Dear all,
I am trying to carry out a comparison between hand calculation and Caesar computed results. I have certain doubts to clear.
I have considered : Simply supported beam with no external force, no insulation. Code : B31.3, Pipe = 4” Wall Thk = 6.0198, Temp Design = 65 deg & Press. Design = 14.36 bar g, Length = 1000mm(Node 10-15 = 500mm , 15-25 500mm), Fluid density = 670kg/m3

A)LOADCASE 4 (SUS) W

Highest Stresses: ( KPa )
Ratio (%): 0.5 @Node 15
Code Stress: 641.6 Allowable Stress: 137895.1
Axial Stress: 0.0 @Node 15
Bending Stress: 641.6 @Node 15
Torsion Stress: 0.0 @Node 15
Hoop Stress: 0.0 @Node 15
Max Stress Intensity: 641.6 @Node 15

B)LOADCASE 7 (SUS) P1

Highest Stresses: ( KPa )
Ratio (%): 5.8 @Node 15
Code Stress: 8020.3 Allowable Stress: 137895.1
Axial Stress: 8020.3 @Node 15
Bending Stress: 0.0 @Node 15
Torsion Stress: 0.0 @Node 15
Hoop Stress: 16729.5 @Node 15
Max Stress Intensity: 18914.1 @Node 15

C)LOADCASE 8 (SUS) W+P1

Highest Stresses: ( KPa )
Ratio (%): 6.3 @Node 15
Code Stress: 8662.0 Allowable Stress: 137895.1
Axial Stress: 8020.3 @Node 15
Bending Stress: 641.6 @Node 15
Torsion Stress: 0.0 @Node 15
Hoop Stress: 16729.5 @Node 15
Max Stress Intensity: 18914.1 @Node 15


1) The axial stress mentioned in all load case above is the stress due to longitudinal pressure force alone. By the Caesar default equation, the results found to match. As per B31.3, axial force due to sustained structural loads (F/A) has to be also added to compute the code stress. But none of the case above, F/A term has been considered. From Configuration >>SIF& Stress>> changing “Add F/A in stress” to ‘default/yes/no’ make no difference in the axial stress above. Please clarify !!!!

2) Also what is the purpose of “Use PD/4t” in the configuration settings ? Does it mean that Caesar will default to this eq. for calculation of axial stress when turned ‘ON’ ? If yes, turning it ON doesn’t change the computed axial stress above…


3) As per code para 304.1.2 - eq. (3a), D = Outside diameter of the pipe. Why does Caesar default to ‘hoop stress calculation based on ID’ for B31.3?

http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Number=58570



But here in all the above load cases, for calculation of Hoop stress, ID & nominal thickness less allowances are used (cross checked using manual calculation)



Please clarify my doubts.
Many thanks in advance

1) I don't think you have any axial force in your analysis - you are looking at weight alone (giving a shear load and bending moment at 15). That's why your axial stress = 0 and bending stress = Code stress.
2) Setting PD/4t to TRUE will use that (more conservative) formula. CAESAR II defaults to the more accurate P(A-inside)/(A-cross section). This would change results in Load Cases 2 & 3 but not 1.
3) You are pointing to a thickness for pressure equation in B31.3. While having the same approach as a hoop stress calculation, they are not the same. There are many ways to calculate hoop stress; we offer 4 variations in CAESAR II (ID, OD, mean & Lame). Pick one.

Yes I have not considered any axial force on the system.

As per para S302.6.1,
SL= [(Sa+Sb)^2+4St^2]^1/2

Force used in Sa = Sustained axial "mechanical" force (F/A)+ longitudinal pressure force
As per Caesar manual, Sustained axial "mechanical" force = structural loads in the piping system = F/A

1)So, doesn't the "Sustained axial mechanical force” or “structural loads in the piping system” means axial force due to effect of - pipe weight+ insul. weight + fluid weight ?
If 'NO', then as you said, I applied an axial force F to my system @ node 15 (my system is in Z direction). Load case under comparison = (SUS)W+P1 and (SUS)W+P1+F1
Axial stress increased from 8020 (SUS)W+P1 to 9276 Kpa in (SUS)W+P1+F1


a)Now, setting ‘add F/A in stress’ to NO – makes no difference in axial stress of (SUS)W+P1 or (SUS)W+P1+F1


b)I applied an axial force F to my system – But practically, how can a system have such an external axial force in SUS condition other than longitudinal pressure force ?






2) PD/4t result should be 8362 kpa.... As you said, Setting PD/4t to TRUE - makes no change in axial stress of load case B&C(still 8020 kpa)


3) OK about diameters. But does caesar uses nominal thk or nominal thk less allowances to compute hoop stress ? The quote in first post says 'nominal thk'.. But the calculations show the use of nominal thk less allowance


please correct me if I’m wrong ..

1a) That F/A switch only works when the piping code in use does NOT have a complete equation for the stress. B31.3 used to be in this position but now B31.3 has an equation for sustained stress (and now included axial load in the expansion stress range equation).
1b) Rotate your cantilever into a vertical orientation and you will now have a deadweight force in the axial direction. This force will be included in your calculated stress. Do not confuse moments and forces.
2) I do not believe CAESAR II will allow you to use PD/4t in place of an explicit (sustained) stress formula found in a piping code. B31.3 now has a formula for sustained stress and it referenced F-axial/Area where F-axial included effects of pressure; no PD/4t there.
3) You are referencing a post that was questioning the use of "allowances" in calculating load and stiffness as opposed to use of "allowances" in calculating stress. Obviously, according to your research, such allowances are included hoop stress calculation displayed by CAESAR II.

Thanking you Mr Dave

So far, its pretty clear for me