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ACI 318-19 Rectangular Column — Axial Load and Biaxial Bending

Exact strain-compatibility check with an inclined neutral axis (the method used by spColumn), with φ from the extreme tension strain; the Bresler reciprocal load and load-contour approximations are shown alongside for comparison.

Section

b (x) in.

h (y) in.

Face to bar centre in.

Bars per face along b

Bars per face along h

Bar size

f′c psi

f_y psi

Factored loads

P_u kip

M_ux kip-ft

M_uy kip-ft

Results

P_o · φP_n,max · ρ_g—

Exact: φ · P_n · c · neutral-axis angle—

Exact contour capacity (M_nx, M_ny)—

Exact: |M_u| / φ|M_n| at constant P—

Bresler P_n: constant e (Bresler) · constant M (PCA/SP reading)—

Linear load contour (α = 1)—

ACI 318-1922.2.2 (strain compatibility, εcu 0.003, 0.85f′c block, β₁), Table 21.2.2 (φ from ε_t), Table 22.4.2.1 (φP_n,max), 10.6.1.1 (ρ_g). Approximate methods: Bresler (ACI Journal, 1960) via PCA Notes Ch. 7. Clause numbers only.

VerifiedStructurePoint biaxial design-procedure example (ACI 318-19): exact neutral-axis solution (c 25.11 in., 25.24°: P_n 1846, M_nx 608, M_ny 245.4 kip-ft) within 0.5%; Bresler P_n 2100 kip; linear contour 0.96. Note: the example reads P_ox, P_oy at constant moment; Bresler's definition (constant eccentricity) gives about 6% less.

Preliminary calculation tool. Final design requires the full text of ACI 318 and review by a licensed PE. This tool follows ACI 318-19, the edition referenced by the 2021 and 2024 IBC; ACI CODE-318-25 was published in January 2025 and is expected in the 2027 IBC (changes include development length equations and seismic provisions).