Capacity reduction factors (AS 3600)

About this AS 3600 Capacity Reduction Factors Calculator

This page gives the capacity reduction factors φ of AS 3600:2018 Table 2.2.2 for a reinforced concrete member in bending without axial force and in shear. The bending factor depends on the neutral axis parameter k_uo of the section and on the ductility class of the tension reinforcement, so the page is built to be referenced from a design check.

• Structural engineer links the bending and shear factors into beam and slab checks so φ updates with the section.
• Design reviewer confirms which φ a calculation used and that it matches the reinforcement class.
• Engineering student sees how ductility in the section earns a higher bending factor.

It is an engineering-grade CalcTree page: map its inputs to the values your check works out and the factors flow through.

More info on AS 3600 Capacity Reduction Factors

Bending without axial force

With Class N tension reinforcement the bending factor rises as the neutral axis parameter k_uo falls, between the lower and upper limits of Table 2.2.2, so a ductile section earns the higher factor. With Class L reinforcement the factor is fixed at the lower value.

Shear

The factor for shear in Table 2.2.2 is a single value and does not depend on the section.

Inputs

The neutral axis parameter k_uo is the value at ultimate strength in pure bending, worked out by the check that uses these factors. A selector chooses Class N or Class L for the tension reinforcement.

Scope

Members with axial force, torsion, bearing, plain concrete, tension members, prestressed members and strut-and-tie models are not covered; take those factors from Table 2.2.2 directly.

Common Calculation Errors to Avoid

• Using the Class N formula with Class L reinforcement overstates the bending factor. Class L takes the fixed lower value.
• Taking k_uo from a section with axial force applies the pure bending factor where the interpolation towards the compression factor is needed.
• Using the bending factor for shear, or the reverse, mixes two separate rows of Table 2.2.2.
• Ignoring the ductility of the section misses the reward for a low k_uo. A section with a small neutral axis depth earns a higher factor.

FAQs

Why does the bending factor depend on k_uo?

A low neutral axis parameter means a ductile section that yields well before the concrete crushes. AS 3600:2018 Table 2.2.2 rewards that ductility with a higher capacity reduction factor for Class N reinforcement.

What changes with Class L reinforcement?

With Class L reinforcement the bending factor is fixed at the lower value of Table 2.2.2, whatever k_uo is.

Does this cover columns?

No. Members with axial force need the interpolation towards the compression factor, which this page does not do.

How do I use it in my own check?

Reference the page from your beam or slab calculation and map k_uo and the reinforcement class to the values your check works out.