The principal objective in compiling the material for this CONCRETE PIPE DESIGN MANUAL was to present data and information on the design of concrete pipe systems in a readily usable form. The Design Manual is a companion volume to the CONCRETE PIPE HANDBOOK which provides an up-to-date compilation of the concepts and theories which form the basis for the design and installation of precast concrete pipe sewers and culverts and explanations for the charts, tables and design procedures summarized in the Design Manual.
Special recognition is acknowledged for the contribution of the staff of the American Concrete Pipe Association and the technical review and assistance of the engineers of the member companies of the Association in preparing this Design Manual. Also acknowledged is the development work of the American Association of State Highway and Transportation Officials, American Society of Civil Engineers, U. S. Army Corps of Engineers, U. S. Federal Highway Administration, Bureau of Reclamation, Iowa State University, Natural Resources Conservation Service, Water Environment Federation, and many others. Credit for much of the data in this Manual goes to the engineers of these organizations and agencies. Every effort has been made to assure accuracy, and technical data are considered reliable, but no guarantee is made or liability assumed.
The design and construction of sewers and culverts are among the most important areas of public works engineering and, like all engineering projects, they involve various stages of development. The information presented in this manual does not cover all phases of the project, and the engineer may need to consult additional references for the data required to complete preliminary surveys. This manual is a compilation of data on concrete pipe, and it was planned to provide all design information needed by the engineer when he begins to consider the type and shape of pipe to be used. All equations used in developing the figures and tables are shown along with limited supporting theory. A condensed bibliography of literature references is included to assist the engineer who wishes to further study the development of these equations.
Chapters have been arranged so the descriptive information can be easily followed into the tables and figures containing data which enable the engineer to select the required type and size concrete pipe without the lengthy computations previously required. All of these design aids are presently published in engineering textbooks or represent the computer analysis of involved equations. Supplemental data and information are included to assist in completing this important phase of the project, and illustrative example problems are presented in Chapters 2 through 4. A review of these examples will indicate the relative ease with which this manual can be used.
The revised Chapter 4 on Loads and Supporting Strengths incorporates the Standard Installations for concrete pipe bedding and design. The standard Installations are compatible with today’s methods of installation and incorporate the latest research on concrete pipe. In 1996 the B, C, and D beddings, researched by Anson Marston and Merlin Spangler, were replaced in the AASHTO Bridge Specifications by the Standard Installations. A description of the B, C, and D beddings along with the appropriate design procedures are included in Appendix B of this manual to facilitate designs still using these beddings.
HYDRAULICS OF SEWERS
The hydraulic design procedure for sewers requires:
- Determination of Sewer System Type
- Determination of Design Flow
- Selection of Pipe Size
- Determination of Flow Velocity
DETERMINATION OF SEWER SYSTEM TYPE
Sanitary sewers are designed to carry domestic, commercial and industrial sewage with consideration given to possible infiltration of ground water. All types of flow are designed on the basis of having the flow characteristics of water.
DETERMINATION OF DESIGN FLOW
In designing sanitary sewers, average, peak and minimum flows are considered. Average flow is determined or selected, and a factor applied to arrive at the peak flow which is used for selecting pipe size. Minimum flows are used to determine if specified velocities can be maintained to prevent deposition of solids.
Average Flow. The average flow, usually expressed in gallons per day, is a hypothetical quantity which is derived from past data and experience. With adequate local historical records, the average rate of water consumption can be related to the average sewage flow from domestic, commercial and industrial sources. Without such records, information on probable average flows can be obtained from other sources such as state or national agencies. Requirements for minimum average flows are usually specified by local or state sanitary authorities or local, state and national public health agencies. Table 1 lists design criteria for domestic sewage flows for various municipalities. Commercial and industrial sewage flows are listed in Table 2. These tables were adapted from the “Design and Construction of Sanitary and Storm Sewers,” published by American Society Civil Engineers and Water Pollution Control Federation. To apply flow criteria in the design of a sewer system, it is necessary to determine present and future zoning, population densities and types of business and industry.
Peak Flow. The actual flow in a sanitary sewer is variable, and many studies have been made of hourly, daily and seasonal variations. Typical results of one study are shown in Figure I adapted from “Design and Construction of Sanitary and Storm Sewers,” published by the American Society of Civil Engineers and Water Pollution Control Federation. Maximum and minimum daily flows are used in the design of treatment plants, but the sanitary sewer must carry the peak flow that will occur during its design life. This peak flow is defined as the mean rate of the maximum flow occurring during a 15-minute period for any 12-month period and is determined by multiplying average daily flow by an appropriate factor. Estimates of this factor range from 4.0 to 5.5 for design populations of one thousand, to a factor of 1.5 to 2.0 for design population of one million. Tables 1 and 2 list minimum peak loads used by some municipalities as a basis for design. Minimum Flow. A minimum velocity of 2 feet per second, when the pipe is flowing full or half full, will prevent deposition of solids. The design should be checked using the minimum flow to determine if this self-cleaning velocity is maintained.
SELECTION OF PIPE SIZE
After the design flows have been calculated, pipe size is selected using Manning’s formula. The formula can be solved by selecting a pipe roughness coefficient, and assuming a pipe size and slope. However, this trial and error method is not necessary since nomographs, tables, graphs and computer programs provide a direct solution. Manning’s Formula. Manning’s formula for selecting pipe size is:
Q = AR S (1) 1.486 2/3 1/2
A constant C1 = AR 1.486 2/3 n which depends only on the geometry and characteristics of the pipe enables Manning’s formula to be written as:
Q = C1S (2) 1/2
Tables 3, 4, 5 and 6 list full flow values of C1 for circular pipe, elliptical pipe, arch pipe, and box sections. Table A-1 in the Appendix lists values of S1/2.
Manning’s “n” Value. The difference between laboratory test values of Manning’s “n” and accepted design values is significant. Numerous tests by public and other agencies have established Manning’s “n” laboratory values. However, these laboratory results were obtained utilizing clean water and straight pipe sections without bends, manholes, debris, or other obstructions. The laboratory results indicated the only differences were between smooth wall and rough wall pipes. Rough wall, or corrugated pipe, have relatively high “n” values which are approximately 2.5 to 3 times those of smooth wall pipe.
All smooth wall pipes, such as concrete and plastic, were found to have “n” values ranging between 0.009 and 0.010, but, historically, engineers familiar with sewers have used 0.012 and 0.013. This “design factor” of 20-30 percent takes into account the difference between laboratory testing and actual installed conditions. The use of such design factors is good engineering practice, and, to be consistent for all pipe materials, the applicable Manning’s “ ” laboratory value should be increased a similar amount in order to arrive at design values.