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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.