Biaxial bending update

The reinforced concrete biaxial bending spreadsheet has now been updated:

  • Stress block and capacity reduction factors updated for AS 3600-2018
  • ACI factors for US customary units used when stress is entered in ksi or psi units
  • Example data updated

The revised spreadsheet is included in the file:

ULS Design

I have checked the spreadsheet results against a paper published by Enercalc:

Reinforced Concrete Columns in Biaxial Bending

Enercalc results:

ULS Design Functions-biax results:

After adjusting the Enercalc moments to the centroid of the uncracked concrete section, the results are in near exact agreement.

The results below compare the spreadsheet results with the simplified procedures given in AS 3600 and Eurocode 2, using a rectangular section and plotting moment capacity results under different axial loads:

The AS 3600 and Eurocode provisions for biaxial bending take exactly the same form:

The two codes give significantly different results however because:

  • In AS 3600 the Phi factor is defined as 0.65 for all axial loads, whereas the Eurocode uses the design Mux and Muy values for the applied axial load.
  • The definition of the exponent alpha is different, resulting in AS 3600 having a lower value for low axial loads, but a higher value for high axial loads.

For the purposes of comparison, the Eurocode Mx and My values used the design moment capacities under the applicable axial load to the AS 3600 rules.

For an axial load well below the balance load both codes have an alpha value of 1, resulting in conservative capacity values under biaxial loading.  The AS 3600 requirement to use a Phi factor of 0.65 results in a further substantial conservatism:

Increasing the axial load to close to the balance load increases the alpha values above 1, and brings the AS 3600 Phi factor down close to 0.65, reducing the conservatism of the simplified approach:

A further increase in the axial load increases the alpha value for AS 3600 to close to 2, so that the AS 3600 results are no longer conservative.  The Eurocode alpha value is lower, and it is still conservative:

A further increase in the axial load shows the same trend, with the AS 3600 results increasingly unconservative, and the Eurocode results closer to the detailed calculation, but still conservative:

For very high axial loads, where the base of the rectangular stress block is outside the concrete section, there is a large reduction in the moment capacity from the detailed calculation.  This is not reflected in the simplified method from either code, which both have an alpha value of 2 at these loads:

Further complications arise with non-rectangular sections:

In this case the simplified method from both codes gives results that are highly unconservative for some moment combinations, even for axial loads well below the design limit:

It should be noted that all calculations assumed a rectangular stress block to the AS 3600 code.  Use of a parabolic linear stress block would result in further significant reductions in the design moment capacity when the concrete compression zone was triangular.

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18 Responses to Biaxial bending update

  1. Bill Harvey says:

    Where are you based Doug?

    On Thu, 21 Nov 2019 at 14:01, Newton Excel Bach, not (just) an Excel Blog wrote:

    > dougaj4 posted: “The reinforced concrete biaxial bending spreadsheet has > now been updated: Stress block and capacity reduction factors updated for > AS 3600-2018 ACI factors for US customary units used when stress is entered > in ksi or psi units Example data updat” >


  2. Angelo says:

    Thank you for the post. As someone who is studying section analysis through Excel VBA, I find your content to be the most helpful reference so far.


  3. DonB says:


    been attempting this myself , If you don’t mind sharing what method are you using to solve for the NA depth and angle for the interaction point?

    Method I’ve been trying works well for some configurations but breaks down in the tension side of the P v M curve and with shapes having large aspect ratios.

    Also I wasn’t able to get the reinforcement table in the ULS biaxial sheet to work, do you have a post explaining that?



    • DonB says:

      ended up trying out a bisection method approach which almost always gets it right but is a fraction slower. Still breaks down in the tension side for unsymmetric reinforcement layouts because the entire interaction surface no longer falls into one of the coordinate quadrants.

      If you feel like taking a peek my attempt is here:


    • dougaj4 says:

      Don – for more background on how it works see:
      In brief:
      For any specified NA angle the section is split into trapezoidal layers parallel to the X axis.
      The depth of the NA is found using a closed form solution (but could be found by iteration).
      The moment about the X and Y axes is found for that NA location.
      That process is repeated with different angles of the NA to find the NA angle that has a resultant angle of the reaction forces in the same direction as the input moments, using Brent’s method.

