Analysis and Design of Buried Prestressed Concrete Pipes

Abstract

Due to rapid urbanization and the ever-increasing population of urban areas, domestic water demands of these areas are increasing at a fast rate. This phenomenon has been resulting into increasing shortage of traditional nearby sources of water for many urban areas, and thus necessitating development of distant sources of water, requiring transport of water from source to the cities. Of late, use of long transport pipelines of large diameters has become inevitable for urban water supply scheme. The applicability of different pipe materials varies with each site and the system requirements. The pipe material also must be compatible with the soil structure and topography of the site, which affects the pipe location and depth, the supports necessary for the pipe fill material, and the required strength of the pipe material. Reinforced concrete pipes are used for lines where pressure is up to 0.4 N/mm2, beyond this pressure the pipe is not considered suitable due to pressure limitations. Metallic pipes such as cast iron, ductile iron and mild steel are suitable for high pressures. Cast iron and ductile iron pipes are costlier and are commercially not available when diameter exceeds 900 mm. In case of large pipelines the choice narrows down to mild steel or prestressed concrete pipeline. Mild steel pipeline is quite expensive and require outer coating to prevent corrosion and maintaining water carrying capacity. Rigid pipe, such as prestressed concrete, is much economical compared to metallic pipes. The pipe has strength to withstand the combined internal pressure and external load it is subjected to along the barrel as well as at the joint. The pipe has smooth inner surface and hence has an excellent hydraulically efficiency. It is also highly corrosion resistant. Because of these reasons PSC pipes are widely used in water supply and lift irrigation. This study deals with the analysis and design of prestressed concrete pipes. The analysis takes into account various design loads due to water load, dead load, earth load and live load for various bedding angles and installation conditions. The moments and thrusts are then calculated for earth load, water load and self weight of the pipe by three approaches namely AWWA guidelines, manual approach and STAAD Pro software. The moments and thrusts so calculated are then used for calculating the stresses in the pipe. Prestressed concrete pipes are then designed for the maximum stresses obtained among the three approaches. The design of prestressed concrete non cylinder pipe covers the determination of circumferential and longitudinal prestressing steel wires where as in cylinder prestressed concrete pipes only the circumferential prestressing steel wire is to be determined. The stresses developed at different stages of pipe are then checked for the permissible stresses specified by the Indian standards IS: 784-2001.