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 to eliminate any leakage
Permeability of concrete is directly proportional to water cement ratio. Optimum water cement ratio to be used.
Cement content ranging from 330 Kg/m
3to
530 Kg/m
3is recommended in order to keep
shrinkage low.
Design Philosophy
A water retaining structure should be design using working stress method but all relevant limit states must be considered in design to ensure an adequate degree of safety and serviceability.
The maximum calculated surface width of cracks for direct tension and flexure must not exceed 0.2 mm.
This can be achieved if the stress in steel under service conditions does not exceed 150 Mpa in high strength deformed bars.
The minimum grade of concrete used is M25. Higher the grade, 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]
Stress
Grade 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)
Stress Grade 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 shear 150
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 of reinforcement may be placed in one face.
When reinforcement is placed in two layers, the two layers of reinforcement steel shall be placed near each face of the section to make up the minimum reinforcement.
For liquid faces of parts of members either in contact with the liquid or enclosing the space above the liquid, the minimum cover to all reinforcement should be 25 mm or diameter of the bars, which ever is greater.
In wall slabs less than 200 mm in thickness. the calculated amount of reinforcement may all be placed in one face . For ground slabs less than 300 mm thick the calculated reinforcement should be placed in one face as near as possible to the upper surface consistent with the nominal cover. Bar spacing should generally not exceed 300 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 three heads:
(a) Movement joints (b) Constructions joints (c) Temporary open joints.
Movement joints: These require the incorporation of special materials in order to maintain water-tightness while accommodating relative movement between the sides of the joints. 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 discontinuity in both reinforcement and concrete, and is intended to accommodate either expansion or contraction of the structure. In general such a joint requires the provision of an initial gap between the adjoining parts of a structure which by closing or opening accommodates the expansion or contraction of the structure. The initial gap is filled with joint filler.
Joint fillers are usually compressible sheet or strip materials used as spacers. They are fixed to the face of the first placed concrete and against which the second placed concrete is cast. With an initial gap of 30 mm, the maximum expansion or contraction that the filler materials may allow may be of the order of 10 mm. Joint fillers, as at present available cannot by themselves function as water-tight expansion joints. But they can only be relied upon as spacers to provide the gap in 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.
