Pipe Stiffness (PS) value for PVC pipes 3

[yahoo123]

AWWA M23 shows how to calculate the Pipe Stiffness for buried pvc pipes. However, it doesn't say what to do with this value. Does anybody know where this PS value is used?

 

[stanier]

Pipe stiffness is used in the engineering design of a buried pipeline.

In Australia we use AS 2566.1 to determine the trench/pipe structural design for buckling, deflection, combined loading, strain and pressure.

Refer to books by Moser or Watkins for the design approaches if your country does not have standards to cover this aspect of design. AWWA has other standards for the design of buried pipelines. M23 I assume is a material standard. Much the same as AWWA has C950 & M45 for FRP.

UniBell has a design manual for PVC that covers this topic.

 

[yahoo123]

Unfortunately, the unibell design manual only shows how to calculate the ring deflection but does not provide guidance on selecting a ring deflection limit.

 

[stanier]

The ring deflection limit is set by the standards. Suggest you search the internet sites such as Techstreet.com and purchase the appropriate standard. Your local authority will dictate what standard to use if it is for water or sewage service.

 

[rconner]

Ah, you have asked what may be (say in old American game show parlance) “a $64,000 question”. In perhaps about enough over-simplification of the “Iowa” (named after/where much early research near Iowa State University, my alma mater a few years later, took place) ring deflection formula for buried flexible pipes to evoke argument, I believe at least the dependable pipe stiffness value was originally intended to be used as follows:

Ring Deflection = (a Factoring of Buried Load)/[(Pipe Stiffness) + (a Factoring of Sidefill Soil Stiffness)], and of course all to be in compatible units

Now, moving beyond that and looking in some more detail at only the one term you are curious about from the denominator, “pipe stiffness” has traditionally been determined either from ring loading tests (such as “three-edge bearing” or “parallel plate loading” tests) or from strength of materials ring (essentially “curved beam”) formulae relationships involving the modulus of elasticity (E) of the piping material. If I might misquote from Hamlet’s soliloquy, I believe “therein lies the rub”, at least with regard to low stiffness, viscoelastic piping materials.

Specifically, experimental pipe stiffness or P.S. is basically determined per ASTM D2412 standard test as the recorded laboratory load e.g. in pounds, distributed e.g. per axial inch on the pipe ring divided by the measured vertical ring deflection e.g. in inches at that load. Now in this regard, when ASTM D2412 external plastic pipe loading standard was founded, the developers of that standard required the pipe rings to be tested at a quite rapid rate of (e.g. of vertical) load application between parallel plates. The prescribed rate of laboratory (normally vertical) load application has been ½ inch diameter (12.5 mm) per second. I do not know why the founders of the ASTM standard chose such a rapid rate (perhaps more learned folks and maybe even in the industry will explain); however, I believe quick loading rates of course minimize testing times and cost and also perhaps more importantly maximize the determined stiffness value, even when some types of plastic pipes are formulated with (less expensive than resin derived e.g. from crude oil) fillers.

While significantly slower rates of load application would result in significantly lower values of pipe stiffness, I think many in the plastic pipe industry in general (and/or maybe even a well-known third-party researcher or two they have employed) has traditionally or historically basically advocated that this ASTM quite rapid-loading (or arguably short-term) stiffness or modulus is what should be plugged into the aforementioned buried formula/relationship, even though earth load in reality is of course MUCH more inexorably applied, in some cases for a buried design life of 50 or more years (or at least until the pipe fails)!

If you are curious as to the magnitude of difference (that may be surprising to many designers) between some typical short-term and longer-term pipe stiffness or modulus values for plastics, I think you could find some representative values now with at least some in-depth searches (as I don’t think this has traditionally been heavily advertised by the plastic pipe industry). E.g. see Section H 211.41 DESIGN OF PLASTIC PIPE in the manual of a very large USA utility at

http://eng.lacity.org/techdocs/str-man/h-200.pdf

and Table 2.3 page 9 at

http://irc.nrc-cnrc.gc.ca/pubs/fulltext/nrcc42868/nrcc42868.pdf

etc.

With regard to my opening statement, I believe there have been a great many problems involving flexible plastic pipes and lofty expectations of pipe/soil strength and/or available construction/inspection quality to achieve same, and many with far more than “$64,000” at stake, and it appears that this may be a factor in the involvement of a former president of the American Society of Civil Engineers in some pipe matters as are reflected in a presentation not long ago at

http://t2.ce.ufl.edu/facers/2008/Role_Risk_Responsibility.ppt#322,52,Conclusions-in.

I believe as is mentioned in that presentation, "profile-walled" pipes with invevitably some even higher strained local areas than solid-walled pipes can even further complicate the picture.

 

[yahoo123]

thanks for the very detailed reply rconner!

 

[stanier]

I must agree with rconner in that the properties of thermoplastic materials are strain rate dependent.

However considerable research in Europe and SE Asia has shown the long term viability of thermoplastic materials in well designed trench conditions.

PVC-U pipes that have been buried for 50 years have been exhumed and have been shown to have the same properties as when installed. They did not fail.

Design standards such as AS 2566.1 are inherently conservative.

In the design methods of a flexible pipeline the soil in the trench and the native soil takes most of the load from external application. Soil load does not have a significant effect as the vertical deflection transmits the load into the horizontal.

The design has to be carried out to ensure that the pipe does not buckle.

One concern is that when a large diameter thin wall pipe is installed the pipe is made out of round. This occurs when the soil at the side of the pipe is compacted too much. This creates an oval section. When the soil on top of the pipe is compacted the the top of the pipe is squared with tighter radii in the upper corners. These may then exceed the strain criteria for the material.

Suggest you read Watkins and Moser. Also get a suite of applicable standards. Australian standards AS2566.1 & .2 and also the Commentary are good guide.

This website allows you to download a tool for design of pipe /trench for PVC & PE.

http://www.iplex.com.au