Elevated Water Tank

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Chapter 8: Elevated Water Tank 126

8.1 INTRODUCTION:

Reinforced Concrete and eventually prestressed concrete are generally the most convenient material for liquid tank and containers.

Due to the internal pressure of the liquid stored in such structure the walls and floors are mainly subjected to tensile force. Bending moments and eccentric tension which cause in most causes critical tensile stresses on the surface of the different elements facing the liquid. If such elements are designed according to the general principle adopted in ordinary reinforced concrete. Cracks will be developed and the liquid contained in tank has the possibility to penetrate under its hydrostatic pressure through the cracks and cause rusting of the steel reinforcement. Therefore, special provisions must be taken to prevent the formation of such cracks. Such provisions generally lead to an increased thickness of the walls towards their foot and at their other concerns. If the effect of this increase is not to be considered. It may lead to serious defects so that at a thorough investigation is absolutely essential.

Porous concrete or concrete containing honey combing or badly executed joints lead to the same possibility of rusting with all its ill effect to the structure. Therefore, dense, water-tight concrete is one of the essential requirements of liquid tanks and containers, the necessary provision required in the careful design of the mix and in the execution of the structure must be taken.

The protection of the finished concrete structure by convenient plastering, painting or casing as well as its thorough curing must be carefully studied.

The previous investigation gives some points showing that liquid containers are delicate structure and need, due to their intensive use in structural engineering, special care and knowledge in the design execution and protection.

1.1 Elevated Storage:

In elevated areas which cannot be supplied from ground reservoir it will be necessary to pump the average day demand and this can be achieved with or without storage.

Pumping without storage is the least desirable method of distribution, since it provides no reserve flow in the event of power failure and pressures will fluctuate substantially with variations in flow. Since the flow must be constantly varied to match an unpredictable demand, sophisticated control systems are required. Peak water use and thus peak power consumption are likely to coincide with periods of already high power use, increasing power costs. Systems of this kind have the advantage of permitting

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increased pressure for fire fighting, although individual users must then be protected by pressure reducing valves.

Pumping with storage is the most common method of distribution. Water is pumped at a more or less uniform rate, with flow in excess of consumption being stored in elevated storage tanks distributed throughout the system. During periods of high demand, the stored water augments the pumped flow, thus helping to equalize the pumping rate and to maintain more uniform pressure in the system.

1.2 Jordan's Practice Regarding Elevated

Storage

The current practice in Jordan for supplying elevated areas which cannot be supplied from ground storage reservoirs is to pump to a standard 500 cubic meter elevated tank and supplying the consumers directly from the tank. The tank is always 500 cubic meters regardless of the size or nature of population served or the estimated current or future demand. The tank provides head only and not storage. Consequently, the pumps must be sized to provide the demand flow in excess of the stored volume for the expected life of the pumps. Alternatively, pumps of different sizes or variable speed pumps may be used for different demand periods. This alternative of variable speed motors involves costly equipment, sophisticated controls, and well trained operators and is totally dependent on a reliable power supply. Replacement and maintenance are also frequent and costs high The reliability of supply is directly dependent on power supply and also on a steady and reliable supply of water. This system is also highly sensitive to future power costs increases, and would therefore be the least desirable system for any country without long term energy resources or one that is dependent on importation of energy resources. If pumping is done with adequate storage or with different size pumps for different demand periods then the pumping station would be smaller, and annual power costs lower, pump life longer, and maintenance and replacements fewer.

9 In this project; the tank with 1500 m3

capacity is expected to achieve both storage, and head that needed to satisfy the expanded 2025 population water demand.

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8.2

PRODUCTION OF WATER TIGHT CONCRETE:

Dense concrete, free from or honey combing is the main requirement for water-tightness. Porous concrete having cracks on the liquid side allow the liquid in the in the tank, under its hydrostatic pressure to penetrate thorough the concrete and cause rusting of the steel reinforcement leading to all its serious effects on the structure. Dense water-tight concrete can be achieved through careful selection of aggregate, suitable granular composition, use of low water cement ratio, sufficient cement content and thorough mixing, compaction and curing.

We give, in the following, a short accurate about the main factors affecting the density and water- tightness of concrete.

2.1 Composition, Mixing and

Compaction:

Dense concrete can be produced if the voids are reduced to a minimum, such a provision can be attained through the choice of a convenient mix composed of fine aggregate (smaller than 5 mm), medium aggregate ( between 5 mm and 10 mm). The maximum grain size is to be according to the thickness of the element in which it’s used and preferably not more than 30 mm in reinforced concrete water structure.

In general cases, the cement content in the mix is generally 350 kg. per cubic meter finished concrete, in small tanks and in cases of low stresses, the cement dose may be reduced to 300 kg/m3. Richer doses with a maximum of 400kg/m3 may be used for big under-ground tanks in wet medium. The use of high cement doses in dry weather under normal condition is not recommended because the shrinkage tensile stresses causing cracks in the concrete increases of the cement concrete.

