Iron Pipe Joint Development down through the Years - Part 3 of 5

This is an edited version of a presentation and paper at the 2017 American Society of Civil Engineers' Pipelines conference. This is the third in a series of five brief articles and on the sixth week, it will publish in its entirety with a complete listing of references. Today, we begin the topic of thrust and how joints were developed to resist it. Today's restrained joint has come a long way.

An Overview of Pipe Thrust and Thrust Restraint

Since these articles deal with joints in general and their innovative development, we will not get into details concerning the various methods of determining restrained joint lengths, but will briefly address the factors involved.

Thrust to be resisted is the product of pressure and cross-sectional area. Variations in the resultant thrust are determined by the degree of the bend within the pipeline. That resultant thrust is countered by the friction between the pipe and its surrounding environment. Factors contributing to the frictional resistance include primarily the coefficient of friction between the soil and the pipe and the depth of soil overburden above the pipe. 

Not to be too elementary, but imagine the force required to pull a rope. The deeper that rope is buried, the longer the rope is, the rougher the rope's outside surface, and the fatter the rope is, the more force will be required to pull it through the soil. The objective of thrust restraint systems is to connect together with sufficiently strong joints a long enough length of pipe to engage with the surrounding soil so that thrust from internal water pressure and surges can be more than counter-balanced. In other words, make a rope long enough to not be pulled through the soil. One well-accepted method of determining that length is offered by the Ductile Iron Pipe Research Association, and the program is available on-line in an easy-to-use computer model located here. (DIPRA, 2016)

Early Iron Pipe Restrained Joint Development and Performance

A common method of stabilizing unbalanced forces within a pipeline is the use of thrust blocks to transfer the unbalanced forces to stable soil. When the size required becomes too large or the construction site is not otherwise conducive to thrust blocks, restrained joints are called for. (Wilber and Stroud, 2003)

The earliest restraint systems used threaded rods connecting each joint as depicted in Figure 8. (York, 1949)

Figure 8. Tie rods connecting pipe and fitting joints. (York, 1949).

These tie rods were expensive, labor intensive, and subject to failure over time. The necessity of an alternative resulted in the development of joints that were self-restraining. Flanged joints met that description, but their rigidity subjected them to bending moment forces often resulting in leaks and other problems. 

Next week, we will see how the industry developed increasingly innovative restrained joints to meet the demanding requirements of efficiency and effectiveness. To let others know of this, Share and Like today's article, and feel free to ask any questions.