What is the Purpose of a Pipe Stiffness Test?
Why should you do a pipe stiffness test, regardless according to which standard, constant speed (ISO9969) or constant load (DIN16961)? As a part of a paper related to stiffness testing this little article will answer the upper mentioned question.
The pipe stiffness or a stiffness class will not give any confirmation for a pipe that it is suitable for a specific pipe project.
The only proof that a pipe is suitable for a specific project is a structural calculation (or in some cases experience).
The stiffness test will only prove that the produced pipes are correct according to given specifications. To be specific, the proposed specific bending stiffness is proven as the product of the short-term Modulus of Elasticity E for the used material and the specific area moment of inertia I with dimension mm4/mm. It is an integrated part of the production quality!
To explain it on a short example:
A given project (gravity pipeline for drainage, DN/ID1600mm installed under a hard shoulder of a highway, with certain installation conditions, short term ground water, traffic load, etc.) required a structural calculation regarding the American AWWA M55 standard. The idea of this calculation is to be sure that the proposed pipes-material combination will take all load of the project. A structural calculation will always check and proof multiple load-cases and not only deflection/stiffness.
The result of the calculation (considering all site conditions and installation conditions) is a profile type in combination with a raw material, in our example it should be Profile No. PR75-011.95 (profile distance 140mm, waterway wall thickness 6mm and profile height 83mm, etc.) with a theoretical area moment of inertia of 11.95mm4/mm and a raw material PE100 with a short-term E-Modulus of 1.100N/mm2 – for a pipe diameter of DN/ID1600. So, the manufacturing company knows exactly what to produce.
The product EI is called specific bending stiffness of the profile, where the area moment of inertia is normalized with respect to the profile width.
The corresponding stiffness (according to ISO9969) is given by the specific bending stiffness divided by the third power to the mean diameter of the pipe. So that the testing machine can do a proper quality control test.
The theoretical pipe stiffness for this combination of profile, material and diameter should be approximated.
SN 2.8 (rounded)
Remark: A SN2 lass pipe would be too weak, whereas a SN4 class pipe would be over-designed.
Now, this theoretical approach should be “re-checked” by a physical stiffness test according to ISO9969. Following the test procedure of ISO9969 three pipe samples should be checked by the constant speed method, where the average value should be taken as result. Note that the pipe need to be exposed to a proper temperature within the test as well as before the test. Moreover, a correct storing of the pipe is of most importance to avoid ovality.
The actual testing result (SN-value), according ISO9969, will be compared with the theoretical value.
If the measured SN-value is greater or equal to the theoretical value – the real pipe fulfilled the theoretical approach (in this specific case it was 3.1 and all was more than ok).
If not, problems could be:
- usage of unsuitable material
- improper temperature condi- tions within the last 24 hours
- deviating dimensions of the pipe
- improper storage of the pipe
- incorrect cutting of the pipe sample
- incorrect placing and testing of the pipe
The problem needs to be analysed using another raw material test as well as a detailed measurement and check of the profile geometry.
Checking the production settings to exclude e.g., an overheating of the material.
Depending on above mentioned issues, for spiral wound pipes like Krah-Pipes especially the cutting and positioning of the pipe sample allows a variation of -5% which can appear and is not critical. Note that due to the profile structure of standard double wall corrugated pipes which are not spiral produced, these tolerances do not appear.
Other solutions and problem finding methods are available. One is sending a thin cut of the profile to Krah Germany making a real analysis of the actual profile to certify the actual area moment of inertia.
Conclusion: The basis of the decision which pipe can fulfil the requirements on a real project, is never a stiffness test, or even a stiffness class, it is always a structural calculation (e.G. AWWA M55 or ATV A127). A stiffness test only confirms that the theoretical values of material properties in terms of E and the profile geometry in terms I are reached. A stiffness test is only a quality control test for the production, additional tests according to the standards should be checked, too. In our example the theoretical approach was approved by the physical deflection/stiffness – test.
Alexander Krah,
Krah GmbH