Pipeline Stresses; How to get 2d model forces into stress

[CEcoug]

Hi all,
I hope I can explain this well enough, here it goes.

I have modeled a pipeline using SAP2000 and have moment,shear, and axial forces along the frame element. I am trying to superimpose an internal pressure (a 3D model could do this, but I am trying to avoid that). At this point I have the hoop and longitudinal stress due to internal pressure and the longitudinal stress due to bending. How would I get the shear stresses? The steel pipeline is 66" dia with a 0.375" wall. What I am trying to do is break down all the forces into sigma 1,2,3 and τ 1,2,3 and then use Von Mises equations to get a total stress (worst case).

So how do I get the shear force (peak value of 272 kip) into τ 1,2,3?

(See attached ppt)

Thanks for any help

 

[IDS]

[URL unfurl="true"]http://www.eng-tips.com/viewthread.cfm?qid=90897[/url]

looks like a good place to start.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[Everynameistaken]

I think you should be able to use a mohr circle.

For a pipeline of this thickness and diameter I'm not sure you need vary the stress throughout the wall thickness? So then you would only need sigma 1 and 2 and tau 1 and 2. From this you can draw the mohrs circle and get the two principal stresses (since you have got to combine the different axial stresses and hoop stress and you have shear, I'm pretty sure your principal stresses are not in line with the pipe axis) to put into your von mises.

 

[BigInch]

Shear stress = VQ/I/t and adds algebraically to the wall stress in the same plane as hoop stress. It is a maximum at the neutral axis of the pipe. If hoop stress is 2000 psi and shear stress is +/- 150 psi, then wall stress at one end of the free body is 2000+150=2150 psi and 2000-150=1850 psi on the other end.

von Mises is not used for ASME B31.4 or B31.8 pipeline designs. Hopefully this is not a liquid hydrocarbon or natural gas pipeline in the USA, or you may be on a short course to violating a number of federal regulations.

 

[JStephen]

Seems like that would get involved.
If you're using a frame analysis, treating the pipe as a beam, then when you add in pressure, the length of the pipeline segments would try to change, which will affect the framing forces in the adjacent sections. IE, if it's all connected, you can't determine the axial load in Segment N without considering the effect of pressure in Segment M, etc.
I would think you'd get some rotation of ells and wyes due to pressure which is also not considered in a beam analysis.