Hertz Contact Stress in Pipe Support
Hertz Contact Stress in Pipe Support
(OP)
I have an FEA model of a generic pipe support problem I face and I am looking to see if anyone has any insight as to how to possibly run calcs by hand? Using Roark's for stress on a hollow cylinder as a line will fail the pipe due to local stress but an FEA model shows a lower stress as the distribution is not a line.





RE: Hertz Contact Stress in Pipe Support
What you need to consider is whether the local combined stress conditions existing at the pipe/support contact surface post yielding actually presents a fatigue or stress problem.
RE: Hertz Contact Stress in Pipe Support
RE: Hertz Contact Stress in Pipe Support
RE: Hertz Contact Stress in Pipe Support
RE: Hertz Contact Stress in Pipe Support
RE: Hertz Contact Stress in Pipe Support
RE: Hertz Contact Stress in Pipe Support
The only other variable that would have a significant effect on your analysis results would be the boundary conditions used. The structural stiffness characteristics of the adjacent pipes and supports could have a significant effect on the contact bearing stresses between the pipe surface and the support bracket.
RE: Hertz Contact Stress in Pipe Support
I'm starting to believe that the supporting structure is far more critical than initially believed. It is rigidly supported currently. I may model the HSS underneath it instead of the plate to see what we get in terms of a reduction in stress.
RE: Hertz Contact Stress in Pipe Support
RE: Hertz Contact Stress in Pipe Support
140kips sound like one hell of a pipe reaction, on a flat plate support. What’s in this pipe and what is the spacing btwn. supports? I don’t normal do much pipe analysis, but I think you might be going about this the wrong way. What’s causing that kind of gravity reaction? Are there other forces on the pipe, in play here? Show us some sketches with loads, forces, and dimensions. They’ve been supporting pipes on beams, on pipe racks, for years and haven’t had this kind of problem, what are they doing right that you are doing wrong. I haven’t even bothered to look at Roark’s formulas tonight, but I suggest you read some of the verbiage around the formula that you have applied, so you have a vague idea of what the Hertz stress is all about. The upshot is that you have to do something to alleviate that line load on the pipe. What do the pipe dia. on a flat plate have to do with the problem? Why would you use a saddle of some sort, why would you put a thick rubber pad, or some such, btwn. the pipe and the saddle?
RE: Hertz Contact Stress in Pipe Support
RE: Hertz Contact Stress in Pipe Support
http://www.pipingtech.com/blog/2007/06/13/42-large...
RE: Hertz Contact Stress in Pipe Support
"Theoretically" you do have a flat plate supporting a "flat" surface (the HSS upper wall under a flat plate) supporting a round pipe, so without local deformation of the "round" pipe, you would see very high local stresses. Which is what that local deformation is "responding to" as the pipe wall yields slightly.
I agree with the several observers above who recommend a off-the-shelf simple commercial pipe saddle to spread the load out both radially (rigidly wrapped around the pipe) and axially (the saddle will have slightly longer support surface down the length of the pipe). otherwise, your only contact surface is the width of the HSS wall and the deformed "spread" of the pipe wall against the (unreinforced, itself-subject-to-bending) thin flat plate: That would be a final area of 1/8 inch, maybe 3/16 inch of contact surface? To paraphrase what one writer here uses in his signature, "When everybody does it that a way, there is usually a reason why everybody does it that a way ...".
RE: Hertz Contact Stress in Pipe Support
RE: Hertz Contact Stress in Pipe Support
We carry out FEA on the pipe shoes and even they have high stress in the pipe. There is no way we could support the pipe with out a shoe.
After a few thermal cycles a directly supported pipe will wear and lose wall thickness.
Kevin
RE: Hertz Contact Stress in Pipe Support
RE: Hertz Contact Stress in Pipe Support
The Hertz stresses are real, they don’t go away. As the pipe or the HSS wear, the bearing line becomes wider and the bearing stress goes down. The FEA program will show these high stresses too. I think what Kevin might be talking about as a shoe, in its crudest form might be; take a 16" length of the same 20" pipe, make two longitudinal cuts on the pipe so you end up with a 90̊ arc of pipe. Clean this up, shape the corners with nice radii, press this to the pipe and weld it to the pipe as a wear pl. Weld the other wear pl. to the HSS, and let em wear.
RE: Hertz Contact Stress in Pipe Support
Have you ever noticed how pipe support Companies are eager to sell their pipe shoes/supports and quote allowable loads for these items. However it is up to the piping engineer to check what the pipe supports do to the pipe and what "local" stresses are developed in the pipe wall. There are many piping engineers that do not consider this. They select the pipe support from the catalogue - check the actual loading is below the allowable in the catalogue - and hey presto specify that support. No checks of how the loading is transferred between the support and the pipe wall - no check of the local stresses in the pipe wall and combine these with other stresses developed in the pipe. End of gripe!!!
On the present subject. There is a local stress due to the contact between the two items and generally the softer of the two items yields slightly until the surface area is sufficient to sustain the contact stresses. So there is some indentation of the weaker material. This indentation is normally checked by assuming a "reasonable" bearing length (which is small). It should be remembered that this indentation can affect the "free" movement of the pipe in certain cases. Using contact stress formulae will result in unrealistic stress levels due to the actual "local" yielding.