Reinforced Concrete Pressure Pipe Stress Distribution
What are the appropriate coefficients to use for the design of reinforced concrete pressure pipe (RCPP) in light of recent code trends of reduced load factors?
Concrete pipe design is based upon assumed stress coefficients for moment, shear, and thrust loadings. The coefficient values and the location of the stresses for typical loadings on buried installations were developed in the 1950s. Modern installation techniques, including Standard Installation Direct Design (SIDD), provide additional support that results in reduced reinforcement requirements.
One of the major loadings on buried concrete pipe is from earth cover. The embankment condition loading typically assumed is sensitive to design assumptions, installation, and the backfill construction processes.
Limited field tests have been conducted to verify that pipe stresses predicted during design are comparable to the actual stresses following construction.
RCPP is subjected to unusual concrete design loading conditions due to the fact that the pipe often carries high internal pressure, which results in axial (circumferential) tension. This high internal pressure tension significantly affects the ability of the concrete to resist shear stresses.
Although procedures for reinforced concrete design have evolved over the years, design procedures for reinforced concrete pressure pipe, with the exception of stirrups, have not changed significantly since the procedures were first established.
Professional society (ASTM, ASCE, AWWA, AASHTO) committees are re-evaluating design parameters such as load factors for this type of pipe because the stresses are not well understood.
Designers are left to combine what they believe are the applicable sections of several design codes in order to design a pipe structure that will be safe and durable.
Need and Benefit
RCPP is commonly used in Bureau of Reclamation (Reclamation) projects for cross drainage structures, in-line siphons, pipeline distribution systems, and other features.
Traditional ultimate strength concrete design load factors used by several standards (Reclamation, ASTM, AWWA) are larger than modern codes and standards (ACI building code, ASCE, AASHTO). Traditional concrete design load factors used for RCP pipe is 1.8 (since about 1940) for all loads and phi factor of 1.0. ASCE (since 1993), and AASHTO have used load factors as low as 1.3 for the design of gravity (very low pressure) pipe. ASTM is in the process of reviewing their C361 design document, with one of the objectives being to implement revised load and phi factors. The reduced factors will result in significant reinforcement and concrete cost savings while retaining adequate structure durability and safety.
The RCPP project proposed for use to gather field behavior is a cast-in-place construction and installation. The project is adding a second barrel to an existing system to increase water carrying capacity. The project has five additional structures proposed. The information will be correlated to predicted and computed values to determine the degree of reduced reinforcement that may be used. There will be some unique structural behaviors with this installation when compared to the more common precast installation. However, much of the information will be directly transferable.
Refined design approaches can reduce construction costs. However, Reclamation should endeavor to maintain durable and safe facilities.
Independent Peer Review
The following documents were reviewed by qualified Bureau of Reclamation employees. The findings were determined to be achieved using valid means.
Discussion of Earth pressure measurements on buried HDPE pipe (interim, PDF,
By Dr. Ashok Chugh
Report completed on June 22, 2012
with interest because of the U.S. Bureau of Reclamation's
(USBR's) involvement in analysis, design and construction of
buried pipelines for water conveyance related to hydro projects.
Laboratory load tests, similar in detail to the ones
presented in the paper, were performed in the 1960s by the
USBR to investigate soil–structure interaction effects on
buried pipelines, and the pressure cells used to measure radial
earth pressure wer