ANALYSIS AND DESIGN OF

WATER TANKS

Dr. Hasaan Irtaza, Professor

Department of Civil Engineering, A.M.U., Aligarh – 202002, India

LEARNING OUT COME

Review

Types Of Tanks

Design Of Rectangular Water Tank Resting On

Firm Ground

Design of Circular Water Tank with Flat Bottom

Design Of Intze Type Water Tank (Over Head

Tank)

INTRODUCTION

Tanks are built for storing water, liquid petroleum/petroleum products and similar liquids

Tanks are designed as crack free structures toeliminate any leakage

Permeability of concrete is directly proportional towater cement ratio. Optimum water cement ratio tobe used.

Cement content ranging from 330 Kg/m3 to530 Kg/m3 is recommended in order to keepshrinkage low.

Design Philosophy

A water retaining structure should be design using workingstress method but all relevant limit states must beconsidered in design to ensure an adequate degree of safetyand serviceability.

The maximum calculated surface width of cracks for directtension and flexure must not exceed 0.2 mm.

This can be achieved if the stress in steel under serviceconditions does not exceed 150 Mpa in high strengthdeformed bars.

The minimum grade of concrete used is M25. Higher thegrade, lesser is the porosity of concrete.

The impermeability of concrete is basic requirement for liquid

retaining structures.

The impermeability of concrete not only have direct effect on

the leakage but also affects durability, resistance to leaching,

chemical attack, erosion, abrasion, frost damage and

protection from corrosion of embedded steel.

The permeability of concrete of a given mix proportions is

largely depend upon the water cement ratio.

It is essential to select a concrete mix compatible with the

available particle shape and grading to have a high degree of

workability.

Water Tank Classification

Based on

Placement of Tank

Based on

Shape of Tank

1. Resting on Ground

2. Under Ground

3. Elevated

1. Circular

2. Rectangular

3. Intze Tank

4. Spherical

5. Conical Bottom

Circular Tank Resting on Ground

Rectangular Tank Resting on Ground/Support (Wall)

Underground Water Tank

Spherical/ Conical Water Tank

Intze Tank

Permissible Stresses in Concrete

The permissible tensile stresses in concrete on water face

(MPa) as per IS:3370 [See Part I to IV]

StressGrade of Concrete

M20 M25 M30 M35

Direct Tension 1.2 1.3 1.5 1.6

Bending Tension 1.7 1.8 2.0 2.2

Permissible Stresses

Permissible Compression Stress in Concrete (MPa)

Permissible stresses in reinforcement (MPa)

StressGrade of Concrete

M20 M25 M30 M35

Direct Compression 5.0 6.0 8.0 9.0

Bending Compression 7.0 8.5 10.0 11.5

Stress High Strength Deformed Steel

Tensile stress in direct tension,

bending and shear150

Compressive stress in columns

subjected to direct load 175

Minimum Reinforcement

The minimum reinforcement in walls. floors and roof in

each of two directions at right angles shall have an area

of 0.3% of the concrete section in that direction for

sections upto 100 mm thick. For sections of thickness

greater than 100 mm and less than 450 mm, minimum

reinforcement in each of the two directions shall be

linearly reduced from 0.3% for 100 mm thick section to

0.2% for 450 mm thick section. For sections of thickness

greater than 450 mm , minimum reinforcements in each

of the two directions shall be kept at 0.2%

In walls less than 200 mm thickness, the calculated amount ofreinforcement may be placed in one face.

When reinforcement is placed in two layers, the two layers ofreinforcement steel shall be placed near each face of the section tomake up the minimum reinforcement.

For liquid faces of parts of members either in contact with the liquidor enclosing the space above the liquid, the minimum cover to allreinforcement should be 25 mm or diameter of the bars, which everis greater.

In wall slabs less than 200 mm in thickness. the calculated amountof reinforcement may all be placed in one face . For ground slabsless than 300 mm thick the calculated reinforcement should beplaced in one face as near as possible to the upper surface consistentwith the nominal cover. Bar spacing should generally not exceed300 mm or the thickness of the section, whichever is less

JOINTS IN WATER TANKS

Types of Joints

The various types of joints may be categorized under threeheads:(a) Movement joints(b) Constructions joints(c) Temporary open joints.

Movement joints: These require the incorporation of specialmaterials in order to maintain water-tightness whileaccommodating relative movement between the sides of thejoints. All movement joints are essentially flexible joints.Movement joints are of three types(i) Contraction joint(ii) Expansion joint(iii) Sliding joint.

(i) Contraction joint: A contraction joint is a typical movement joint which

accommodates the contraction of the concrete. The joint may be either a

complete contraction joint in which there is discontinuity of both concrete and

steel, or it may be partial contraction joint in which there is discontinuity of

concrete but the reinforcements run through the joint. In both cases, no initial

gap is kept at the joint, but only discontinuity is given during construction. In

the former type, a water bar is inserted while in the later type, the mouth of

the joint is filled with joint sealing compound and then strip painted. A water

bar is a pre-formed strip of impermeable material (such as a metal, polyvinyl

chloride or rubber). Joint sealing compounds are impermeable ductile

materials which are required to provide a water-tight seal by adhesion to the

concrete throughout the range of joint movement. The commonly used

materials are based on asphalt, bitumen, or coal tar pitch with or without

fillers such as limestone or slate dust, asbestos fibre, chopped hemp, rubber or

other suitable material. This are usually applied after construction or just

before the reservoir is put into service by pouring in the hot or cold state, by

trowelling or gunning or as preformed strips ironed into position.

(ii) Expansion joint: It is a movement joint with complete discontinuityin both reinforcement and concrete, and is intended to accommodateeither expansion or contraction of the structure. In general such a jointrequires the provision of an initial gap between the adjoining parts of astructure which by closing or opening accommodates the expansion orcontraction of the structure. The initial gap is filled with joint filler.Joint fillers are usually compressible sheet or strip materials used asspacers. They are fixed to the face of the first placed concrete andagainst which the second placed concrete is cast. With an initial gap of30 mm, the maximum expansion or contraction that the filler materialsmay allow may be of the order of 10 mm. Joint fillers, as at presentavailable cannot by themselves function as water-tight expansionjoints. But they can only be relied upon as spacers to provide the gapin an expansion joint when the gap is bridged by a water bar.

(iii) Sliding joint: Sliding joint is a movement joint with

complete discontinuity in both reinforcement and concrete

at which special provision is made to facilitate relative

movement.

(iv) Construction joints: A construction joint is a joint in the

concrete introduced for convenience in construction at which

special measures are taken to achieve subsequent continuity

without provision for further relative movement. It is,

therefore, a rigid joint in contrast to a movement joint which

is a flexible joint. Fig. below shows a typical construction

joint between successive lifts in a reservoir wall. The position

and arrangement of all construction joints should be

predetermined by the engineer. Consideration should be given

to limiting the number of such joints and to keeping them

free’ from possibility of percolation in a manner similar to

contraction joints.

(v) Temporary open joints: A temporary open joint is a gap

temporarily left parts of a structure which after a suitable

interval and before the structure is put into use, is filled with

mortar or concrete completely as provided below, with the

inclusion of suitable jointing material. In the former case the

width of gap should be sufficient to allow the sides to be

prepared before filling. Where measures are taken for

example, by the inclusion of suitable joining materials to

maintain the water-tightness of the concrete subsequent to the

filling of the joint, this type of joint may be regarded as being

equivalent to a contraction joint (partial or complete) as

defined.