# Ultimate axial load limit (Concrete wall: EC2)

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*.

The concrete design reduction factor RF originates from EC2 section 3.1.7(3): "Note: If the width of the compression zone decreases in the direction of the extreme compression fibre, the value η_{fcd} should be reduced by 10%"

In Tekla Structural Designer the RF factor is applied in both the axial-moment interaction check and the ultimate axial resistance check (even though there is no extreme compression fibre in this latter calculation) so that the ultimate axial resistance matches the peak position of the interaction diagram - its inclusion creates a conservative result.