N-M interaction chart to Australian Standards 2

[Pretty Girl]

The below is the N-M interaction chart for Eurocode for a biaxial concrete column.

Do Australian standards provide any guidance to come up with the N-M interaction diagram? Which clauses I need to look at if I want to make our chart compliant with AS 3600?
I want to know the calculations which involves with this and especially the limits, caps, and other considerations to make it AS 3600 compliant.

Does it have some different way of calculating strains for pure compression and when column is not in pure compression?

How do I construct a chart which is compliant to Australian standards?

@IDS @Celt83

 

[Retrograde]

Section 10.6.2 is the relevant part of AS3600.

 

[IDS]

The interaction diagram shown is not for biaxial loading. The examples in the book find the capacities about the X and Y axes, and then combine the ratios of applied moment to moment capacity, using the code simplified approach.

The Australian code (AS 3600) has procedures for finding the moment capacity for any specified axial load, so those procedures can be used to generate an NM diagram, as for the Eurocode. The basic analysis procedure is the same as in the Eurocode, but there are significant differences in application:

1. The design ultimate capacity is calculated using the unreduced material yield stresses, and the resulting axial force and bending moment are factored down by a single factor which varies between 0.85 for tension and no axial load down to 0.65 or 0.6 for axial loads above the balance load. This is a similar procedure to that used in ACI 318, although there are differences in calculation of the intermediate values.

2. The code only gives detailed procedures for a rectangular stress block, although it specifically allows other recognised stress blocks to be used.

3. The factors for the depth and design stress of the rectangular stress block are different from the Eurocode values.

4. The specified strain at the compression face is 0.003 for all grades of concrete. This is reduced to 0.0025 for calculation of the "squash load", i.e. for sections in uniform compression.

5. The squash load is reduced by an additional factor between 0.72 and 0.85. See code extract in the previous post for details.

6. The code specifies that a straight line interpolation may be used for calculations when the NA is outside the section.

7. Sections with high axial load have detailed requirement for confinement reinforcement, and the effect of confinement on section capacity.

 

[Pretty Girl]

Thank you @IDS and @Retrograde . I'll go through those and will get back to you.

 

[Pretty Girl]

@IDS
Hi, is there any standard N-M interaction reference chart for Aus standards (for X axis atleast), so I can compare with mine as we did before for the eurocode chart?

 

[IDS]

@IDS
Hi, is there any standard N-M interaction reference chart for Aus standards (for X axis atleast), so I can compare with mine as we did before for the eurocode chart?

I don't know of any printed interaction diagrams for the latest AS 3600, but you can always use the RCInteract function from my RC design functions spreadsheet, which you can download from:

https://newtonexcelbach.com/2025/02/08/rc-design-functions-9-07/

Also see https://newtonexcelbach.com/2010/12/21/uls-design-of-reinforced-concrete-as-aci-and-european-codes/ for a comparison of Australian, European and American codes.

 

[Pretty Girl]

I don't know of any printed interaction diagrams for the latest AS 3600, but you can always use the RCInteract function from my RC design functions spreadsheet, which you can download from:

https://newtonexcelbach.com/2025/02/08/rc-design-functions-9-07/

Also see https://newtonexcelbach.com/2010/12/21/uls-design-of-reinforced-concrete-as-aci-and-european-codes/ for a comparison of Australian, European and American codes.

@IDS thank you for the response and the excel file.
However, I need a standard reference though. No need of latest charts, It's ok if the chart is old. Is there any old standard charts. I need official ones.

 

[EngDM]

Also see https://newtonexcelbach.com/2010/12/21/uls-design-of-reinforced-concrete-as-aci-and-european-codes/ for a comparison of Australian, European and American codes.

Yo yo yo where is my CSA comparison we exist too!

 

[IDS]

@IDS thank you for the response and the excel file.
However, I need a standard reference though. No need of latest charts, It's ok if the chart is old. Is there any old standard charts. I need official ones.

What is the point of referring to old charts that will have different results to the current code?

Also being printed in a book does not make a chart "official".

 

[IDS]

Yo yo yo where is my CSA comparison we exist too!

Sorry about that, you have to stop somewhere.

