Concrete Column

How it calculates

The Concrete Column calculator designs rectangular and circular reinforced concrete columns to AS 3600:2018 (Amendment 2) using limit state design. It generates P-M interaction diagrams for both axes, checks combined axial and bending demands, and evaluates buckling for slender columns.

Section geometry and reinforcement

For rectangular sections, the cross-section is defined by depth D and breadth B. For circular sections, the overall diameter is entered directly. Longitudinal reinforcement is specified by bar size and count, arranged in a circular pattern for circular sections or as corner and face bars for rectangular sections. Fitment (tie or helix) size and spacing are entered separately.

Axial squash and buckling capacity (AS 3600:2018, Cl 10.3 and 10.4)

The squash load is the maximum concentric axial load the section can carry without bending:

phi × N_uo = phi × [alpha_1 × f'c × (Ag - Ast) + fy × Ast]

where alpha_1 = 1.0 - (f'c / 340) per Cl 10.6.2.2. The reduced squash load phi_N_u accounts for accidental eccentricity (Cl 10.3.1).

Buckling capacity is determined from the effective length kL and the section radius of gyration r:

phi × N_uc = pi² × E × I / (kL)² (for slender columns per Cl 10.4)

P-M interaction diagram (AS 3600:2018, Cl 10.6)

The interaction diagram is constructed from three key points per axis:

• Decompression point: axial load at which tensile strain at the extreme fibre is zero (Cl 10.6.2.3)
• Balanced point: simultaneous concrete crushing and steel yielding (Cl 10.6.2.5)
• Pure bending point: zero axial load capacity per Cl 8.1

The rectangular stress block (Cl 10.6.2.4) is used throughout with gamma = 0.85 - 0.007(f'c - 28) for the block depth factor. The phi factor transitions between 0.65 (compression-controlled) and 0.80 (tension-controlled) per Cl 2.2.2.

Biaxial interaction check (AS 3600:2018, Cl 10.6.3)

For combined biaxial bending and axial load, the utilization is checked against the interaction relationship:

(Mu,x / phi × M_ux)^alpha_n + (Mu,y / phi × M_uy)^alpha_n ≤ 1.0

where alpha_n is determined from the axial load ratio N*/N_uo.

Shear capacity (AS 3600:2018, Cl 8.2)

Shear capacity about each axis is computed using the simplified method (Cl 8.2.4.3), valid for f'c up to 65 MPa without prestress, tension, or torsion. Results are reported for reference; columns with significant lateral shear demands should be verified separately.

Assumptions

No torsional demands are considered. Fitments are assumed horizontal and perpendicular to tensile reinforcement. Transmission of axial force through floor systems (Cl 10.8) and crack control are not checked within this calculator.