23-06-2014, 02:04 PM
Design ofWater Tank
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ABSTRACT
Storage reservoirs and overhead tank are used to store water, liquid
petroleum, petroleum products and similar liquids. The force analysis of
the reservoirs or tanks is about the same irrespective of the chemical
nature of the product. All tanks are designed as crack free structures to
eliminate any leakage.
This project gives in brief, the theory behind the design of liquid
retaining structure (circular water tank with flexible and rigid base and
rectangular under ground water tank) using working stress method. This
report also includes computer subroutines to analyze and design circular
water tank with flexible and rigid base and rectangular under ground
water tank. The program has been written as Macros in Microsoft Excel
using Visual Basic programming language. In the end, the programs are
validated with the results of manual calculation given in Concrete
Structure book.
INTRODUCTION
Storage reservoirs and overhead tank are used to store water, liquid
petroleum, petroleum products and similar liquids. The force analysis of
the reservoirs or tanks is about the same irrespective of the chemical
nature of the product. All tanks are designed as crack free structures to
eliminate any leakage. Water or raw petroleum retaining slab and walls
can be of reinforced concrete with adequate cover to the reinforcement.
Water and petroleum and react with concrete and, therefore, no special
treatment to the surface is required. Industrial wastes can also be
collected and processed in concrete tanks with few exceptions. The
petroleum product such as petrol, diesel oil, etc. are likely to leak through
the concrete walls, therefore such tanks need special membranes to
prevent leakage. Reservoir is a common term applied to liquid storage
structure and it can be below or above the ground level. Reservoirs below
the ground level are normally built to store large quantities of water
whereas those of overhead type are built for direct distribution by gravity
flow and are usually of smaller capacity.
DESIGN REQUIREMENT OF CONCRETE (I. S. I)
In water retaining structure 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 M20 is not used. The minimum quantity of
cement in the concrete mix shall be not less than 30 kN/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 structure 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.
A reinforced concrete member of liquid retaining structure is designed on
the usual principles ignoring tensile resistance of concrete in bending.
Additionally it should be ensured that tensile stress on the liquid retaining
face of the equivalent concrete section does not exceed the permissible
tensile strength of concrete as given in table 1. For calculation purposes
the cover is also taken into concrete area.
Cracking may be caused due to restraint to shrinkage, expansion and
contraction of concrete due to temperature or shrinkage and swelling due
to moisture effects. Such restraint may be caused by
SPACING OF JOINTS
Unless alternative effective means are taken to avoid cracks by allowing
for the additional stresses that may be induced by temperature or
shrinkage changes or by unequal settlement, movement joints should be
provided at the following spacing:-
(a) In reinforced concrete floors, movement joints should be spaced at not
more than 7.5m apart in two directions at right angles. The wall and floor
joints should be in line except where sliding joints occur at the base of
the wall in which correspondence is not so important.
GENERAL DESIGN REQUIREMENTS (I.S.I)
2.3.1 Plain Concrete Structures. Plain concrete member of reinforced
concrete liquid retaining structure may be designed against structural
failure by allowing tension in plain concrete as per the permissible limits
for tension in bending. This will automatically take care of failure due to
cracking. However, nominal reinforcement shall be provided, for plain
concrete structural members.
2.3.2. Permissible Stresses in Concrete.
(a) For resistance to cracking. For calculations relating to the resistance
of members to cracking, the permissible stresses in tension (direct and
due to bending) and shear shall confirm to the values specified in Table 1.
The permissible tensile stresses due to bending apply to the face of the
member in contact with the liquid. In members less than 225mm. thick and
in contact with liquid on one side these permissible stresses in bending
apply also to the face remote from the liquid.
(b) For strength calculations. In strength calculations the permissible
concrete stresses shall be in accordance with Table 1. Where the
calculated shear stress in concrete alone exceeds the permissible value,
reinforcement acting in conjunction with diagonal compression in the
concrete shall be provided to take the whole of the shear.
Minimum Reinforcement
(a)The minimum reinforcement in walls, floors and roofs in each of two
directions at right angles shall have an area of 0.3 per cent of the concrete
section in that direction for sections up to 100mm, thickness. For sections
of thickness greater than 100mm, and less than 450mm the minimum
reinforcement in each of the two directions shall be linearly reduced from
0.3 percent for 100mm thick section to 0.2 percent for 450mm, thick
sections. For sections of thickness greater than 450mm, minimum
reinforcement in each of the two directions shall be kept at 0.2 per cent.
In concrete sections of thickness 225mm or greater, two layers of
reinforcement steel shall be placed one near each face of the section to
make up the minimum reinforcement.
(b)In special circumstances floor slabs may be constructed with
percentage of reinforcement less than specified above. In no case the
percentage of reinforcement in any member be less than 0°15% of gross
sectional area of the member.
FLEXIBLE BASE CIRCULAR WATER TANK
For smaller capacities rectangular tanks are used and for bigger capacities
circular tanks are used .In circular tanks with flexible joint at the base
tanks walls are subjected to hydrostatic pressure .so the tank walls are
designed as thin cylinder. As the hoop tension gradually reduces to zero at
top, the reinforcement is gradually reduced to minimum reinforcement at
top. The main reinforcement consists of circular hoops. Vertical
reinforcement equal to 0.3% of concrete are is provided and hoop
reinforcement is tied to this reinforcement.
CONCLUSION
Storage of water in the form of tanks for drinking and washing purposes,
swimming pools for exercise and enjoyment, and sewage sedimentation
tanks are gaining increasing importance in the present day life. For small
capacities we go for rectangular water tanks while for bigger capacities
we provide circular water tanks.
Design of water tank is a very tedious method. Particularly design of
under ground water tank involves lots of mathematical formulae and
calculation. It is also time consuming. Hence program gives a solution to
the above problems.
There is a little difference between the design values of program to that of
manual calculation. The program gives the least value for the design.
Hence designer should not provide less than the values we get from the
program. In case of theoretical calculation designer initially add some
extra values to the obtained values to be in safer side.