16-01-2013, 10:30 AM
WATER TANKS DESIGN
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INTRODUCTION
In general there are three kinds of water tanks-tanks resting on ground, underground
tanks and elevated tanks.
The tanks resting on ground like clear water reservoirs, settling tanks, aeration tanks
etc. are supported on the ground directly. The walls of these tanks are subjected to pressure and
the base is subjected to weight of water and pressure of soil. The tanks may be covered on top.
The tanks like purification tanks, Imhoff tanks, septic tanks, and gas holders are built
underground. The walls of these tanks are subjected to water pressure from inside and the earth
pressure from outside. The base is subjected to weight of water and soil pressure. These tanks
may be covered at the top.
Elevated tanks are supported on staging which may consist of masonry walls, R.C.C.
tower or R.C.C. columns braced together. The walls are subjected to water pressure. The base
has to carry the load of water and tank load. The staging has to carry load of water and tank.
The staging is also designed for wind forces.
From design point of view the tanks may be classified as per their shape-rectangular
tanks, circular tanks, intze type tanks. spherical tanks conical bottom tanks and suspended
bottom tanks.
Design requirement of concrete ( I.S.I )
In water retaining structures a dense impermeable concrete is required therefore,
proportion of fine and course aggregates to cement should be such as to give high quality
concrete.
Concrete mix weaker than M200 is not used. The minimum quantity of cement in the
concrete mix shall be not less than 300 kg/m3.
The design of the concrete mix shall be such that the resultant concrete is sufficiently
impervious. Efficient compaction preferably by vibration is essential. The permeability of the
thoroughly compacted concrete is dependent on water cement ratio. Increase in water cement
ratio increases permeability, while concrete with low water cement ratio is difficult to compact.
Other causes of leakage in concrete are defects such as segregation and honey combing. All
joints should be made water-tight as these are potential sources of leakage.
Design of liquid retaining structures is different from ordinary R.C.C, structures as it
requires that concrete should not crack and hence tensile stresses in concrete should be within
permissible limits.
Walls
(i) Provision of Joints
(a) Sliding joints at the base of the wall. Where it is desired to allow the walls to
expand or contract separately from the floor, or to prevent moments at the base of the wall
owing to fixity to the floor, sliding joints may be employed.
(b) The spacing of vertical movement joints should be as discussed in article 8.3
while the majority of these joints may be of the partial or complete contraction type, sufficient
joints of the expansion type should be provided to satisfy the requirements given in article
(ii) Pressure on Walls.
(a) In liquid retaining structures with fixed or floating covers the gas pressure
developed above liquid surface shall be added to the liquid pressure.
(b) When the wall of liquid retaining structure is built in ground, or has earth
embanked against it, the effect of earth pressure shall be taken into account.
(iii) Walls or Tanks Rectangular or Polygonal in Plan.
While designing the walls of rectangular or polygonal concrete tanks, the following points
should be borne in mind.
(a) In plane walls, the liquid pressure is resisted by both vertical and horizontal
bending moments. An estimate should be made of the proportion of the pressure resisted by
bending moments in the vertical and horizontal planes. The direct horizontal tension caused by
the direct pull due to water pressure on the end walls, should be added to that resulting from
horizontal bending moments.