RCC WATER TANK DESIGN BASICS
RCC WATER TANK DESIGN BASICS
The design of the Reinforced Concrete
water tank is based on IS 3370: 2009 (Parts I – IV). The design depends on the
location of tanks, i.e. overhead, resting on the ground or underground water
tanks. The tanks can be made in different shapes, usually circular and
rectangular shapes are mostly used. The tanks can be made of RCC or even of
steel. The overhead tanks are usually elevated from the rooftop through the
column. On the other hand, the underground tanks are rested on the foundation.
TYPES OF WATER TANK
Basing on the location of the tank in
a building s tanks can be classified into three categories. Those are:
o Underground tanks
(UGT)
o Tank resting on
grounds (Ground Storage Reservoir – GSR)
o Overhead tanks
(OHT)
In most cases, the underground and on
ground tanks are circular or rectangular in shape but the shape of the overhead
tanks are influenced by the aesthetical view of the surroundings and as well as
the design of the construction.
Steel tanks are also used especially
in railway yards. Basing on the shape the tanks can be circular, rectangular,
square, polygonal, spherical and conical. A special type of tank named Intze tank
is used for storing a large amount of water for an area.
The overhead tanks are supported by
the column which acts as stages. This column can be braced for increasing
strength and as well as to improve the aesthetic views.
BASIS OF CONCRETE
WATER TANK DESIGN
One of the vital considerations for
the design of tanks is that the structure has adequate resistance to cracking
and has adequate strength. For achieving these following assumptions are made:
o Concrete is capable
of resisting limited tensile stresses the full section of concrete including
cover and reinforcement are taken into account in this assumption.
o To guard against
structural failure in strength calculation the tensile strength of concrete is
ignored.
o Reduced values of
permissible stresses in steel are adopted in design.
CIRCULAR WATER TANK
The simplest form of the water tank
is a circular tank. For the same amount of storage, the circular tank requires
a lesser amount of material. Moreover, for its circular shape, it has no corner
and can be made watertight easily. It is very economical for smaller storage of
water up to 20000000 liters and with the diameter in the range of 5 to 8 m. The
depth of the storage is between 3 to 4 m. The side walls are designed for hoop
tension and bending moments.
PERMISSIBLE
STRESSES IN CONCRETE
To ensure
impervious concrete mixture linear than M 20 grade is not normally recommended
to make the walls leak proof the concretes near the water face need to such
that no crack occurs. To ensure this, member thicknesses are so designed that
stress in the concrete is lesser than the permissible as given in table 1.
Table 1 Permissible
Stresses in Concrete (For calculations relating to resistance to concrete)
Grade of
Concrete
|
Permissible
Stresses
|
Shear stress
N/mm2
|
|
Direct
Tension
N/mm2
|
Tension due
to bending N/mm2
|
||
M15
|
1.1
|
1.5
|
1.5
|
M20
|
1.2
|
1.7
|
1.7
|
M25
|
1.3
|
1.8
|
1.9
|
M30
|
1.5
|
2.0
|
2.2
|
M35
|
1.6
|
2.2
|
2.7
|
M40
|
1.7
|
2.4
|
2.7
|
THE PERMISSIBLE
STRESS IN STEEL
The stress in steel
must not be allowed to exceed the following values under different positions to
prevent cracking of concrete.
o When steel is
placed near the face of the members in contact with liquid 115 N/ sq mm for
Mild Steel Bars and 150 N/ sq mm for HYSD bars.
o When steel is
placed on face away from the liquid for members less than 225 mm in thickness
same as earlier.
o When steel is
placed on the face away from the liquid for members 225 mm or more in
thickness: 125 N/ sq mm for M.S. bars and 190 N/sq mm for HYSD bars.
BASE FOR FLOOR SLAB
The floor slab should be strong
enough to transmit the load from the liquid and the structure itself to the
ground without subsidence. The floor slab is usually 150 to 200 mm thick and is
reinforced with nominal reinforcement, which may be provided in the form of
mesh both at the top and bottom face of the slab. Before laying the slab the
bed has to be rammed and leveled then a 75 mm thick layer of lean concrete of M
100 grade should be laid and cured. This layer should be covered with tar to
enable the floor slab to act independently on the bottom layer. In waterlogged
soils, the bottom layer of concrete should preferably be of M 15 grade.
MINIMUM
REINFORCEMENT FOR WATER TANK
Minimum
reinforcement required for 199 mm thick sections is 0.3 % of the area of
the concrete section which reduced linearly to 0.2 % for 450 mm thick sections.
In case of floor slab for tank resting on the ground, the minimum reinforcement
from practical consideration should not be less than 0.3% of the gross
sectional area of the floor slab.
If the thickness of the section
(wall, floor or roof slab of the tank) works out to be 225 mm and above two
layers of reinforcing steel shall be placed, one near each of the section to
make up the minimum reinforcement requirements.
To download IS 3370 use following link
IS 3370 Part -1:2009
IS 3370 Part -2:2009
IS 3370 Part -3:1967
IS 3370 Part -4:1967
To download IS 3370 use following link
IS 3370 Part -1:2009
IS 3370 Part -2:2009
IS 3370 Part -3:1967
IS 3370 Part -4:1967
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