Prestressed Concrete Cylinder Pipe (PCCP) Design

Design Parameters

PreStressed Concrete Cylinder Pipe (PCCP) is designed as a rigid structure to resist the simultaneous application of external loads and internal pressures. External dead loads normally encountered are earth loads, foundation loads, or surcharges applied at the ground surface. External live loads are caused by vehicular traffic, railroads, or construction equipment.

The weight of the pipe and the weight of water inside the pipe are also considered in the design procedure. Internal pressures used for design are the  working pressure, transient pressure, and the field hydrostatic test pressure. The working pressure should be the expected steady-state internal pressure for the system. The transient pressure is the expected internal pressure over and above the working pressure that can occur during surge (water hammer) conditions. 

If the purchaser does not include a transient pressure in the specifications, the AWWA C3045 design standard requires that, as a minimum, the transient pressure allowance be 40 percent of the working pressure, or 40 psi (0.27 MPa), whichever is greater. A postconstruction hydrostatic pressure test is usually conducted to confirm the structural integrity and watertightness of the completed system. In the absence of a field hydrostatic test pressure specified by the purchaser, theAWWA C304 design standard requires the use of a test pressure of 1.2 times the working pressure.

Support under the pipe provided by the bedding material must also be used in the design procedure. Various suggested bedding types are shown in the AWWA C304 design standard.

The pipe purchaser’s plans and specifications should contain as a minimum the following design parameters for PCCP:

• Earth cover over the top of the pipe
• Expected live load (normally AASHTO HS20 truck loading configuration)
• Internal working pressure
• Internal transient pressure allowance
• Field hydrostatic test pressure
• Bedding type

The pipe and fittings manufacturer will design and manufacture the pipe and fittings to comply with the pressures and loadings specified.

Hydraulics

Energy use in pipeline operation can be greatly reduced during the design stage. Head losses due to pipe wall friction are among the most manageable causes of energy consumption for pipelines which use pumps. These losses can be minimized with the use of pipe which has excellent long-term hydraulic characteristics and by selecting a large enough pipe diameter to avoid high-flow velocities which accelerate energy costs. Energy savings resulting from these design decisions will help reduce operating costs each year throughout the life of the pipeline.

Flow Formulas

Over the years, many empirical flow formulas have been proposed. The Hazen-Williams formula, shown below, was first published by Allen Hazen and Gardner S. Williams in 1905, and continues to be the most widely used for pressure pipe systems.

Empirical flow formula

 A statistical analysis of 67 flow tests of concrete pressure lines was made by Swanson and Reed and published in the January 1963 AWWA Journal.7 Some of this pipe was manufactured as early as 1895. This report presented a ‘‘best fit’’ mean deviation comparison with the well-known formulas by Hazen-Williams, Morris, Moody, and Scobey. The authors concluded that the Hazen-Williams expression for head loss most closely matched the test results for the range of velocities normally encountered in water transmission. The average mean deviation between calculated and observed losses was lowest for the Hazen-Williams formula. A regression analysis least-squares method was used to develop a correlation equation for the Hazen-Williams ‘‘Ch’’ term for concrete pipe, as follows:

Regression analysis least-squares method

The Hazen-Williams flow formula can be rewritten in a more convenient form where head loss is expressed in terms of flow velocity.

Hazen-Williams flow formula

Head Losses Due to Fittings

While head losses due to fittings are generally a minor portion of the overall head loss in a pipeline, they can be important in certain applications such as treatment plants when the length of a line is short and the number of fittings is high. These head losses occur in elbows, reducers, enlargements, valves, and other fittings in the pipeline. The rational method of calculating these losses assumes full turbulence and expresses the loss in terms of velocity head. This expression is:

Calculating losses assumes full turbulence and expresses the loss in terms of velocity head

Values of ‘‘CL’’ commonly used for design purposes for a variety of fittings and appurtenances, along with a more comprehensive treatment of hydraulics, are included in the AWWA manual ‘‘M9—Concrete Pressure Pipe.’’

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