      You will find more details in the link above, and links from that, or try searching the blog, or use the categories drop-down. But if anything isn’t clear, please ask.

      What was the problem with the reinforcement table? Each row lists the XY coordinates of the bars at the left and right of each row, then the number of bars and diameter. Also see examples on the Example sheet, that you can copy and paste to the Input sheet.


      • DonB says:

        Thank you for the reply!

        Found your paper on the derivations neat stuff, it is interesting seeing different approaches to the problem.

        The reinf input, whenever I change anything it seems it breaks something as all of the calculations on the sheet error to N/A(), may be a problem with the version of excel on my work machine will try on my home PC and see if I have better luck.


        • dougaj4 says:

          I recently changed to 64 bit Excel, so that may causing problems. I’ll try it on an older Office and let you know.


  4. Sergiu says:

    Great work, and the program runs so fast ! Can you recommend a book or an article where i can find formulas for creating my own routine for designing bending with biaxial ?


    • dougaj4 says:

      I can’t think of any text books that cover the process in detail. I have seen a few papers with different approaches, I’ll have a look.
      Also the VBA code in my spreadsheet is all open source, and my response to DonB in the comments above has more details of my procedure, and a link to an earlier post.


  5. Mohd Yousef says:

    i am trying to create a function to resolve the tension in anchor bolts and compression in concrete under axial load and biaxial bending, i imagine the procedure will be much similar to what is presented here, but the main difference that you don’t limit the concrete strain to 0.003 or maximum steel stress to steel yield strength, so the number of variables will increase to be four:
    – The N.A. depth
    – The N.A. Angel
    – The total tension in the bolts
    – The maximum stress in concrete

    i can’t find any reference to help me to start, i have done it already for uniaxial bending but with biaxial i am stuck!!

    any guidance or references will be appreciated.


      • dougaj4 says:

        DonB – I edited your link because it was automatically downloading the pdf and opening it without any prompt. Seems to work OK now.


      • Mohd Yousef says:

        Thanks for sharing, in deed it’s a good reference but i have a “silly” question”, in page 4 it’s mentioned that “The next step is to determine the equivalent forces and moments in the local coordinate system for the concrete compression and anchor rods tension.”

        What i understand that i have to rotate the section an assumed angle and solve the equations based on the rotated coordinates, however in page 3 it says that “The equivalent force, moment about local x axis and moment about local y axis for a line segment between node i and node i+1” so it got me confused, do i have to keep rotating the section and solve the equation for each face!!.

        unfortunately the numerical example skips the important part which is setting the geometry, i understand that this part specially can be done in many different way’s but still it could had helped allot to show an example.


        • DonB says:

          so the general formula for the strain plane is e = c1x+c2y+c3 with c1,c2,c3 being constants. A base assumption in their calculation is that the base plate and anchors are rotated so that the strain and therefore stress would only vary in the y direction this is done to greatly simplify the double integration needed to get the compression resultant force and associated moments.

          the calculation on page 3 utilizes line integrals, Green’s Theorem, to calculate the double integral of the stressed area to get the total force. They also reduced the integration further by doing it parametrically for an arbitrary line with two points and a linear varying stress.


  6. Mohd Yousef says:

    I think i will pass on this, i use a method developed by Horn for square base plates with a circular opening which takes into account individual bolts locations to calculate bolts tension and concrete stresses by taking sum of first moment of area for bolts and compression zone and 2nd moment of area for the same about the eccentricity line and the result of dividing the 2nd moment of area over 1st moment of area will be the distance from N.A. to eccentricity line.

    it works perfect for a uniaxial eccentricity and converge very fast since you iterate only one variable.
    for biaxial eccentricity it seems reasonable to me to rotate the section in an angle equal to ATAN(Mx/My) and calculate the new coordinates (including eccentricity point) by maintaining the N.A horizontal, then solve for also one variable.

    a further iteration can be done to iterate the angle of rotation but in most cases it will not be necessary since it’s impact on results is minor in compare to the depth of N.A. and it will not vary much from the first assumed angle.


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