It is recommended to use the least possible amount of mixing water giving good plastic concrete. The water-cement ratio to be specified depends on the method of compaction – by hand or by mechanical vibration- and on the nature of the concrete constitutes, in this respect figures based on a slump test are recommended. Excess of mixing water is to be avoided as it leads to porous concrete due to the evaporation of the surplus water not needed for the chemical action and increases the shrinkage strains.

In big tanks, the use of mechanical mixers with automatic water control is essential.

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To produce dense concrete, good compaction is necessary as it compensates for the possible gaps in the granulometric composition of the aggregate. The use of source and immersion vibrators gives satisfactory results.

2.2 Admixture

Some admixtures have a mechanical effect on concrete while others have a chemical effect. Admixture having a mechanical lubricants effect increase the workability of concrete mixes, thus allowing a reduction in the water content which in turn results in an increase in the strength and water-tightness of concrete (e.g. baraplast and air entraining agents). Admixture having chemical effects on the concrete mix are to be used only when tests prove that they very no ill effect on the concrete or the steel throughout their lifetimes.

Other admixture help to seal the pores in the concrete, their presence is to be considered only above mentioned steps and not in any way as a replacement.

2.3 Curing

Concrete undergoes a volume change during hardening; it shrinks in dry weather and swells under water. Shrinkage cause tensile stresses in the concrete. If such stresses are developed and act on fresh concrete of low strength, they cause shrinkage cracks. It is absolutely essential to prevent such stresses from being developed until the concrete has gained sufficient strength to resist them. This can be done by intensive curing of fresh concrete (Keeping it continuously wet) starting immediately after the final setting of the concrete and a minimum period of 15 days.

2.4 Surface Treatment, paint And

Casing

The most effective surface treatment is cement mortar plaster composed of 600 to 650 kg cement gun. The thickness may be chosen 1.5 to 2.0 cm. It is recommended to apply such a plaster on side facing the liquid after filling the tank with water for 7 days. The surface should be thoroughly cleaned by wire brushes before the application of the cement gun. In this manner, the preliminary cracks which may appear after the first filling of the tank will be sealed by the plaster. Moreover, the plaster will not be subjected to a big part of the plastic strains due to water pressure.

Paints (e.g. barafluate, baranormal, bituminous paints…. etc) may also be either directly on the concrete surface or on the cement plaster. The use of special

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paints whose is to close the surface pores (such as glass paints, plastic paints watertight casing, lining with metallic sheets-e.g. stainless steel or water tight tiling) may be of advantage.

Any material for water-tightness either as admixture or surface treatment must not be used unless it is proved by experiments to be suitable for the purpose.

3. GENERAL DESCRIPTION

The elevated water tank was modeled using SAP-2000 software with the following properties:

a) 42.5 m height above ground surface,

b) 700 mm thick tapered to 300 mm thick circular hollow shear wall with 30 m height above the ground to the bottom of the cone,

c) 300 mm thick inverse cone with 5 m diameter at 30 m height above the ground and 24 m diameter at 40 m height above the ground,

d) 1.5 m thick circular raft foundation with 13 m diameter lies 2.5 m below the ground surface,

e) 250 mm thick top slab.

4. SEISMIC LOAD ANALYSIS

4.1 Occupancy Category

Î Category No. II: Substantial Hazard Occupancy

4.2 Design response spectrum

1. Site Class: Hard Rock 2. Soil Type: A

3. Basic ground motion parameters (Ss, S1) – Zonation map Ss = 0.57g

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S1 = 0.23g

4. Determine site coefficient adjustment factors (Fa, Fv) – IBC 2006 Fa = 0.8

Fv = 0.8

5. Determine design ground motion parameters (SDS, Sd1) • Maximum expected earthquake coefficients:

SMS= FaSs = 0.8 * 0.57 = 0.456

SM1= FvS1 = 0.8 * 0.23 = 0.18

• Design earthquake coefficients:

SDS= 2/3SMS = 0.3g SD1= 2/3SM1 = 0.12g • Period: To= 0.2 SD1/SDS = 0.08 sec Ts= SD1/SDs = 0.4

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Chapter 8:

4.3 catego

Choose

t acceleration 0.167 S 0.067 S

4.4

For Seismi

4.

** For Ord - Re - Sy Elevated W

Deter

ory

the serve ca n: SDS 0.33 SD1 0.13g

Dete

ic Design C

5 S

P

dinary Bear esponse (str ystem over-Water Tank Figure

rminat

ategory base g ………… g …………

ermina

ategory II; t

Struct

Parame

ring Reinfor rength) mod -strength pa e 8.1: (Design

ion of

ed on short ….. SDC: B …. SDC: B

ation

the IE = 1.2

tural

eters

rced Concre dification co arameter = Ω Response Spe

f seism

Period Acc

of im

5

syste

(R, C

ete Shear W oefficient = Ωo”Wo” = 4 ectrum)

mic de

celeration &

mporta

em and

C

_d

,

Ω

_o

Wall; the sys R = 4.0 .0

SDC: B

esign

& One - Sec

ance f

d syst

)

tem parame 132 cond Period

factor

tem

eters are:

d

r

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- Deflection amplification factor = Cd = 2.5

4.8 Redundancy factor (

ρ )

-

For seismic Design Categories

B: ρ = 1.0

4.9 Effective Seismic Weight of the

structure, W

Total Ultimate Load.

w

1= 1.2 = 1.2 (24) (110) = 3167 kN

w

2 = 1.2 = 1.2 (24) (78.54) = 2262 kN

w

3 = 1.2 = 1.2 (24) (39.27) = 1131 kN

w

4 =

=1.2( ) + 1.6( )

= 1.2 ( γconcrete Vcore 4 + γconcrete Vcore + γconcrete Vtop slab + Area of top slab * qsnow) + 1.6 * (

γ

waterVtank)

= 1.2((24 *39.27) 136.7 24 24 24 0.4 24 0.4 )

+ 1.6(1500 9.81)

= 9530 + 23540 = 33070 kN

4.10 ELF procedure “Lateral Load

Computation”

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Chapter 8: Elevated Water Tank 134 Cs = SDS = . (0.3) = 0.0937 Cs, MIN = 0.044 IE SDS = 0.044*1.25*0.3 = 0.0165 Cs, MAX = SD1

9 Tcomp. = 1.98 sec. by SAP2000 software.

9 Cs, MAX = SD1 = . . (0.12) = 0.019 … USE Cs, MAX = 0.019

9 V

B

= C

s,max

W

= 0.019 (W1 + W2+ W3+ W4) = 0.019 (39630)

= 753 kN

9

V

_B

= 752 kN Using SAP-2000 software.

"

small value; concrete

core can alone resist this shear"

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Figure 8.2: (Undeformed Shape) Figure 8.3: (Mode Shapes One & Two) Figure 8.4: (Mode Shape four) Figure 8.5: (Natural Period Illustration)

8.5 EXTERNAL STABILITY

The elevated water tank might be treated as a cantilever retaining wall the purpose of stability check.

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- It must not slide horizontally. - It must not overturn.

- The resultant of the normal force that acts on the base of the footing must be within the middle third of the footing.

- The foundation must not experience a bearing-capacity failure i.e; Bearing pressure allowable bearing capacity.

- It must not settle excessively.

5.1 Sliding:

F.S “against sliding” =

∑

9 Treat the tank conservatively as cantilever retaining wall. PDriving = Inertia force "seismic force"

Presistimg = Sliding friction along the bottom of the footing (lateral earth pressure.) ¾ γ _f = 20 kN /m3 ¾ : Coefficient of friction. - = tan( 0.7) = tan(32*0.7) = 0.412 - = . . = 0.275 Fx = Cvx VB

k”higher mode effect” = 0.5 T + 0.75 ; for 0.5<T<2.5 sec = 0.5 (1.29) + 0.75 Î k = 1.4

C

vx

=

_∑ ∑ = 3167 (5)1.4 + 2262 (15)1.4 + 1131 (25)1.4 + 33070 ( 36.25)1.4 = 5273000 Cv1= . = 0.006 Cv2= . = 0.019 Cv3= . = 0.019

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Cv4= . . = 0.956 F1= Cv1 VB = 0.006 (752) = 4.51 kN F2= Cv2 VB = 0.019 (752) =14.29 kN F3= Cv3 VB = 0.019 (752) = 14.29 kN F4= Cv4 VB = 0.956 (752) =718.91 kN ∑ 752 kN OK

∑ = lateral earth force + friction under the base of raft = 0.5 γh2 D + μa N = 0.5 * 20 kN/m3 * 2.52 m2 * 13 m + 0.275 * (39630+4778) = 13,024.7 kN

¾

F.S =

, .

= 17

>

1.5 “

ok for sliding

”

5.2 Overturning:

F.S “against overturning” = ∑ ∑ MDriving = ∑ = 4.51 (9) + 14.29 (19) + 14.29 (29) + 718.91 (40.25) = 29,662 kN .m MResisting = ∑ = 812.5 (2.33) + 39630 (6.5) + 4778 (6.5) + 235.6 (6.5) = 292100 kN .m F.S overturning = _, = 9.85 > 2.5

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8.6 STRUCTURAL DESIGN:

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Figure 8.7: (M22)

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Chapter 8: Elevated Water Tank 140 Figure 8.9: (Cross section detailing in Cone) Figure 8.10: (Cross section detailing in Top Slab – Top Reinforcement)

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Figure 8.11: (Cross section detailing in Top Slab – Bottom Reinforcement)