Concrete Column Calculator, ACI 318-19

What it checks

The module designs short tied rectangular concrete columns for combined axial compression and biaxial bending. It applies ACI 318-19 strength design methods, utilizing a Bresler-style straight-line interaction surface for the biaxial case. This approach is conservative, and the source notes that final design should use a full P-M interaction diagram per ACI 22.4. The engine constructs a P-M interaction curve about the strong axis by iterating the neutral-axis depth over 50 log-spaced steps from 0.001·h to 2.5·h. It includes three landmark points: pure axial, balanced, and pure flexure. The results are sorted by decreasing φPn. The module also calculates the minimum eccentricity required for small columns, applying the (0.6 + 0.03·h) formula to ensure non-conservative legacy formulas are not used.

Inputs

Geometry - Section dimensions (in), specifically depth h and width b for strong-axis bending.
Materials - Concrete compressive strength f'c and steel yield strength fy.
Reinforcement - Bar designation (#3..#11, #14, #18) and number of bars, used to calculate nominal area and diameter.
Loads - Axial load Pu and biaxial bending moments for interaction surface evaluation.

What it reports

Interaction Curve - A vector of P-M points sorted by decreasing φPn, covering the neutral-axis depth range.
Capacity Points - φPn (kips) and φMn (kip·ft) for each evaluated neutral-axis depth c.
Strain - The tensile strain eps_t at the extreme compression fiber for each point.
Minimum Eccentricity - The calculated e_min in inches, compared against the legacy 0.1·h formula to verify conservatism.
Minimum Moment - The derived M_min (Pu × e_min / 12) used to check against the applied moment.
Bar Properties - Nominal bar area (in²) and diameter (in) based on the geometric formula for the selected bar size.

Code references

The engine cites ACI 318-19 §22.2 for strain compatibility, §22.4.2.4 for minimum eccentricity, and §21.2.2. It specifically references ACI 22.4.2.1 for pure axial capacity and ACI 22.4 for the full P-M interaction diagram. The minimum eccentricity calculation follows the (0.6 + 0.03·h) clause, replacing the older 0.1·h formula which was found to be under by 23% for small columns.

Limits

Slenderness effects are not yet implemented, so the module is limited to short tied columns. The biaxial bending analysis uses a Bresler-style straight-line interaction surface, which is conservative; the source states that final design should use a full P-M interaction diagram. The bar area calculation uses a geometric formula that does not equal the ASTM A615 nominal area for bars #9 and larger, a conflict flagged in the source. Results are engineering aids; the engineer of record is responsible for the design.

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