SIDD Installation and Direct Design for Reinforced Concrete Low-Head Pressure Pipe

This paper describes the principal technical provisions in a proposed ASCE Standard Practice for the Direct Design of Buried Reinforced Concrete Low-Head Pressure Pipe using the same new standard installations (SIDD) as recommended in ASCE 15–93, Standard Practice for Direct Design of Buried Precast Concrete Pipe Using Standard Installations (for reinforced concrete gravity flow pipe). The proposed standard practice defines installation requirements and structural analyses for moment, thrust, and shear produced by external loads that are the same as given in ASCE-15. It incorporates design procedures for combined circumferential tension produced by internal pressure and flexure produced by external loads that are essentially in compliance with both ASTM C361 and AWWA C302 and as recommended in AWWA Manual M9. These provisions and the cited referenced standards use load factors that are larger than those used in various standards for direct design of gravity flow pipe and they further limit the maximum combined stress in the reinforcement by inclusion of a limiting maximum design yield strength of 40,000 psi, regardless of the actual yield strength of the reinforcement. There are further limits on the reinforcement stress produced by internal pressure alone as well as on the tensile stress in the uncracked concrete pipe wall that are essentially the same as given in the above standards for low-head pressure pipe. In addition to providing the requirements and structural effects of SIDD installations, the proposed standard fills a gap in the existing standards for design of low-head concrete pipe by incorporating rational methods for calculating radial tension and shear strengths. Radial tension strength is required to resist the bending moments at the crown and invert produced by external loads. The presence of internal pressure does not increase the required radial tension strength, and the same methods used for gravity flow pipe may be used for determining the radial tension strength of low-head pressure pipe. Shear strength is another matter. The circumferential tension produced by internal pressure produces a significant effect on shear strength. Research by Dr. M. P. Collins and coworkers in Canada is the basis for new more rational procedures for shear strength of flexural members in the AASHTO LRFD Highway Bridge Specification. These are adopted and simplified for use over the range of wall thickness and strain variations applicable to low-head pressure pipe so that for the first time a rational procedure is available in the proposed standard to calculate the shear strength of low-head concrete pipe. Application of the new procedure to pipe designs for the higher fill heights combined with the highest pressure limits in the design tables of ASTM C361 indicates an adequate factor of safety for those designs, including adequate shear strength.

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