Neither of those calculations have anything to do with bending moments.
For a rectangular stress block with a specified stress the force in the concrete = stress x area.
For a linear-elastic stress block we don't know the stress, but it is proportional to the distance from the neutral axis...
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...
Sorry about that, you have to stop somewhere.
CSA does get a mention in this one:
https://interactiveds.com.au/Publications/Shear%20and%20Torsion-CIA-2023.pdf
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".
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...
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...
We are starting to go round in circles here, but:
It's "acceptable" in the sense that it will always be conservative, and will give the correct result when M = 0, but I don't see the point of doing complex calculations with a lower than required limit on the strain, when a straight line...
Frankly I am not sure how cl. 6.1(5) is supposed to be applied, but since we are not analysing a flanged beam it doesn't seem relevant anyway. There seems to be general agreement, both in the post above and in the other thread you started here, that as the NA goes from just outside the section...
No, we aren't over-estimating the in-between points. The curve if you did the calculation would be slightly convex, so a straight line would be slightly conservative, but the difference is very small.
You only get the step in the line in your graph because you insist on reducing the strain at...
Full calculation? Sure:
Force = Concrete stress x concrete area + (Es x 0.00175 - concrete stress) x steel area.
All forces are symmetrical about the concrete centroid, so moment = 0.
What do you understand by "pure compression"? My understanding is that the compression strain is constant across the section, so the bending moment is zero. Under that condition the strain limit applies across the full section. When the NA is at the base of the section the strain distribution...
Yes, the strains are just for the calculation of the steel stresses. The stress for the rectangular stress block is defined in the code, and it doesn't say anywhere it should be reduced for sections with uniform compression.
The code also does not say there should be a jump in the strain at...
Have another look at Fig. 6.1 in the code:
- When the bottom face is in tension the top face strain stays constant at 0.0035
- When the NA reaches the bottom face the top face strain is still 0.0035, the bottom face is zero, and point C is 0.00175 (or 0.002 for the parabolic stress block).
- For...
The text doesn't say anything about the 0.9 factor, so we can assume they didn't apply it. The charts are for rectangular sections bending about their principal axes, so there is no reason why they would have applied it. My calculations fitting the chart also did not apply it.
By the "reduced at pure compression" clause do you mean the requirement to reduce the maximum compression strain to 0.00175 or 0.002? If so, why do you think I have "neglected" it? The input allows you to enter the strain under maximum compression, and that is used in calculating the maximum...
But the graph has a tiny range of axial loads close to the tensile capacity of the section. The N/bhfck plotted is -0.354 to -0.342, and you are comparing to a range 0 to 1.2. Try a plot with the same axial load range.
