RC Column Designer (ACI 318-19)

Interaction diagram

The tool builds the P–M diagram by strain compatibility (ACI 318-19 22.2): concrete strain 0.003 at the compression face, an equivalent stress block 0.85f′c over a = β1c, and steel stress Es·εs limited to ±fy. Bars inside the stress block have the displaced concrete deducted. φ varies from 0.65 to 0.90 with the strain in the extreme tension steel (Table 21.2.2), and the axial load is capped at φPn,max = 0.65 × 0.80Po (22.4.2.1).

Biaxial bending

For each axis the design moment strength φMn at the given Pu is read from its diagram. The biaxial check uses the Bresler load contour with α = 1: Mcx/φMnx + Mcy/φMny ≤ 1. This straight-line form is conservative; the true contour bulges outwards (α ≈ 1.15–1.5).

Slenderness

For a non-sway column, slenderness may be ignored when klu/r ≤ 34 + 12(M1/M2) and ≤ 40, with r = 0.3 times the dimension in the bending direction (6.2.5). Otherwise moments are magnified: δ = Cm/(1 − Pu/0.75Pc) ≥ 1, with Pc = π²EI/(klu)², EI = 0.4EcIg/(1 + βdns) and Cm = 0.6 − 0.4M1/M2, applied to the larger of M2 and M2,min = Pu(15 + 0.03h) (6.6.4.5). ACI 318-19 uses M1/M2 negative for single curvature.

Detailing and shear

Longitudinal steel must be 1–8% of Ag (10.6.1.1). Tie spacing is limited to 16 bar diameters, 48 tie diameters and the least column dimension (25.7.2.1), with at least Ø10 ties for bars up to Ø32 (25.7.2.2). Shear uses Vc = [0.17√f′c + Nu/(6Ag)]bd, which rewards axial compression (Table 22.5.5.1).

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