The RC Design Functions spreadsheet has been updated (to Version 8.04) with the addition of a CircU function for ULS design of circular reinforced concrete sections under combined bending and axial load.  The new version, including full open-source code, may be downloaded from:

RC Design Functions8.zip

Typical input and output are shown in the screen-shots below:

Output of design moment capacity- axial load interaction diagram to AS 3600 with 40 MPa concrete:

The function has 3 options for the design concrete stress block: rectangular, bi-linear, and parabolic-linear.  For the non-rectangular stress blocks the strain limits and parabolic curve exponent are set to Eurocode 2 requirements.

For 32 MPa concrete to the latest AS 3600 code the rectangular stress block is conservative:

With 50MPa concrete the conservatism is increased:

… but with 80 MPa concrete the rectangular stress block is  significantly  unconservative for mid-range axial loads:

Using the Eurocode 2 partial load factors and rectangular block width factors the rectangular stress block is slightly unconservative for all concrete grades:

Comparing rectangular stress block results for AS 3600, Eurocode 2, and ACI 318, for 32 and 50 MPa concrete the AS 3600 results are significantly more conservative for intermediate axial loads, and the Eurocode results are less conservative above the balance load:

For 80 MPa concrete the AS 3600 and Eurocode 2 results are close up to the balance axial load, with the ACI results being significantly less conservative except for very  high axial loads: