By Andrew M. Jenkins, E.I.
| Saturday, October 01, 2016 | 1832 Views |
Dear Tech Abby,
What does 46 psi pipe stiffness really mean? Do all pipes meet or need 46 psi? Can you help?
- John Q. Civil
J.Q., you are not alone. Where do I start?
It all came about during the 1960’s when solid-wall PVC thermoplastic pipe was becoming more widely used for small diameter sanitary sewers (e.g. laterals and collectors). One of the standards created during this time was a quality control test method first published in 1965 by ASTM International and designated as ASTM D2412. The title of ASTM D2412 is “Standard Testing Method for Determination of External Loading Characteristics of Plastic Pipe by Parallel-Plate Loading.”
Now, one of the plastic pipe characteristics derived from this lab test was unfortunately called “pipe stiffness.” I say “unfortunately” because many engineers think this represents the pipe’s strength; it is very misleading. Basically, the test applies a load (pounds of force) to a specific sample length of pipe (units in inches) that has been placed between two parallel plates. The pipe is deflected between the two plates to a specific percentage from round and is measured (units in inches of deflection from round). Therefore, what you actually get is pounds per inch of pipe length per inch of deflection from round, or pii (lbs./in/in). Yes, “pii.” Not “psi” which has been mislabeled by engineers for years and years, so, don’t feel bad about your initial question, John. One problem is many engineers were never taught flexible pipe design in college and don’t understand how it actually works. I could write an entire blog on that topic alone. Many engineers don’t understand that per this ASTM test the pipe is loaded without backfill side support. This is the key issue, because it is the backfill side support in an actual installation that induces ring compression in the flexible pipe; and resistance to ring compression is what determines the pipe wall’s thrust capacity. This thrust capacity is used to determine ultimate loading or strength capabilities per AASHTO’s structural sections of the design methodology, but pipe stiffness is not. B.T.W…. AASHTO should be considered the go-to for structural design of flexible pipes.
There is no side support creating ring compression like there would be in an actual direct buried installation.
Now back to the 1960’s, when PVC pipe took over the small diameter sanitary sewer world, there were various PVC wall thicknesses used and they were standardized using the ratio of diameter to wall thickness. You know the ratio as Standard Dimension Ratio or “SDR”. Take a guess which ratio or SDR became the go-to wall thickness in most specifications. Does SDR 35 ring a bell? If you look at the vast majority of specifications across the country, you will see it everywhere. Now, take a guess as to what the minimum pipe stiffness is for solid wall SDR 35 PVC pipe. Bingo! You guessed correctly, 46 pii.
So… SDR 35 PVC pipe worked well enough and, when tested per a specific QC test, got a minimum stiffness value of 46 pii. Now many people believe 46 pii should correlate to other material types and pipe designs in the flexible pipe world, but they should not. 46 pii is not the bar for all pipe types and does not represent load capacity or ultimate strength. It does relate to a pipe’s effectiveness in resisting handling, installation and construction loads, but it is also known that larger diameter pipes require much lower “pipe stiffness” for proper installation than do smaller diameter pipes.
Somehow over the past 50 years the industry has forgotten, become confused or simply distorted the meaning and use of this QC test and the number 46 as a requirement for “stiffness.” Again, the problem is that the test does not accurately represent actual loading conditions of a flexible pipe since there is no backfill side support. The test has predetermined parameters such as the target deflection (5%), the rate of loading (0.5 inches per minute) and the atmospheric testing temperature (73 degrees Fahrenheit). Had the test originators picked different values for these test variables, something other than 46 pii would have become the standard. If you start making changes to the test’s parameters, the pipe stiffness can be drastically different for various material and pipe types. For example:
- Temperature: Some materials can have up to 50% reduction in stiffness at elevated atmospheric temperatures or after periods of solar absorption.
- Soil Interaction: The test is performed on two smooth plates. Soil interaction in an actual installation will increase the load carrying capacity of the pipe, and this increase will vary based on the pipe’s mechanical and physical section properties.
- Strain Rates: The test applies a load or strain at 0.5 inches per minute. Non-reinforced plastics and high visco-elastic materials would have significant reductions in stiffness at reduced rates.
- Pipe Configuration: Pipe types that are helically wrapped have increased pipe stiffness results when longer sample lengths are tested.
What is D2412 & pipe stiffness used for? The two are used to check quality and estimate deflection respectively. Pipe stiffness also relates to handling and installation characteristics of a flexible plastic pipe during the very early stages of soil consolidation, per ASTM D2412.
So, what should we use to determine load capacity or compare structural capacity? I would use AASHTO Section 12 design methodology and the stub compression test per AASHTO T341. ASTM D2412 and its resulting “pipe stiffness” is simply a QC check that does not fully incorporate long-term properties associated with the AASHTO methodology. Therefore, it is not considered representative of a pipe’s structural strength. Look for more on AASHTO design methodology in another blog.