water tanks - ecajmer.ac.in tank 1.pdf · design a circular water tank with flexible connection at...
TRANSCRIPT
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WATER TANKS
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Learning out Come
• REVIEW
• TYPES OF TANKS
• DESIGN OF CIRCULAR WATER TANK RESTING ON GROUND WITH FLEXIBLE BASE
• DESIGN OF CIRCULAR WATER TANK RESTING ON GROUND WITH RIGID BASE
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INTRODUCTION
• Storage tanks are built for storing water, liquid
petroleum, petroleum products and similar liquids
• Designed as crack free structures to eliminate
any leakage
• Permeability of concrete is directly proportional
to water cement ratio.
• Cement content ranging from 330 Kg/m3 to 530
Kg/m3 is recommended in order to keep shrinkage
low.
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RESTING ON GROUND
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UNDERGROUND
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ELEVATED
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CIRCULAR
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RECTANGULAR
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SPHERICAL
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INTZ
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CONICAL BOTTOM
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Circular Tanks Resting On
Ground
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• The tank has tendency to increase in
diameter due to hydrostatic pressure
• This increase in diameter all along the
height of the tank depends on the
nature of joint at the junction of slab
and wall
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• When the joints at base are flexible,
hydrostatic pressure induces maximum
increase in diameter at base and no
increase in diameter at top
• When the joint at base is rigid, the base
does not move
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Design of Circular Tanks resting on
ground with flexible base
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• When the thickness ≤ 225 mm, the steel placed atcentre.
• When the thickness > 225mm, at each face Ast/2 ofsteel as hoop reinforcement is provided
• The stress in concrete is computed as
If c cat, where cat=0.27fck , then no crack appearsin concrete
ststc
cA)1m(t1000
2/HD
A)1m(A
T
−+
=
−+=
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• While designing, the thickness of concrete wallcan be estimated as t=30H+50 mm, where H is inmeters
• Distribution steel in the form of vertical bars areprovided such that minimum steel arearequirement is satisfied
• As base slab is resting on ground and nobending stresses are induced hence minimumsteel distributed at bottom and the top areprovided
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• While designing, the thickness of concrete wallcan be estimated as t=30H+50 mm, where H is inmeters
• Distribution steel in the form of vertical bars areprovided such that minimum steel arearequirement is satisfied
• As base slab is resting on ground and nobending stresses are induced hence minimumsteel distributed at bottom and the top areprovided
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Problem on Circular Tanks resting
on ground with flexible base
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Design a circular water tank with flexible
connection at base for a capacity of 4,00,000
liters. The tank rests on a firm level ground.
The height of tank including a free board of
200 mm should not exceed 3.5m. The tank is
open at top. Use M 20 concrete and Fe 415
steel. Draw to a suitable scale:
Plan at base
Cross section through centre of tank.
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Step 1: Dimension of tank
Depth of water H=3.5 -0.2 = 3.3 m
Volume V = 4,00,000/1000 = 400 m3
Area of tank A = 400/3.3 = 121.2 m2
Diameter of tank 13 m
The thickness is assumed as
t = 30H+50=149 160 mm
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Step 2: Design of Vertical wall
Max hoop tension at bottom Area of steel Minimum steel to be provided
Ast min=0.24%of area of concrete
= 0.24x 1000x160/100 = 384 mm2
The steel required is more than the minimum required
Let the diameter of the bar to be used be 16 mm, area of each bar =201 mm2
Spacing of 16 mm diameter bar=1430x 1000/201= 140.6 mm c/c
Provide #16 @ 140 c/c as hoop tension steel
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Step 3: Check for tensile stress
Area of steel provided Ast
provided=201x1000/140 = 1436.16 mm2
Modular ratio m=Stress in concrete
Permissible stress cat=0.27fck= 1.2
N/mm2
Actual stress is equal to permissible
stress, hence safe.
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Step 4: Curtailment of hoop steel:
Quantity of steel required at 1m, 2m, and
at top are tabulated. In this table the
maximum spacing is taken an 3 x 160 =
480 mm
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Step 5: Vertical reinforcement:
For temperature and shrinkage
distribution steel in the form of vertical
reinforcement is provided @ 0.24 % ie.,
Ast=384 mm2.
Spacing of 10 mm diameter bar =
78.54x1000/384=204 mm c/c 200 mm
c/c
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Step 6: Tank floor:
As the slab rests on firm ground, minimum
steel @ 0.3 % is provided. Thickness of
slab is assumed as 150 mm.
8 mm diameter bars at 200 c/c is provided
in both directions at bottom and top of
the slab.
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Design of Circular Tanks resting on
ground with Rigid base
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• Due to fixity at base of wall, the upper
part of the wall will have hoop tension
and lower part bend like cantilever.
• For shallow tanks with large diameter, hoop
stresses are very small and the wall act more
like cantilever
• For deep tanks of small diameter the
cantilever action due to fixity at the base is
small and the hoop action is predominant
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• The exact analysis of the tank to determinethe portion of wall in which hoop tension ispredominant and the other portion in whichcantilever action is predominant, is difficult
1. Simplified methods of analysis are
2. Reissner’s method
3. Carpenter’s simplified method
4. Approximate method
5. IS code method
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IS code method
• Tables 9,10 and 11 of IS 3370 part IV gives coefficients for computing hoop tension, moment and shear for various values of H2/Dt
• Hoop tension, moment and shear is computed as
T= coefficient ( wHD/2)
M= coefficient (wH3)
V= coefficient (wH2)
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• Thickness of wall required is computed from
BM consideration
where,
Q= ½ cbcjk
j=1-(k/3)
b = 1000mm
Qb
Md=
stcbc
cbc
m
mk
+
=
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IS code method
• Over all thickness is then computed as
t = d+cover.
• Area of reinforcement in the form of vertical
bars on water face is computed as
• Area of hoop steel in the form of rings is
computed as
jd
MA
st
st
=
st
1st
TA
=
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IS code method
• Distribution steel and vertical steel for outer
face of wall is computed from minimum
steel consideration
• Tensile stress computed from the following
equation should be less than the
permissible stress for safe design
st
cA)1m(t1000
T
−+=
the permissible stress is 0.27 fck
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IS code method
• Base slab thickness generally varies
from 150mm to 250 mm and minimum
steel is distributed to top and bottom
of slab.