This limit is when the section is under pure compression (i.e. no moment is applied). It is observed that for non-symmetric arrangements, applying a small moment in one direction may increase the maximum axial load that can be applied to a section because the peak of the N-M interaction diagram is shifted away from the N-axis (i.e. the zero moment line). Checking that the axial load does not exceed the ultimate axial load limit of the section ensures that there is always a positive moment limit and a negative moment limit for the applied axial load for the section.

The ultimate axial load limit of the section, assuming a rectangular stress distribution, is calculated from:

N_max = (RF * A_c * f_cd * η) + ∑(A_s,i * f_s,i)

Given that,

A_c = A - ∑A_s,i

f_s,i = ε_c * E_s,i

Where

RF is the concrete design reduction factor, (this is a fixed value of 0.9 which cannot be changed)

A is the overall area of the section,

A _c is the area of concrete in the section,

A_s.i is the area of bar i,

f_cd is the design compressive strength of the concrete,

η is a reduction factor for the design compressive strength for high strength concrete for the rectangular stress distribution,

ε_c is the strain in the concrete at reaching the maximum strength,

f_s,i is the stress in bar i when the concrete reaches the maximum strength,

E_s,i is the modulus of elasticity of the steel used in bar i.