Check whether a column section can carry a factored axial load and moment acting together. Enter the shape (rectangular tied or circular tied/spiral), dimensions, cover, concrete and steel strengths, and longitudinal reinforcement, and the tool sweeps the neutral axis through the full range of strain-compatibility states — from pure axial compression through balanced failure to pure flexure and pure tension — building the complete φPn–φMn interaction diagram per ACI 318. Enter your factored demand, Pu and Mu, and it tells you immediately whether that point falls inside the diagram.
Any longitudinal bar count is supported (4 up to 32, or a custom count), with bars laid out automatically around the section — forced corner bars for rectangular columns, even spacing around the circle for circular columns. The compression-block concrete force is found by numerical integration rather than a closed-form approximation, so the result is exact for both rectangular and circular sections alike.
What you get
- Rectangular tied columns and circular tied or spiral columns
- Full φPn–φMn interaction diagram built by sweeping the neutral axis with strain compatibility (ACI 318 22.2), not a simplified point check
- Variable strength-reduction factor φ, interpolated between the compression-controlled base value (0.65 tied / 0.75 spiral) and 0.90 per 21.2.2
- Maximum axial capacity cap at 0.80φPo (tied) or 0.85φPo (spiral) per 22.4.2.1
- An immediate OK/NG check of your factored (Pu, Mu) demand point against the diagram, plus key points (pure compression, balanced, near-pure moment) in a summary table
- A section sketch and the interaction-diagram plot with your demand point marked
- Metric (mm, kN) or Imperial (in, kip) units
Notes and limitations
This tool checks uniaxial bending only — it does not check biaxial bending, slenderness (second-order / P-Δ) effects, or shear, tie or spiral capacity. It complements the RCC Column & Beam Section Drawing Generator, which lays out and dimensions a section but does not check its capacity. Final acceptance of the design rests with the responsible engineer.