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Column Required and Design Strength per ACI 318-19 with ideCAD

How does ideCAD calculate concrete column required, and design strength according to ACI 318-19?

  • Column required strengths are calculated automatically.

  • Column design strengths are calculated automatically.


D = dead load
E = earthquake load
L = live load
Lr = roof live load
Mn = nominal flexural strength at section, in.-lb
Mu = factored moment at section, in.-lb
Pn = nominal axial compressive strength of member, lb
Pu = factored axial force; to be taken as positive for compression and negative for tension, lb
R = rain load
S = snow load
Tn = nominal torsional moment strength, in.-lb
Tu = factored torsional moment at section, in.-lb
U = strength of a member or cross section required to resist factored loads or related internal moments and forces in such combinations as stipulated in ACI 318-19
Vn = nominal shear strength, lb
Vu = factored shear force at section, lb
W = wind load
ϕ = strength reduction factor

Required Strength

The required strength is calculated in accordance with the factored load combinations in Load Factors and Combinations per ACI 318-19 with ideCAD title. Combined Axial and Flexural Required strengths of a column Pu and Mu occur simultaneously for a column. Therefore, for each applicable factor load combination specified in ACI Table 5.3.1, the most unfavorable condition of Pu and Mu occurring simultaneously is considered.

Design Strength

Design strength in all sections should satisfy all conditions given below;

  • ϕPn ≥ Pu

  • ϕMn ≥ Mu

  • ϕVn ≥ Vu

  • ϕTn ≥ Tu

Strength reduction factors ϕ is determined according to using ACI Table 21.2.2. Interaction between load effects is considered.

Strain, εt

Section Classification



Compression Controlled Moment


εty < εt < (εty + 0.003)

Transition region

0.65 + 0.25[(εt - εty)/0.003]

εt ≥ (εty + 0.003)

Compression Controlled Moment


Pn and Mn is calculated in accordance with Axial strength or Combined Flexural and Axial Strength per ACI 318-19 with ideCAD title. Combined Axial and Flexural strengths create a three-dimensional interaction failure surface. In addition to axial compression and biaxial bending, the formulation allows for axial tension and biaxial bending considerations. An interaction surface of a column is shown below.

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