6.13.2 Temperature Induced Expansion and Contraction
When the conduits are not permitted to change length due to variations in temperature longitudinal stresses are created in the conduits, which is calculated as shown below.
- (i) Change in pipe length with temperature
/\ = C01
Where 0= change in temperature and C=coefficient of expansion of conduit per Degree Centigrade and is equal to 11.9 x 10-6 for steel,8.5 x 10-6 for Cast Iron, and 10x10-6 for concrete length of pipe.
- (ii) Resulting longitudinal stress S=C0 E
for pipe line with fixed ends, and E= Youngs Modulus of electricity ,2,10,000 N/m2 for steel 1,00,000 N/m2 for cast iron, and 1400-40,000 N/m2 for concrete.
- (iii_ Resulting longitudinal force P = (d+t) t s
6.13.3 Cross Section :
The selection of the optimum corss section of a transmission main depends upon both hydraulic performance and structural behaviour because hydraulic capacity is a direct function of the hydraulic radius, full circles or half circles posses the highest hydraulic capacity by virtue of their largest hydraulic radius or smallest firctional surface for a given area. Hence circular corss sections are prefferred for closed conduits and the semicircular ones for open conduits whenever structural conditions permit. The cross sections preferred next are those in which cirlces or semicircles can be inscribed. The following corss sections are generally used :-
- (a) tapezoids approaching half a hexagon as nearly as maintainable slopes permit, for canals in earth;
- (b) recetangles with widths equal to twice the depths for canals in rock and flumes of masonry or wood;
- (c) semi-circles for flumes of wood staves or steel;
- (d) horse shoe for grade adequates and grade tunnels.
Material high in tensile strength with circular cross sections withstand satisfactorily the internal prssures; external pressures due to pressures are resisted best by horse shoe sections of materials possessing high compressive strength. The hydraulic properties of horse shoe sections are only slightly poorer than those of circles. Moreover their relatively flat invert makes for easy transport of excavation and construction material, in and out of the aqueduct.
6.13.4 Depth of Cover
One meter cover on pipe line is normal and generally sufficient to protect the lines from external damage. When heavy traffic is anticipated, depth of cover has to be arrived at taking into consideration the structural and other aspects as detailed in 6.13.2. When freezing is anticipated 1.5 m cover is recommended asa discussed in 10.12.