6.13 STRENGTH OF PIPES
The stesses in a pipe are normally induced by internal pressure external loading,surge forces and change of temperature, although torsional stresses can also arise. Internal pressure induces circumferential and longitudinal stresses, the latter developing where the line changes in size or direction, or has a colsed end. A pipe is usually chosen so as to carry the circumferential stress without extra strengthening or support but if the joints cannot safely transmit the logitudinal stress, anchorages or some other measns of taking the load must be provided. Longitudinal stress is absorbed by firction between the outside surface of the pipe and the material in which the line is buried.
A pipe must withstand the highest internal pressure it is likely to be subected, the general provisions for which have been discussed in 6.4.4, while surging or water hammer is discussed in 6.17.
External loads generally arise from the weight of the pipe and its contents and that of the trench filling from superimposed loads including impact from traffic, from subsidence and from wind loads in the case of pipes laid above ground. If a pipe is laid on good and uniform continuous bed and the cover does not greatly exceed the normal, on special strengthening to resist external loading is generally necessary. Loading likely to airse from subsidence is best dealt with by the use of flexible joints and steel pipes. External loading becomes important usually when a line is laid on a foundation providing uneven support (e.g. across a sewer, trench or in rock under deep cover) or is subjected to heavy superimposed surface loads at less than normal cover. The necessity of stronger pipes can often be avoided by careful bedding and trench-filling to give additional support. The importance of good bedding under and around the pipe upto at least the horizontal diameter cannot be overemphasized and in some cases concreting may be required.
Execussive distoration of a steel pipe may casuse failure of its protective coating but can be limited by the use of strengthening rings. This problem is only likely to arise in very large mains. Distorations at flexible joints can cause leakage.
When a pipeline has to be laid above ground over some obstraction, such as water way or railway, it may either be carried on a pipe-bridge or be supported on pillars. In the latter case, the pipe ends must be properly designed to resist shear, if the full strength of the pipe as a beam is to be realised. A small diameter pipe is usually thick enough to span short lengths with its ends simply supported, but as diameters and lengths of span increase the problem becomes more complex and the ends must be properly designed to resist shear, if the full strength of the pipe as a beam is to be realised. A small diameter pipe is usually thick enough to span short lenghths with its ends simply supported but as diameter and lengths of span increase, the porblem becomes more complex and the ends must be supported in saddles or restrained by ring girders. For pipes of more than 900mm in dia the ring girder method will probably provide the most economical design. Structural design of buried pipes is discussed in detail in the comanion volume. "Manual on sewarage and sewage treatment."
The temperature of the water in a transmission main varies during the year. If the water is derived from underground sources the variation is relatively small, but if it is obtained from surface sources and is filtered through slow sand filters, the variation may be as much as 20 C during the year. Furthermore, the temperature changes may take place fairly quickly and for these and other reasons, long lengths of rigid mains are to be avoided. Provision of expansion joints to take care of these stresses is necessary. Thrust and anchor blocks are provided to keep the pipe curve of these stresses is necessary. Thrust and anchor blocks are provided to keep the pipe curve in position. In small mains i.e. the mains with spigot and socket lead joints, the joints themselves allow sufficient movement, although some recaulking mayh be ocasionally necessary. On large steel pipe lines with welded joints expension can be allowed to give a longitudinal stress in the pipes, when first laid. In about foru years or so, the ground normally consolidates sufficiently around the pipe so that the stress is transferred to the ground. valves require to be bridged by steel or reinforced concrete blocks so that the valve bodies are not stressed, as this could affect their water tightness.
In case of PVC pipe lines, it should be noted that the co-efficient of expansion of PVC is eight times greater than steel and considerable movement can take place in long lengths of rigidly jointed pipelines.