design of water tank.pdf

Design of Water Tanks:

Part (2) Underground Tanks

Prof. Dr. Hamed Hadhoud

Cairo University Prof. Dr. Hamed Hadhoud

1

Design Steps


Stability

2

Design of Critical Sections

Check of Stresses on Soil

Uplift Check

Strength

1. Wide Underground Tanks

(e.g. Swimming Pools)

Stability Checks


1) Uplift check (in case of ground water, during maintenance)

GWT

hw

gwhw

2) Stresses on soil (in case of full tank, just after construction)

fmax fmax

4

Dead loads > Uplift loads

Stresses on soil < allowable stress

Empty

Full

Uplift Check


5

GWT

hw

gwhw

5.1

2.1

Uplift

WFOS Tank

1. Calculate total weight including walls 2. Calculate total uplift 3. Check FOS 4. If unsafe

)( anyifWWWW roofwallsfloorTank

AreaFloorhUplift ww g

a) Increase floor thickness b) Use plain concrete inside tank (above RC floor) c) Use plain concrete below RC floor ( connected

with steel dowels) d) Use toe to include soil weight e) Use tension piles

hw

Wsoil

Toe

“For maximum level of water table” “If water table can rise”

empty (maintenance)



6

Tanks resting on firm soil (rock or coarse sand)

1. No need to check stresses on soil 2. Distance (L) of floor that carry moment reversed

from wall (unsupported length) is calculated as;

w

ML 2

M w

L

M

w= floor weight + water weight M is calculated considering wall fixed @ bottom

b

Proof

Rotation @ point b=0.0

qb= qM + qw=0.0

+

M

wL2/8

EI

MLM

6

q

EI

wLw

24

3

q

0.0246

3

EI

wL

EI

ML

w

ML 2



7

Tanks resting on medium soil (medium sand, silt or clay) 1. Stresses on soil must be checked 2. L= 0.4 H (for sandy soil) 3. L= 0.6 H (for clayey soil) 4. Consider 1m strip 5. Calculate “w”

6. Calculate “Wwall”

7. Check normal stresses on soil

M w

L

1 m

f1 f2

Wwall

Htw wfRC gg

wallwallRCwall htW g + roof reaction(if any)

2221

21

6&

6

&

L

M

L

Nf

L

M

L

Nf

or

yI

M

A

Nfy

I

M

A

Nf

Where; N= Wwall + wL M=M + Wwall (L/2) I=1*L^3/12 Y=L/2 A=1*L

)(2/

)(0

12

2

1

clayforff

sandforf

capacitybearingsoilf



8

Tanks resting on medium soil (medium sand, silt or clay) 8. If stresses on soil are unsafe; • Use toe • Use deep foundations (piles)

• Make soil replacement



9

Case of water pressure

Sec. 1 W.S.S. (M & R) Sec. 2 W.S.S. (R only) Sec. 3 A.S.S. (M3 only)

M 1 2

M R R

hwg

M 1 2

M R R

hwg

3 M3

6

3hM wg

2

2hR wg

24.0 hR wg15

3hM wg

5.33

3

3

hM wg



10

M 1 2

Case of earth pressure M

Sec. 1 WSS (M & -R) R R

“If no ground water or surcharge”

as Khg

hhe

hhKhe

ww

ass

g

g3

2

3

12

pressurewaterspressureearths

ws

AA

ee

))3

2

3

2

R

“If there is a ground water or surcharge”

2hwg

h1

h2

as Kh1g

asubmergedsas KhKh 2)1 gg

Calculate M and Calculate As) earth pressure

Surcharge “p”

aKp

Detailing


pressurewatersA )

pressureearthsA )

Usually Minimum RFt

Deep beam Min RFT for potential relative settlement

11

Example (1)


12

Design a swimming pool resting on a clayey soil with a gross bearing capacity of 120 kN/m2, with no ground water and with the dimensions shown below

3 m

15m

40 m


13

Case (1): Just after construction

M mkNh

M ww .456

310

6

33

g

3 m

mmM

t 400387103

4510

3

44

mHL 8.16.0

200 mm

400 mm

400 mm 200 mm

Check of soil stresses

1.8 m

Wwater Wwall)1

Wwall)2

Wfloor

mkNoM

kNN

kNW

kNW

kNW

kNW

floor

wall

wall

water

.37.692.04277.05.76.015)4.13032

1(@

5.82185.71542

18)8.14.0(25

5.7)32.05.0(25

15)32.0(25

42)4.13(10

2)

1)

o


14

1.8 m

1.0 m

),(838.1

37.696

8.1

5.82

),(1748.1

37.696

8.1

5.82

22

21

unsafetensionf

unsafeBCf

Use toe with length= 1.0 m

mkNoM

kNN

kNW

kNW

kNW

kNW

floor

wall

wall

water

.1.377.04227.05.71.015)4.13032

1(@

5.92285.71542

28)8.24.0(25

5.7)32.05.0(25

15)32.0(25

42)4.13(10

2)

1)

1.8 m

Wwater Wwall)1

Wwall)2

o

1.0 m

Wfloor

),(6.48.2

1.376

8.2

5.92

),(5.618.2

1.376

8.2

5.92

22

21

safenCompressiof

safeBCf


15

Design of critical sections


16


17

Case (2): Maintenance Case

1.8 m

Wwall)1

Wwall)2

o

1.0 m

Wfloor

Wsoil

Same steps are followed


Detailing

6F10/m

5F16

10F10/m

6F10/m 10F

10/m

18

6F10/m

6F10/m

2. Short Underground Tanks


Tanks with relatively short width or length

L L B

L L B

L L B

L L

B

If B 2L walls and floor behave as one unit ≤

20

Uplift Check


21

5.1

2.1

Uplift

WFOS Tank

1. Calculate total weight including walls 2. Calculate total uplift 3. Check FOS 4. If unsafe

)( anyifWWWW roofwallsfloorTank

AreaFloorhUplift ww g

a) Increase floor thickness b) Use plain concrete inside tank (above RC floor) c) Use plain concrete below RC floor ( connected

with steel dowels) d) Use toe to include soil weight e) Use tension piles

“For maximum level of water table” “If water table can rise”

GWT

hw

gwhw

empty (maintenance)



GWT

)( anyifWWWWW roofwallsfloorwaterTotal Full

capacitybearingsoilAreaBase

Wf Total

gross

If stresses on soil are unsafe; • Increase floor (use toe) • Use soil replacement • Use deep foundations



(1) If no tension piles used

B

Note: if both the width and the length of the tank are short;

B

stripmforwallsfromloadsw

)1(1

hw

)()(

1 GrashofftoaccordingorLBareafloor

wallsallfromloadsw

wwfRC ht gg

wwfRC ht gg

23

B


)1(1


hw

Free end Top Beam

24


(1) If no tension piles used



(2) Using tension piles

B

Note: if both the width and the length of the tank are short;

hw

)()(

1 GrashofftoaccordingorLBareafloor

wallsallfromloadsw

wwfRC ht gg

wwfRC ht gg

25

fRCww thw gg 1

B


)1(1


hw

Free end Top Beam

26


(2) Using tension piles


Example (2)

27


Example (2)

28


Example (2)

29


Example (2)

30


Example (2)

31

design of water tank.pdf

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