Concrete Column Load Capacity
Estimate the axial load capacity of a reinforced concrete column using ACI 318 simplified formula for tied and spiral columns.
Results
What is it?
Based on ACI 318-19 Section 22.4.2, the maximum design axial strength of a column is: φPn = φ × [0.80 or 0.85] × [0.85f'c(Ag−Ast) + fyAst]. The factor 0.80 applies to tied columns and 0.85 to spiral columns, with strength reduction factors φ = 0.65 and 0.75 respectively.
How to use
Enter the gross cross-section area (e.g. 300 mm × 300 mm = 90,000 mm²), total reinforcement area Ast, concrete compressive strength f'c, and steel yield strength fy. Select tied or spiral to apply the correct reduction factors.
Example scenario
A 300×300 mm tied column with 4-Ø20 bars (Ast = 1256 mm²), f'c = 30 MPa, fy = 420 MPa. Nominal: 0.80 × (0.85×30×(90000−1256) + 420×1256) = 0.80 × (2259 kN + 527 kN) = 2229 kN. Design: φPn = 0.65 × 2229 ≈ 1449 kN.
Pro tip
ACI 318 requires the steel ratio ρg (Ast/Ag) to be between 1% and 8%. If your ratio falls outside this range, adjust the bar size or count. Spiral columns allow higher loads (φ = 0.75 vs 0.65) due to better confinement — useful when column size is constrained.