The 5th Edition of AWWA M11 has been out since 2017 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 expert’s 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. Steel pipe manufactured to AWWA standards are usually considered an engineered product and require a certain amount of engineering input to be functional for a water facility. This article will review Good Engineering Practice and how some design criteria has evolved from Good Engineering Practice.
Good Engineering Practice
Wikipedia defines Good Engineering Practice as:
"Good engineering practice" or "GEP" is engineering and technical activities that ensure that a company manufactures products of the required quality as expected (e.g., by the relevant regulatory authorities). Good engineering practices are to ensure that the development and/or manufacturing effort consistently generate deliverables that support the requirements for qualification or validation. Good engineering practices are applied to all industries that require engineering.
How does that apply to performance specifications for water projects? Most people would consider engineering to be based on data according to statistical rules such as yield strength of steel or well-established formula using well-known material properties. These properties or formula have been tested and used for numerous designs and have proven acceptable. These have been refined and are consistent and can be very accurate.
In some instances, the variables are not consistently defined or can not be totally accounted for and a subjective approach is necessary. In this case, a subjective engineering approach could be based on one’s observation of numerous repetitions of a common task with numerous variables and achieving an expected outcome.
A pipe design engineer’s belief in the outcome is based on the designer’s experience and belief on a probable outcome. This could be reinforced by establishing objective rules that are applicable in similar instances. These engineering applications are less exact than proven formulas established by years of testing but do prove useful as Good Engineering Practice.
Good Practice - Steel Pipe Handling
AWWA Manual M11 has through the years offered Good Engineering Practice as part of the manual. These practices usually are given based on using standard effort and care. They usually represent some assumed average conditions, normal equipment and reasonable effort by workers.
In the first edition of M11 (1964), Good Practice for wall thickness was addressed in Chapter 6 for handling of unstiffened pipe placed above ground. It was given as a minimum wall of 0.0025(d+20) inches with d being the diameter. For example, this would be a D/t ratio of 282 for 48” diameter pipe. However, this was for bare pipe and no consideration was given to assure the necessary integrity of linings or coatings from time of fabrication, during shipping and handling efforts, and throughout the installation process.
In the first edition of M11, Chapter 15, handling of the pipe with linings and coatings was discussed. An allowance of 2% out-of-roundness for coal tar enamel linings and 1% for cement mortar linings was written as the guidelines. Stulling was suggested if the out-of-roundness was greater than the above values during the shipping and field handling operations.
In the second edition of M11 (1985), D/t was introduced as a Good Engineering Practice for minimum wall thickness. The values suggested were t= D/288 up to 54” diameter pipe and t = (D+20)/400 for greater than 54” pipe.
In the fourth edition of M11 (2004), a minimum wall thickness suggestion was given for cement mortar lined and flexible coated pipe of t = D/240. This parameter was to limit cracking of the cement mortar lining during handling beyond the limits of AWWA C205, again as a Good Engineering Practice.
These handling suggestions have some engineering basis with regards to material properties as D/t is used in some form in above-ground support and vacuum calculations. A D/t of 144 was using for Good Engineering Practice for control of vacuum in above-ground piping for many years until a more defined formula based on testing was made available. Limiting deflection to certain diameters during shipping tie down is also based on observed history and for protection of the lining and coating system.
To assure that a project is completed using “Good Engineering Practice”, specifications are often written with very clear compliance requirements for items that are based on these practices. These are good engineering guidelines and will prevent damage during manufacturing, normal handling and transportation of steel pipe. There are numerous variables that can go into handling such as distance traveled, type of transport, road conditions, and weather that can affect the condition of the pipe. Because of these variables, engineers must use their knowledge when evaluating if anything more is necessary.
Conversely, engineering judgment should be used when considering the accuracy of the suggested values and if there is concern when there is noncompliance. When cement mortar lined pipe is slightly more out-of-round than the suggested values, then a check of the condition of the cement mortar lining will give you an idea of if there is any problem.
In summary, design of steel pipe is accomplished using a combination of well-established formula and Good Engineering Practice but must also include good engineering understanding and judgment of the background and applicability of the design practice.
Pipe Defined Clarified #5 will further discuss this topic with a discussion of buried pipe deflection.