circular HSS with confinement:

Cr = phi × [As × Fy + A_c × 0.85 × f'c × (1 + 0.50 × t/D × Fy/f'c)]

Composite Column Resistance Factors

Per CSA S16 Clause 18:

Parameter Symbol Value
Steel resistance factor phi 0.90
Concrete factor — 0.85 × f'c (Clause 18.2.2)
Concrete confinement factor (circular) alpha_c 0.50
Concrete confinement factor (rectangular) alpha_c 0.00

Capacity Comparison — HSS vs Composite (f'c = 30 MPa)

HSS Section HSS Cr (kN) Composite Cr (kN) Increase
HSS 127×127×8 437 713 +63%
HSS 152×152×9.5 768 1235 +61%
HSS 203×203×9.5 1439 2280 +58%
HSS 254×254×9.5 1775 3140 +77%
HSS 254×254×12.7 2673 3905 +46%
HSS 305×305×12.7 3274 5300 +62%

The concrete fill adds 45-80% to axial capacity depending on the steel ratio As/A_c and concrete strength.

Slenderness Limits

Per CSA S16 Clause 18.3:

Section Classification

Limit Expression For 350W For f'c=30 MPa
Maximum D/t (square) 67000/Fy 191 191
Maximum D/t (circular) 22000/Fy 63 63
Minimum t 2.5 mm (practical) — —

Column Slenderness

Non-dimensional slenderness ratio for composite columns:

lambda_c = (KL/r_m) × sqrt(Fy_comp / (pi^2 × E))

Where:

But Clause 18 simplifies this for rectangular sections with Es/Ec ratio, providing tabulated Cr values in the CISC Handbook.

Construction Sequence Checks

During construction (before concrete gains strength), the steel HSS alone must resist:

  1. Self-weight of steel: HSS section weight per metre
  2. Wet concrete load: Full hydrostatic pressure of fresh concrete
  3. Construction live load: Typically 1.0-1.5 kPa

Per CSA S16 Clause 18.4, the steel HSS during construction must have:

Worked Example — Concrete-Filled HSS 254×254×9.5

Given: HSS 254×254×9.5, Grade 350W, filled with 30 MPa concrete. KL = 5000 mm. K = 1.0, pinned ends.

Step 1 — Section Properties: HSS: As = 8,830 mm^2, D = 254 mm, t = 9.5 mm Concrete core size: 254 - 2×9.5 = 235 mm Ac = 235^2 = 55,225 mm^2

Step 2 — Axial Capacity (Simplified, no confinement for square): Cr = 0.90 × (8,830 × 350 + 55,225 × 0.85 × 30) / 1000 Cr = 0.90 × (3,090,500 + 1,408,238) / 1000 Cr = 0.90 × 4,498,738 / 1000 = 4,049 kN

Step 3 — Slenderness Check: r (HSS alone) = 98.9 mm (from HSS properties table) KL/r = 5000/98.9 = 50.6 — composite column intermediate slenderness For composite columns, the modified slenderness is less than the HSS alone (concrete increases stiffness).

lambda = 50.6 × sqrt(350 / (pi^2 × 200,000)) = 50.6 × 0.01332 = 0.674

For HSS (n = 2.24): Cr_reduced = 4,049 × (1 + 0.674^(2×2.24))^(-1/2.24) = 4,049 × (1 + 0.674^4.48)^(-0.446)

0.674^4.48 = 0.674^2 × 0.674^2.48 ≈ 0.454 × 0.358 = 0.163 Cr_reduced = 4,049 × (1.163)^(-0.446) = 4,049 × 0.934 = 3,782 kN

Step 4 — Check axial load: Cf = 3,000 kN (factored) ≤ Cr = 3,782 kN. Ratio = 0.79. OK.

Step 5 — Construction Check (HSS alone before concrete cures): Cf_construction = 2,000 kN (steel + wet concrete only) KL/r = 50.6, lambda = 0.674 Cr_HSS_alone = 0.90 × 8,830 × 350 × (1 + 0.674^4.48)^(-0.446) / 1000 = 2,780 × 0.934 = 2,597 kN 2,000 ≤ 2,597. OK during construction.

Result: HSS 254×254×9.5 filled with 30 MPa concrete provides Cr = 3,782 kN, representing a 77% increase over the bare HSS compression capacity of 1,775 kN.

Column Base Connection

The base of a composite column must:

  1. Transfer the full axial load to the foundation — typically via a base plate with stiffeners
  2. Provide concrete confinement at the base — the base plate and HSS act together
  3. Allow concrete placement — access holes in the HSS at the top and vent holes at the base

Base Plate Design

Base plate area required: A1 ≥ Cf / (0.85 × phi_c × f'c × sqrt(A2/A1))

For Cf = 3,000 kN, f'c = 30 MPa: A1 ≥ 3,000 × 1000 / (0.85 × 0.65 × 30 × 2.0) = 90,500 mm^2 → 301×301 mm min. Use 350×350×25 mm plate.

Frequently Asked Questions

What is the advantage of concrete-filled HSS composite columns? Concrete fill increases axial compression capacity by 45-80% without increasing the steel section size. The concrete also: (a) provides inherent fire resistance (up to 2 hours without additional fire protection), (b) increases stiffness, reducing KL/r, (c) adds damping to the structure, and (d) provides restraint against HSS wall local buckling. The main trade-off is: increased weight, longer construction time (concrete curing), and the need for concrete placement planning.

How is confinement accounted for in circular vs rectangular HSS? Circular HSS provides concrete confinement (the steel hoop tension restrains concrete dilation under load), increasing concrete effective strength by the factor (1 + 0.50 × t/D × Fy/f'c). Rectangular/square HSS provides negligible confinement (flat walls bulge outward rather than providing hoop restraint), so the confinement term is zero per CSA S16 Clause 18. Circular composite columns therefore have a 10-20% higher capacity than equivalent rectangular sections.

Does the concrete core need vertical reinforcement? Per CSA S16 Clause 18.2, vertical reinforcement is not required for composite columns because the steel HSS provides the longitudinal reinforcement. However, for columns in seismic zones or with high moment demands, additional rebar inside the HSS can be specified. The minimum steel reinforcement ratio per CSA A23.3 does not apply because the HSS serves as the longitudinal reinforcement.

What fire rating do concrete-filled HSS columns provide? Concrete-filled HSS columns typically achieve 1-2 hour fire resistance ratings without additional fire protection. The concrete core acts as a heat sink, absorbing thermal energy. The water content of the concrete (free + chemically bound) provides evaporative cooling. For a 2-hour rating or higher, additional fire protection (spray-applied fireproofing or intumescent coating) is required per NBCC 2020.

Related Pages

This page is for educational reference. Composite column design per CSA S16-19 Clause 18. Verify concrete strength and HSS dimensions with project specifications. Construction sequence checks per Clause 18.4. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent PE/SE verification.

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