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AS3600 8.1.3 Rect Stress Block

AS3600 8.1.3 Rect Stress Block

AS3600 8.1.3 Rect Stress Block

Can anyone confirm the correct equations for defining the rectangular stress block in 8.1.3? IDS (Civil/Environme) raised in a previous post that the formula given for γ2 was wrong. Are the following correct?

α2 =1.00 − 0.003 f′c     (0.67 ≤ α2 ≤ 0.85)     ... 8.1.3(1)
γ2 =1.05 − 0.007 f′c     (0.67 ≤ γ ≤ 0.85)     ... 8.1.3(2)

RE: AS3600 8.1.3 Rect Stress Block


Yes, that is correct.

The code has it right in clause (why it has to be defined twice is beyond me. I will try to have it changed for the 2*** version of the code!

RE: AS3600 8.1.3 Rect Stress Block

It was defined twice to increase the chance of getting at least one correct version.

RE: AS3600 8.1.3 Rect Stress Block


Actually it is defined 3 times, but the 3rd one has an extra factor included so that it looks different again (!). I will see if I can get that factor defined separately and all 3 moved top the materials properties section as 1, thus giving the chance to egt it wrong again!

RE: AS3600 8.1.3 Rect Stress Block


Yes, there should be no bf term in the second part of equation 8.4.3. It should simply be kco f'ct. In previous versions of AS3600, this formula was in force units, so f'ct was multiplied by b d. It has been changed to stress units and in the final editing the b term was not removed for some reason.

This is also in the coming amendment.

RE: AS3600 8.1.3 Rect Stress Block

I had intended to wait until the revised code was issued before updating my reinforced concrete design functions for Excel, but since the current code gives the intended formulae in Section 10 I have done the update now, using Section 10 as the default.

I have also added an option to use the Eurocode 2 parabolic-rectangular stress block, since AS 3600 allows the use of any recognised curvilear forms.  The updated spreadsheet (including comparative output from the different stress blocks) can be downloaded from:


The parabolic-rectangular stress block gives significantly higher bending capacity in the region of the balance moment for concrete strengths up to 50 MPa, and significantly lower for grades 65 and 80.  At 100 MPa the two approaches are very similar.

In my opinion it makes sense to use the parabolic-rectangular stress block all the time.

Doug Jenkins
Interactive Design Services

RE: AS3600 8.1.3 Rect Stress Block

Even the notation γ2 is wrong. So it should read

γ =1.05 − 0.007 f′c     (0.67 ≤ γ ≤ 0.85)     ... 8.1.3(2)

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