Lecture on Design of Water Tanks 1

7/30/2019 Lecture on Design of Water Tanks 1

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Presented By:Manish Bhutani

Assistant Professor,Deptt. Of Civil Engg.

DAVIET, Jalandhar

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Learning out Come

REVIEW

TYPES OF TANKS

DESIGN OF RECTANGULAR WATER TANK

RESTING ON GROUND WITH RIGID BASE

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INTRODUCTION

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Storage tanks are built for storing water, liquidpetroleum, petroleum products and similarliquids

Designed as crack free structures to eliminateany leakage

Permeability of concrete is directlyproportional to water cement ratio.

Cement content ranging from 330 Kg/m3 to 530Kg/m3 is recommended in order to keepshrinkage low.


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INTRODUCTION

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Use of high strength deformed bars of gradeFe415 are recommended for the construction ofliquid retaining structures.

Correct placing of reinforcement, use of smallsized and use of deformed bars lead to adiffused distribution of cracks.

A crack width of 0.1mm has been accepted as

permissible value in liquid retainingstructures.


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INTRODUCTION

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Code of Practice for the storage of Liquids-IS3370 (Part I to IV)

Fractured strength of concrete is computedusing the formula given in clause 6.2.2 of IS 456

-2000 ie., fcr=0.7fck MPa.Allowable stresses in reinforcing steel as per

IS 3370 are

st= 115 MPa for Mild steel (Fe250) andst= 150 MPa for HYSD bars(Fe415)


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INTRODUCTION

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In order to minimize cracking due toshrinkage and temperature, minimumreinforcement is recommended as:

For thickness 100 mm = 0.3 %

For thickness 450 mm = 0.2% For thickness between 100 mm to 450

mm = varies linearly from 0.3% to 0.2%

For concrete thickness 225 mm, two layersof reinforcement be placed, one near waterface and other away from water face.


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INTRODUCTION

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Cover to reinforcement is greater ofi) 25 mm,

ii) Diameter of main bar

For tension on outer face:st=140 MPa for Mild steel andst=230 MPa for HYSD bars For concrete thickness 225 mm, two layers

of reinforcement be placed, one near waterface and other away from water face.


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TYPES OF WATER TANK

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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 heightof 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 at centre.

When the thickness > 225mm, at each face Ast/2 of steel ashoop reinforcement is provided

The stress in concrete is computed as

If c cat, where cat=0.27fck , then no crack appears inconcrete

ststc

c

A)1m(t1000

2/HD

A)1m(A

T


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While designing, the thickness of concrete wall can beestimated as t=30H+50 mm, where H is in meters

Distribution steel in the form of vertical bars areprovided such that minimum steel area requirement issatisfied

As base slab is resting on ground and no bending stressesare induced hence minimum steel distributed at bottomand the top are provided


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While designing, the thickness of concrete wall can be

estimated as t=30H+50 mm, where H is in meters

Distribution steel in the form of vertical bars areprovided such that minimum steel area requirement issatisfied

As base slab is resting on ground and no bending stressesare induced hence minimum steel distributed at bottomand the top are provided


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Problem on Circular Tanks resting on

ground with flexible base


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Prob. No. 1:

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:

a. Plan at base

b. 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 mThe 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 beprovided

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 mm2Spacing 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 mm2Modular ratio m=Stress in concrete Permissible

stress cat=0.27fck= 1.2 N/mm2Actual 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 maximumspacing 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 isassumed 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 hoopaction is predominant


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The exact analysis of the tank to determine theportion of wall in which hoop tension is

predominant and the other portion in whichcantilever action is predominant, is difficult

1. Simplified methods of analysis are

2. Reissners method

3. Carpenters simplified method4. Approximate method

5. IS code method


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IS code method

Tables 9,10 and 11 of IS 3370 part IV givescoefficients 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= cbc

jk

j=1-(k/3)

b = 1000mm

QbMd

stcbc

cbc

m

m

k


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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 onwater face is computed as

Area of hoop steel in the form of rings is computedas

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 equationshould 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 from150mm to 250 mm and minimum steel is

distributed to top and bottom of slab.

Documents

Lecture on Design of Water Tanks 1