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Design of Water Tanks:
Part (2) Underground Tanks
Prof. Dr. Hamed Hadhoud
Cairo University Prof. Dr. Hamed Hadhoud
1
Design Steps
Cairo University Prof. Dr. Hamed Hadhoud
Stability
2
Design of Critical Sections
Check of Stresses on Soil
Uplift Check
Strength
1. Wide Underground Tanks
(e.g. Swimming Pools)
Stability Checks
Cairo University Prof. Dr. Hamed Hadhoud
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
Cairo University Prof. Dr. Hamed Hadhoud
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)
Check of Stresses on Soil
Cairo University Prof. Dr. Hamed Hadhoud
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
Check of Stresses on Soil
Cairo University Prof. Dr. Hamed Hadhoud
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
Check of Stresses on Soil
Cairo University Prof. Dr. Hamed Hadhoud
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
Design of Critical Sections
Cairo University Prof. Dr. Hamed Hadhoud
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
Design of Critical Sections
Cairo University Prof. Dr. Hamed Hadhoud
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
Cairo University Prof. Dr. Hamed Hadhoud
pressurewatersA )
pressureearthsA )
Usually Minimum RFt
Deep beam Min RFT for potential relative settlement
11
Example (1)
Cairo University Prof. Dr. Hamed Hadhoud
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
Cairo University Prof. Dr. Hamed Hadhoud
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
Cairo University Prof. Dr. Hamed Hadhoud
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
Cairo University Prof. Dr. Hamed Hadhoud
15
Design of critical sections
Cairo University Prof. Dr. Hamed Hadhoud
16
Cairo University Prof. Dr. Hamed Hadhoud
17
Case (2): Maintenance Case
1.8 m
Wwall)1
Wwall)2
o
1.0 m
Wfloor
Wsoil
Same steps are followed
Cairo University Prof. Dr. Hamed Hadhoud
Detailing
6F10/m
5F16
10F10/m
6F10/m 10F
10/m
18
6F10/m
6F10/m
2. Short Underground Tanks
Cairo University Prof. Dr. Hamed Hadhoud
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
Cairo University Prof. Dr. Hamed Hadhoud
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)
Check of Stresses on Soil
Cairo University Prof. Dr. Hamed Hadhoud
GWT
)( anyifWWWWW roofwallsfloorwaterTotal Full
capacitybearingsoilAreaBase
Wf Total
gross
If stresses on soil are unsafe; • Increase floor (use toe) • Use soil replacement • Use deep foundations
Cairo University Prof. Dr. Hamed Hadhoud
Design of Critical Sections
(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
stripmforwallsfromloadsw
)1(1
Cairo University Prof. Dr. Hamed Hadhoud
hw
Free end Top Beam
24
Design of Critical Sections
(1) If no tension piles used
Cairo University Prof. Dr. Hamed Hadhoud
Design of Critical Sections
(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
stripmforwallsfromloadsw
)1(1
Cairo University Prof. Dr. Hamed Hadhoud
hw
Free end Top Beam
26
Design of Critical Sections
(2) Using tension piles
Cairo University Prof. Dr. Hamed Hadhoud
Example (2)
27
Cairo University Prof. Dr. Hamed Hadhoud
Example (2)
28
Cairo University Prof. Dr. Hamed Hadhoud
Example (2)
29
Cairo University Prof. Dr. Hamed Hadhoud
Example (2)
30
Cairo University Prof. Dr. Hamed Hadhoud
Example (2)
31