The 5th edition of AWWA M11 has been available for over a year and the benefits to engineers have been recognized for the additional value it has created for steel pipe design. There were numerous changes and additions that are intended to clarify important criteria and procedures when designing steel water pipe.
The nature of engineering design is that the knowledge used to design large facilities is learned and remembered mostly during the work on a project. This series of articles will highlight many of the major changes in the new AWWA M11.
One of STI/SPFA’s goals with our experts’ design tips is to point out important changes in the new edition of AWWA M11 and to provide background of why such revisions were made. Steel pipe systems have been designed, fabricated, and installed with decades of experience, achieving reliable and long-lasting operational service, while establishing proven safety factors that the engineering community has trusted. This article will discuss changes made to
- harnessed coupling design and the associated explanatory material included within Appendixes B and C,
- changes in design to thrust anchor rings, and
- expansion of the guidance provided with the use of stulling to handle large diameter steel pipe.
Harness Design for Gasketed Joints and Non-Self-Restrained Flexible Couplings
Joints can be subject to the forces of thrust due to internal pressure. Thrust forces can result due to change of direction, valves, or dead ends. If non-self-restrained joints exist in the length of pipe affected by the forces of the resultant thrust, then joints can pull apart. Mechanical couplings are the most prominent example of this condition on steel pipe.
Bolted harnesses have been included for many years in AWWA M11 starting with the 1st edition in 1964. The first harnesses were simple U-shaped lugs (See Figure 1) with bolts designed to resist the thrust. However, these designs resulted in dimpling of thinner walled pipe in certain cases. Therefore, when M11 was revised in 1985, in the 2nd edition of M11, front and back rings were added to harness assemblies for larger diameters to better distribute the loads. (See Figure 2)
In the 3rd edition, a new requirement for minimum wall thickness under the harness assembly was added. This was based on a finite element analysis performed to evaluate the stresses in the pipe. In the newest edition, the requirement for a minimum wall thickness was reevaluated when it was discover that the original FEA program utilized back in the late 1980s did not consider localized yielding of the steel interface, which changed the overall evaluation. It was found that the original evaluation completed for the 3rd edition over-estimated the stress at the interface of the harness assembly and the pipe. The new 5th edition has restated the requirement of the need for a minimum wall thickness under the harness assembly based on these findings. The new table was also updated to include nominal diameters of 102” to 144” and pressures of 275 psi and 300 psi.
Since the new evaluation was done, it was decided to include an example to demonstrate the design procedure in the manual. This allows the designer to custom design a harness assembly rather than simply using the values shown in the tables, which might not be applicable to the project. It also allowed the user to change the diameter of the pipe for their specific condition, facilitating the use of cement-mortar lined pipe, such as when there is a need for a minimum internal diameter for water flow after application of the liner. The design example is presented in Appendix B.
Appendix C is an addition to the harness assembly subject. It provides some historic information and guidelines for design and harness rod placement when using multiple mechanical couplings to accommodate differential settlement. See Figure 3.
Pipe Bracing and Stulling Configurations
Handling has always been an important consideration with flexible pipe. This is particularly true when handling cement-mortar lined pipe. In this edition of M11, a table and a figure were added in Chapter 12 to provide best practices for stulling steel pipe. See Figures 4 & 5 below.
Anchor rings are steel rings attached to the steel pipe and imbedded in concrete. (See Figure 6) Their purpose is to resist thrust and to transfer the thrust to the concrete wall of a structure or to a concrete anchor block. In previous editions of AWWA M11, anchor rings were simply a table of ring sizes for pipe up to 36” diameter, and for two specific pressure applications. It was based on a wall strength using less than 1000 psi compressive strength concrete. This data was helpful, but it was limiting in the scope of its use. The current manual offers a design procedure that allows input for diameter, design pressures of the pipe, and design strength for the concrete. The procedure now allows for a wide variety of designs and conditions along with including new, expanded tables of anchor ring designs.
The next article in the series will discuss flange joint design.
STI/SPFA is always looking for ways to further the dissemination of information to the engineering communities and to owners. If there are any subjects that you would like to have covered by one of our articles, please contact STI/SPFA at (847) 550-3829. The association is always looking for ideas and feedback on matters of interest to the steel water pipe industry.