Rectangular Concrete Column

How it calculates

The Rectangular Concrete Column (ACI 318-19) calculator uses the same fundamental design approach as the ACI 318-14 version - interaction diagrams from the rectangular stress block, Bresler biaxial method, and non-sway moment magnification. The core structural logic is equivalent between the two editions.

Interaction diagram

The P-M interaction diagram is generated using εcu = 0.003 and the ACI rectangular stress block factor β1 per ACI 318-19 Table 22.2.2.4.3, which varies with f'c:

• f'c ≤ 4000 psi: β1 = 0.85
• f'c > 4000 psi: β1 = 0.85 - 0.05 × (f'c - 4000) / 1000 ≥ 0.65

The φ factor transitions from 0.65 (compression-controlled) to 0.90 (tension-controlled) per ACI 318-19 Section 21.2. Failure mode and the governing φ value are reported for each axis check.

Slenderness (non-sway frame)

For slender non-sway columns, moment magnification is applied via the ACI magnification method:

Mc = δs × Mu where δs = Cm / (1 - Pu / 0.75Pc)

with Cm = 0.6 + 0.4 × (M1/M2) derived from the user-entered end moment ratio.

Biaxial bending (Bresler method)

The Bresler reciprocal load method determines the biaxial capacity:

1/φPn,biaxial = 1/φPnx + 1/φPny - 1/φPo

The biaxial utilization Pu / φPn,biaxial ≤ 1.0 must be satisfied. The calculator conservatively evaluates the interaction at the maximum simultaneous Pu, Mu,x, and Mu,y from the governing LRFD combination.

ACI 318-19 clause reference updates (φ factor transitions in Section 21.2, shear table changes) are reflected, but the structural design logic confirmed from the template JSON is equivalent to the ACI 318-14 version for the checks covered.