Buried steel pipe design criteria 1

[steve1]

The American Lifelines Alliance published a design guideline for buried steel pipes in July 2001.

http://www.americanlifelinesalliance.org/pdf/buried_pipe.pdf

In their suggested acceptance criteria, (Appendix A), they provide an allowable stress for through wall bending but indicate that the allowable deformation criteria is Not Applicable.

I was reviewing a casing calculation (under RR track) where the engineer based his design on a vertical deflection limit of 3%, per API 1102.

The through wall bending, calculated per the reference above clearly shows the pipe is overstressed.

I'm unsure what to do here. One reference says no good, the other says ok.

Any suggestions?

 

[BarryEng]

Some references are: -
* AWWA M11 (use the 2004 edition - it corrects a lot of items in the last edition)
* Steel Penstocks - ASCE Manuals & reports on engineering practice No. 79
* Tyco Water Design Manual (Australian company)
* Australian Standard Buried flexible pipelines
(AS 2566) Part 1: Structural Design
Essentially, deflections are based on pipe materials & details of joint configuration. Tyco recommendation is 3% of the pipe diamter for a mild steel pipe, cement mortar lined & welded joints. Rubber ring steel pipe, 2% of the pipe diameter. Unlined steel pipe with welded joints, deflection 5%.
AS 2566 gives deflection limits of 7.5% for ABS, 6% for GRP, 7.5% for PVC & PE & 2% for DI. You need to contact pipe manufacturers for their criteria.
AS 2566 gives design methods for most pipe materials in most types of soils, with most types of loadings (including buckling loads). Design includes deflection, strength, strain, internal & external pressure, combined loadings & buckling.
AS 2566 is based on the well documented Marston & Spangler theories with updates of the latest info on Europe etc.
Barry Eng

 

[purdue86]

Since this pipe is under railroad tracks, it should be designed in accordance with the American Railway Engineering and Maintenance of Way Association Manual for Railway Engineering. Part 5 of Chapter 1 covers pipelines. You should also contact the railroad that you are planning to pass under because some railroads have additional guidelines.

Howard Swanson

 

[steve1]

Thanks for your replies. I do have the RR specs for the crossing. My question is really about the appropriateness of using a stress based criteria as opposed to the traditional deflection based criteria for buried steel piping. As I stated the pipe meets the RR spec for deflection but fails to meet the stress based criteria presented in the guidelines I noted above. Are those guidelines really a requirement? Should the wall thickness of buried pipe be based on through wall bending or vertical deflection? I will also post to Mechanical forum.

 

[BarryEng]

When you design a beam, you check the strength (strength limit state) & also the deflection (servicability limit state).

The same applies with a pipe (whether it is a pressure pipe or a 'carrier pipe').

Check strength for bending stresses, hoop stresses (none in a carrier pipe), membrane stresses such as longitudinal stresses, & buckling. Then check deflection for the servicability limit state of pipe joints, lining & coating limitations.

A steel pipe with a flexible lining can sustain more deflection than a steel pipe with a cement mortar lining (limit state of local damage).

Steel pipes manufactured in Australia for water, are usually lined with cement mortar & a crack of one or two mm will re-seal (autogenous healing) during operation. The same pipe used as a carrier pipe may have a reduced allowable crack due to the lack of autogenous healing.

Deflection criteria are usually more demanding under a railway because of the vertical deformation limitations (settlement of the rail sleepers etc). A carrier pipe under a road may not have the same deflection limits as a railway.

 

[steve1]

According to this article I found in the ASCE data base

http://www.pubs.asce.org/

http://WWWdisplay.cgi?0411969

the modified Iowa equation is being misused as a design equation to determine the thickness of buried steel piping. I have not read the article, just the abstract. Has anyone read this article, and if so could they comment on it? The modified Iowa equation is used as the basis for through wall bending stress that is the subject of my original post.

 

[BarryEng]

Steve1

I read the abstract that you provided, but I have not read the article yet. From the abstract, it appears to me that this article is a long awaited explanation of the factors used in pipe design.

One of the first jobs that I give to an engineer, new in pipe design, is the Iowa formular values of the separate contribution of the pipe stiffness & soil stiffness, to the final pipe deflection. In this way the importance of the pipe/soil interaction is indicated.

Designing soil supported structures is not a simple as steel or concrete, because we are not dealing with a homogeneous material.

In the Australian Standard 2566 (buried flexible pipelines), the design of buried pipes is based on deflection, strength from external loadings (ring bending strain), strength from internal pressure, combined loading & buckling. Deflection is only one criteria.

 

[steve1]

BarryEng,

Thanks for your thoughtful replys. I have given you a star!

As I stated earlier I am reviewing someone else's work. I have apparently caused quite the uproar over my comments that we need to specify both a thicker and higher strength carrier pipe. We have an upcoming project meeting where I'll have to justify my comments. The consensus, in my office, seems to be that the American Lifelines Alliance document is not binding on this project and since the casing pipe meets the project specifications, as written, than the oiginal design is satisfatory.

Again, thanks for your insights into this not so cut and dry problem.