CSA does get a mention in this one:
https://interactiveds.com.au/Publications/Shear and Torsion-CIA-2023.pdf

 

[Pretty Girl]

What is the point of referring to old charts that will have different results to the current code?

Also being printed in a book does not make a chart "official".

@IDS Thank you for the response.

You're right, it makes no sense to compare the old chart of a book if the code is significantly changed.
However, how do we both know if yours and mine is correct if we do not compare it to a established chart?
Even both of our charts did not fully align with concrete centre charts (it's possible they used some old method, but their books are not that old though, so how our charts do not match theirs). Although our chart is acceptable, may be eurocode expected some other method to calculate strains. I'm still not sure if the method we did was the method eurocode expected. I'll comply eurocode, but was that the method they mentioned?
It's possible that your Australian chart is completely wrong and may be you just simplified it to be conservative (absolutely not demeaning your expertise, just trying to make a point).
So, isn't that a good idea to have some sort of official reference?
Is it possible that we find anything from official sources? how did you know your chart is correct?
So, australian standards has not given any specific method? so they accept usual strain compatibility method we did for the eurocode with the limitations which applicable to aus standards you mentioned previously?

That's why I'm looking for an official reference so we can compare it to.

 

[IDS]

It's possible that your Australian chart is completely wrong and may be you just simplified it to be conservative (absolutely not demeaning your expertise, just trying to make a point).

You should absolutely assume everything you read in this forum is wrong, until proved otherwise.

But the same applies to "official sources".

As for sources of information, rather than accepting numbers presented in chart form in a book I think it is better to work through calculation examples and make sure that:
1. The intermediate and final results agree with your calculations, and other sources.
2. The procedure and the results are consistent with the underlying theory.
... and that is essentially what we have been doing in this thread.

I'm not sure what you mean about Australian Standards not giving a specific method. They state the basics of the method, which are based on strain compatibility, plane sections remaining plane, and specified simplifications of the stress-strain behaviour, as are all other codes that I know of, but they also have significant differences from the Eurocode approach, particularly in the way that limit state strength factors are applied.

As for good sources of information, my basic reference is Concrete Structures by Warner, Rangan, Hall and Faulkes, but that is pretty out of date now, and probably the best current reference is Reinforced Concrete Basics by Foster, Kilkpatrick and Warner, https://www.cies.unsw.edu.au/news/third-edition-of-reinforced-concrete-basics

 

[Pretty Girl]

You should absolutely assume everything you read in this forum is wrong, until proved otherwise.

But the same applies to "official sources".

As for sources of information, rather than accepting numbers presented in chart form in a book I think it is better to work through calculation examples and make sure that:
1. The intermediate and final results agree with your calculations, and other sources.
2. The procedure and the results are consistent with the underlying theory.
... and that is essentially what we have been doing in this thread.

I'm not sure what you mean about Australian Standards not giving a specific method. They state the basics of the method, which are based on strain compatibility, plane sections remaining plane, and specified simplifications of the stress-strain behaviour, as are all other codes that I know of, but they also have significant differences from the Eurocode approach, particularly in the way that limit state strength factors are applied.

As for good sources of information, my basic reference is Concrete Structures by Warner, Rangan, Hall and Faulkes, but that is pretty out of date now, and probably the best current reference is Reinforced Concrete Basics by Foster, Kilkpatrick and Warner, https://www.cies.unsw.edu.au/news/third-edition-of-reinforced-concrete-basics

@IDS Thank you for the response.
You're right. the "official sources" as we think they are, might also be wrong, but still my brain says what we did was not Eurocode expects us to do with that 0.00175 limit. The way we did it correct in calculation, so chart is in safe region, but the method they expect is not the way we did it as I feel. However I still have some doubt (about the method Eurocode expects) as we didn't make any calculation for the Eurocode pure-compression one, we just used 0.00175 as a cap. If we replace 0.0035 with 0.00175 and do the calculation that "drop/discontinuity" happens. Since we used 0.00175 as a cap, it smoothly ended at 0.00175 at the M=0. But Eurocode may not be expecting us to calculate in that way though. I should move on it seems, or else we're encircling at the same place.

I'll come up with the Australian chart and seek your verification.