Pipe stiffness is likely the most commonly referenced and least understood performance parameter in the pipe industry.

When most people think of pipes or try to describe pipes, they first describe it by the material it’s manufactured with (i.e. concrete, HDPE, steel, PVC, etc.).  They may also describe it by some physical attributes such as by it’s joint type (i.e. bell-spigot, welded, flanged) or whether it’s a solid wall (SDR) or profile wall.

However, none of those items would be considered properties that would describe the pipes performance.  Quantitative descriptions of performance properties are harder to identify.  If the pipe is designed for pressurized usage, then the pressure rating would qualify as one of those terms.  However, most pipes don’t have a pressure rating.

That’s likely why pipe stiffness has been latched onto by many engineers as a property that is intended to describe the performance of a pipe.  And pipe stiffness does describe the performance of a pipe – sort of. 

Pipe stiffness measures the load required to deflect an unsupported pipe a distance equal to 5% of its diameter at a specific temperature and at a prescribed rate of loading.  Period.  Pipes with high stiffnesses can resist deformations during the handling and installation of the pipes, but they have little impact on pipe performance once the pipe is installed.

Understanding the pipe stiffness of a pipe is good information to know, but because it’s one of the few quantitative performance measures available, the relevance of the measured stiffness is sometimes overstated.  For instance, some people believe that a pipe with a higher pipe stiffness can carry more load than another pipe with a lower pipe stiffness.  This is not true.  There are plenty of examples of pipes with lower measured pipe stiffnesses that can carry higher cover depths than similar pipes that have a higher pipe stiffness.  Corrugated steel pipes can support fill depths much deeper than plastic pipes, but their pipe stiffnesses are frequently similar to the plastic pipes.

There is also some thought that a pipe with a higher pipe stiffness will perform better over time as compared to a pipe with a lower measured pipe stiffness.  This is technically true, but the degree of difference in performance is so slight that it is essentially negligible in most cases.  Highlighting the limited impact that pipe stiffness has on long term performance of pipelines was the topic of an article I recently wrote for Informed Infrastructure.  In the article, I provide quantitative evidence of the limited impact that pipe stiffness has on long term pipe performance.

Pipe stiffness is a meaningful property for pipes.  A pipe needs to have enough stiffness to resist the transportation, handling and installation of the pipe without deforming excessively.  However, pipe stiffness has little impact when describing the strength of the pipe in carrying soil loads or on the long term performance of the pipe.