Buried steel pipe design: pressure influence??

[carletes]

Dear all,

I am designing a bruried steel pipe of 100" which can be subjected to a vacuum of 10 psi. I was going to use the Iowa-Spangler deflection formula in order to choose its thickness. But I have a doubt: The internal pressure of the pipe (in this case vacuum as I have told) shouldn't affect the deflection and, therefore, the thickness? I find it quite strange..

Perhaps it would be more appropiate to calculate it as a vessel subjected to an external pressure equivalent to the weight of backfill plus the internal vaccum of the pipe?

Any help or indication of any other calculation method will be welcomed.

Best regards,

 

[pmover]

please note this thread:

thread161-101299

perhaps further explanation of problem can be provided, for example, fluid, application, service, etc.

good luck!
-pmover

 

[stanier]

Carletes,

Deflection isnt the only criteria. The controlling criteria for this diameter of steel pipe will be buckling.

 

[carletes]

Thanks for the help. I have a couple of doubts about this matter, afeter reviewing more documentation:

Stanier says that "the controlling criteria for this diameter of steel pipe will be buckling". I suppose that it refers to the fact that this steel pipe must be considered as flexible.Is there any criteria (depending on thickness, diameter, elasticity modulus etc) to know if a steel pipe is flexible or rigid? I have to design other smaller buried pipes and I am not sure if they will be flexible enough...

And finally another question regarding AWWA: At first, I thought that the design of buried pipes would be similar to the design of vessels under external pressure, but I see that AWWA M11 says nothing about stifening rings, for example. Are not useful these stiffening rings for avoiding buckling of buried steel pipes? I do not understand it. Is there any reason not to use them in buried pipes?

Thanks a lot and best regards.

 

[pennpoint]

carletes

I am confused, what is your pipe dimensions? 100" does not say enough.
How deep are you going to bury the pipe?
Whats the service?
More data is needed

Regards
pennpoint

 

[carletes]

Well, the main pipe has an outside diameter of 100", thickness 0,5". It is buried between 6 and 18 feet. It is the main cooling water of a power generation plant. Ther are vehicular loads as well (75000 lb)

It has got other smaller branches. One of them is, for instance a 10" Sch. 40 pipe.

As you see they are very different, so my doubt is if the calculation method must be different (the bog one is likely to be flexible but the other one may be rigid, I do not know), and i do not know if using stiffening rings is useful according to AWWA M-11.

Thanks you all.

 

[carletes]

Re-reading my previous post, there is another point I'd like to add.

I see that Spangler equation includes a factor of soil resistance (0,061E'R^3). As far as I know E' dependes on the soil properties and the pipe radius, so this factor does not depend in any way on the "deformability" of the pipe.Is it reasonable? The reaction forces of the soil do not depend on pipe flexibility?

This matter is related to my previous question: when a steel pipe can be considered flexible and when it must be calculated as rigid, so, Spangler equation would not be applicabble?

Thanks and best regards,

 

[stanier]

E' is the effective combined soil modulus which is zeta times the embedment modulus. The property zeta does not only depend upon soil modulus and radius. It also contains a design factor, ratio of embedment modulus to native soil modulus and ratio of trench bredth to depth.

Suggest you purchase AS 2566.1 & AS 2566.2

 

[Cockroach]

I have used the Von Mises-Hencky Equation in the past, and as you stated, regard the pipe as a pressure vessel of infinite length (i.e. no end cap reactions means longitudinal stress is zero). You can then redo the above model for biaxial stress and pressure loading equal to the vacumn pressure PLUS external pressure experienced by the soil loaded over the bearing area of the pipe itself.

In practice, my results were very equal to the civil boys who use much more complicated analysis. I just consider the soil to act as an elastic foundation, that way you can use beam analysis to see exactly where the greatest moment occurs, hence the point of fracture.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada