Chapter (8) Design of water tank.doc

Chapter 8 : Design of Water Tank

C h a p t e r

Design of water tanks .Introduction 8.1 .Types

of water tanks 8.2 Cracks in water tanks 8.3 8.4 A Selected example for design of a water tank .

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:Introduction 8.1 Water tanks are the structural elements that store water, liquid petroleum, petroleum products and similar liquids. Their shapes may be circular, rectangular spherical etc. The force analysis of the reservoirs or tanks is about the same irrespective of the chemical nature of the product.The designer has to design all the tanks as crack free structures to eliminate any leakage. Industrial wastes can also be collected and processed in concrete tanks with few exceptions.The petroleum product such as petrol, diesel oil, etc. are likely to leak through the concrete walls, therefore such tanks need special membranes to prevent leakage.

8.2 Types of water tanks: 1) Concrete Tanks : These are referred to as underground tanks that are built on the site itself, but they can be used above-ground. Concrete tanks are generally long-lasting but are subject to cracking if placed underground in clay soil. The above ground versions have the advantage of keeping the water cooler than other tank types since light can't penetrate.

Figure 8.1: (Concrete Tank).

2) Metal Tanks: This variant can either be made from stainless steel or galvanized steel. They can be transported and assembled easily and can be situated in any place you desire. Metal tanks normally come in standard shapes. If you are having a Metal tank installed you should inspect it thoroughly when it is in place, as they 124


are prone to transport damage (including the breaking of welds if the tank flexes too much). Metal tanks are usually not used underground.

Figure 8.2: (Metal Tank).

3) Rainwater Tanks: Rainwater tanks are installed for the purpose of collecting all the rainwater that runs off your roof via installation of roof gutters. The water collected can be used for many things like watering your crops, feeding animal, washing your car and the like. However, the water collected by this means needs to undergo a certain process before it can be used for drinking. 4) Fiberglass Tanks: these tanks are strong, durable and able to withstand extreme temperatures. However, their popularity has suffered with the advent of cheaper poly tanks. They are mostly used for above-ground applications

Figure 8.3: (Fiberglass Tank).

8.3 cracks in concrete water tanks:

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Cracking may be caused due to restraint to shrinkage, expansion and contraction of concrete due to temperature or shrinkage and swelling due to moisture effects. Such restraint may be caused by : (i) The interaction between reinforcement and concrete during shrinkage due to drying. (ii) The boundary conditions. (iii) The differential conditions prevailing through the large thickness of massive concrete.

To minimize the possibilities of cracking use small size bars placed properly, leads to closer cracks but of smaller width. The risk of cracking due to temperature and shrinkage effects may be minimized by limiting the changes in moisture content and temperature to which the structure as a whole is subjected. The risk of cracking can also be minimized by reducing the restraint on the free expansion of the structure with long walls or slab founded at or below ground level. Restraint can be minimized by the provision of a sliding layer. This can be provided by founding the structure on a flat layer of concrete with interposition of some material to break the bond and facilitate movement. In case length of structure is large it should be subdivided into suitable lengths separated by movement joints, especially where sections are changed the movement joints should be provided. Where structures have to store hot liquids, stresses caused by difference in temperature between inside and outside of the reservoir should be taken into account.

:An

example for design of water tank 8.4 126


L x

=

4.1m , L y

f y

=

420 Mpa

E

c=

E s

4700

=

×

=

3.5m

, h

( f c ' )

=

=

, γ w

4.0m

3

=

9.81 KN / m ,

f

c'

=

24 Mpa

, t = 400mm , d = 360mm

4700 × ( 24)

=

23025 Mpa

200000 Mpa

Figure 8.4: (Water Tank dimensions). • •

2-way direction Capacity = LxBxh = 4.1 x3.5x4 = 57.4 m3

* Design of Roof Slab:

Lb/La = 4.1/3.5 = 1.17<2 "Two-way solid slab" Assume H=300mm then d=260mm. Dead load Calculation:

Total Dead load = 8.78 KN/ m 2 Live Load = 2.5 KN/ m 2 Wu = 1.2 DL+ 1.6 LL = 1.2 (8.78) + 1.6 (2.5) =14.53 KN/ m2 M = La / Lb = 3.5/4.1 = 0.854

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From table of bending coefficient moment find Ca and Cb for dead and live load :

M "short direction " = Ca xWDxl 2 + CaxWLx l2 =0.028x1.2x8.78x3.52+0.045x1.6x2.5x3.52 = 5.82 KN.m M "Long direction " = Cb xWDxl2 + CbxWLx l2 =0.009x1.2x8.78x4.1 2+0.014x1.6x2.5x4.1 2 = 2.54 KN.m

* According to ACI thickness of two way slab : L/30 = h " From table 9.5 C " h= 4100/30 = 136.66mm take it 200mm h=200mm then d=160mm * Steel Reinforcement: 1. Short Direction :

2. Long Direction :

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* Design of Tank Walls: Loading Case: The water tanks empty with external earth pressure:

= tan2 (45-(30/2)) = 0.333 Ps = 20x4x0.333 = 26.64 KN/m * Analysis using computer program " prokon " : Required slab depth : assume h=400mm then d=360mm. Mu = 72.23 KN.m

Secondary steel horizontal:

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In wall design we use As = 0.002 x 1000 x 400 = 800 mm2

* Check crack: n

=

Es Ec

200000 =

23168 .66

=

8.6

* Designs for Base of tank:

Load roof slab weight = L B t = 4.2x3.5x.4x25 =147 KN

Total weight = 147 + 616 = 763 KN. Actual bearing capacity " q " = weight / area = 763 / (4.7x4) = 40.58 KN/ m2 M =La/Lb = 4/4.7 = 0.85 KN.m From table of bending coefficient moment find Ca and Cb for dead and live load : M = 0.04 x 40.58 x 4 2 = 25.97 KN.m

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* According to ACI thickness of two way slab: L/30 = h "From table 9.5 C" h= 4700/30 = 157 mm take h = 200mm then d = 200-70= 130 mm .

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Chapter (8) Design of water tank.doc

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