Calculation of Code Combined Stress result

Hi there,

I'm trying to reconcile the stress results shown in AutoPIPE with what I would expect the code stresses to be based on the shown values.

The AutoPIPE code used set to ASME B31.4

Please see the screen shot below with reference to the highlighted line, the results of interest shown are:
- Hoop stress (Sh)= 180.41 N/mm2
- Max Long  (SL)= 317.05 N/mm2
- Combined (Se)= 362.84 N/mm2
- Shear stress (Ss)= 1.01 N/mm2

I'm currently doing a design to PD8010 so need to check for the equivalent Von Mises stress based on:

Se = (Sh^2 + SL^2 - Sh.SL + 3Ss^2)^0.5

Therefore using the numbers above - I calculate the Se to be 275.4 N/mm2, however as you can see the Combined stress reported by AutoPIPE (which I would have expected to be the VM stress as well) is much higher at 362.84 N/mm2.

Can you please help with clarifying how AutoPIPE calculates the Combined stress?



  • Please note that Von Mises stress is calculated for at least 3 locations, neutral axis, maximum bending and minimum or negative bending. Only the maximum of these is reported.

  • Hi Karim,

    Thanks for responding.

    Is it possible for me to use the reported (Hoop), (Shear) and (Max Long)  stress component values to manually calculate the Maximum Von Mises Stress that matches the value shown in the (Combined) column?

    The VM stress that I calculate using the component values are lower that the (Combined) value calculated by AutoPIPE - so are you saying that the reported stress component values are not the maximum values for ALL locations (i.e. NA, bending, -bending) so can't be used to manually determine the same (Combined) stress?



  • You should be able to calculate the Max Von Mises stress using three different values of SL.

    SL1 = Axial (Neutral axis)

    SL2 = Axial + Bending (Max Long if Axial is positive)

    SL3 = Axial - Bending

    Se = (Sh^2 + SL^2 - Sh.SL + 3Ss^2)^0.5

    Then get the maximum Von Mises stress from all three.

Reply Children
  • Hi Karim,

    The screencapture of the results grid in my message at the top only has one column for Max Long, so I'm unclear how to obtain the 3 SL values that you mention.  Could you please advise how I can obtain the 3 different values of SL?

    ie. What options do I need to set? Where in the stress output does it appear?



  • If you are doing a PD8010 code check you should be extracting the longitudinal stresses from the general stress output. This reports Lmax and Lmin, I am unaware of where the neutral axis axial stress is reported though and would be curious to find out. Also, for performing a PD81010 code check you require the hoop stress to be calculated using tmin (I generally do this with a hand calc outside of autopipe and then extract the longitudinal and shear from the general stress output).

  • Hi Shimmy,

    Thanks for the response and the advice on the PD8010 stress checks.  We will adopt this process.

    It is a little disconcerting however, as we had been using the ASME 31.4 output instead of the 'General' output and the maximum longitudinal stress values in the B31.4 are significant lower than the Lmax in the 'General' values e.g. 257MPa (B31.4) as opposed to 337MPa (General).

    Do you know why the values are so different in their reporting of the longitudinal stress? How is AutoPIPE calculating the longitudinal stress for ASME B31.4?



  • You need to use "Axial" and "Bending" columns to get the three SL values.

    As far as the diffrence between B31.4 offshore code stress and general stress:

    1. General stress uses corrosion plus mill tolerance for hoop stress. If you set mill tol=0, the hoop stress will match

    2. General stress uses rigorous shear stress (evaluate stress at every 15 degress along circumference). If you set "Direct Shear=R" in result options, the B31.4 offshore code stress will do the same.

    If you set mill=0 and Direct shear=R, the results will match.