ACI 318-19 Section 22.4 | Axial Strength & P-M Interaction of Columns
Concrete columns carry simultaneous axial compression and bending moments. Because minimum accidental eccentricity is always present in real construction, ACI 318-19 caps the maximum usable axial capacity at 80% of pure squash load Po for tied columns (85% for spiral columns) and evaluates combined capacity across a continuous P-M interaction curve.
Governing Formula
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Notation & Variables
Worked Example
Compute section areas
For an 18" × 18" tied column (Ag = 324 in²) reinforced with 8-#8 bars (Ast = 6.32 in²), f'c = 4,000 psi, fy = 60,000 psi.
Calculate pure axial squash load Po
Evaluate concrete core and longitudinal steel contribution at zero eccentricity.
Apply tied column eccentricity cap & phi
Multiply by 0.80 for tied columns and φ = 0.65 (compression-controlled).
When does this govern?
Governs all reinforced concrete building columns, bridge piers, and pedestal supports.
Common Mistake
Forgetting to subtract Ast from Ag in the 0.85 f'c (Ag - Ast) term, which double-counts the concrete area displaced by heavy longitudinal rebar.