Dear Friends,
I had a doubt as to why in Caesar II the component of Hoop stress is included in calculation of Octahedral stress. Assume that in a pipe subjected to Internal Pressure,P and subjected to following stresses:
1) Hoop stresses, Shoop = PDo/2t (Thin shell)
2) Axial stress due to P, Slong = P* ID^2/(OD^2-ID^2)
3) Axial stress dur to Fy , Saxial = Fy/ Across-sec
4) Bending stress = Mb / Z (Assume pipe is along Y then Mb = SQRT (Mx^2+Mz^2)
5) Torsional stress = Mt/J (Mt= My)
My query is related to the fact that when we provide nominal pipe wall for taking care of Hoop stress component why do we need to additionally include it in calculation of Octahedral stress as given in CII User guide. Isn't it better to use the following:
(Sqrt(2)/3) x SQRT((Axial stress due to P + Fy/A + Mb/Z)^2 + 3(Mt/J)^2)= Octahedral stress component for evaluation
Best Regards
Benoy

it says " 3D Octahedral stress" this means a third dimension is also accounted for the radial pressure stress. you get that information so you can make sure the "total stress" in any elements of your calculation will never exceed SQRT(2)/6 times the yield strength at the corresponding temperature.

Dear Friend,

Thanks for the reply but once we have provided sufficient nominal wall thickness to satisfy the hoop stress component due to Radial growth of pipe when subjected to Fluid internal pressure it is similar to de-coupling its effect from the total stress developed due to other components in the pipe system i.e. we have ensured Pressure intergrity.

Hence, I still feel the total stress or Octahedral stress component should be based on the stresses developed due to the other components i.e.

(Sqrt(2)/3) x SQRT((Axial stress due to P + Fy/A + Mb/Z)^2 + 3(Mt/J)^2)



Best Regards

Benoy Abraham

you tending to neglect one force/dimension does not defy the existence of it.
I do not know any design code which suggests accounting for the 3rd dimension of stresses. what CII offers you in the column "Max 3D shear/Octahedral" is just a bonus information for the user to be able to compare calculated values as per the code with "real stresses" applying to the elements.

Dear Friend,

Please note that I am not denying the existence of Hoop stress in Pipe but what I am trying to point out is the fact that even B31.1 code for that matter when it compares the stress due to sustained loads does not account for the same in the Code stress i.e.

ASME B31.1 reference Sec 104.8.1 Stress due to sustained loads is as follows: SL = PDo/4tn + 0.75iMA/Z < Sh

ASME B31.1 does not account for the effect of hoop stress in the evaluation of Code stress for Stress due to Sustained loads even though Hoop stress is developed due to P.

So my point is if the ASME Code is decoupling the effect of Hoop stress from the remaining components listed in my previous post then why do we need to include the Hoop stress component in calculation of Ocatahedral stress and not use the below for computation of Combined stress or Total stress.

(Sqrt(2)/3) x SQRT((Axial stress due to P + Fy/A + Mb/Z)^2 + 3(Mt/J)^2)

Best Regards

Benoy

Piping Codes are simplified.
In my opinion, once B31 sets wall thickness, safety regarding hoop stress is satisfied.
While an "exact" evaluation of Tresca stress (maximum shear stress) in the plane normal to radial must include hoop, longitudinal and shear stress, a simpler and more conservative calculation ignores the hoop term. That's why it is called longitudinal stress - the focus is on the (remaining) non-hoop terms: i.e., longitudinal and shear.
But don't go too far with your octahedral shearing stress calculations. This equivalent stress calculation may be the root of this stress evaluation but it is not used today. The Code defines the stress calculation and the allowable limit.
CAESAR II calculates many stresses. One is octahedral shearing stress and it includes hoop, longitudinal, radial and shear terms. The one called Code Stress is the one with direct Piping Code implications.