Domes and Shell theory

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The details of dome and shell theory
  A water tank is used to store water to tide over the daily requirement. In the construction of concrete structure for the storage of water and other liquids the imperviousness of concrete is most essential .The permeability of any uniform and thoroughly compacted concrete of given mix proportions is mainly dependent on water cement rat io .The increase in water cement rat io results in increase in the permeability .The decrease in water cement rat io will therefore be desirable to decrease the permeability, but very much reduced water cement ratio may cause compact ion difficulties and prove to be harmful also. Design of liquid retaining structure has to  be based on the avoidance of cracking in the concrete having regard to its tensile strength. Cracks can be prevented by avoiding the use of thick timber shuttering which prevent the easy escape of heat of hydration from the concrete mass the risk of cracking can also be minimized by reducing the restraints on free expansion or contraction of the structure. 1.   Objective: a)   To make a study about the analysis and design of water tanks.  b)   To make a study about the guidelines for the design of liquid retaining Structure according to is code. c)   To know about the design philosophy for the safe and economical design of water tank. d)   To develop programs for the design of water tank of flexible base and rigid base and the underground tank to avoid the tedious calculations. e)   In the end, the programs are validated with the results of manual calculation given in concrete Structure. 2.   Sources of water supply: The various sources of water can be classified into two categories: Surface sources, such as a)   Ponds and lakes,  b)   Streams and rivers, c)   Storage reservoirs, and d)   Oceans, generally not used for water supplies, at present.  Sub-surface sources or underground sources, such as a)   Springs,  b)   Infiltration wells, and c)   Wells and Tube-wells 3.   WATER TANKS 3.1   CLASSIFICATIONS: Classification based on under three heads: a)   Tanks resting on ground  b)   Elevated tanks supported on staging c)   Underground tanks. Classification based on shapes a)   Circular tanks  b)   Rectangular tanks c)   Spherical tanks d)   Intze tanks e)   Circular tanks with conical bottom 4.   DESIGN REQUIREMENT OF CONCRETE (I. S. I) In water retaining structure a dense impermeable concrete is required therefore, proportion of fine and course aggregates to cement should be such as to give high quality concrete. Concrete mix lesser than M 20 is not used. The minimum quantity of cement in the concrete mix shall be not less than 30 KN/m 3 .The design of the concrete mix shall be such that the resultant concrete is su efficiently impervious. Efficient compact ion preferably by vibration is essential. The  permeability of the thoroughly compacted concrete is dependent on water cement ratio. Increase in water cement ratio increases permeability, while concrete with low water cement ratio is difficult to compact. Other causes of leakage in concrete are defects such as segregation and honey combing. All joint s should be made watertight as these are potential sources of leakage.  Design of liquid retaining structure is different from ordinary R.C.C. structures as it requires that concrete should not crack and hence tensile stresses in concrete should be within permissible limits. A reinforced concrete member of liquid retaining structure is designed on the usual  principles ignoring tensile resistance of concrete in bending. Additionally it should be ensured that tensile stress on the liquid retaining ace of the equivalent concrete section does not exceed the permissible tensile strength of concrete as given in table 1. For calculation purposes the cover is also taken into concrete area. Cracking may be caused due to restraint to shrinkage, expansion and contract ion of concrete due to temperature or shrinkage and swelling due to moisture effects. Such restraint may be caused by. a)   The interaction between reinforcement and concrete during shrinkage due to drying.  b)   The boundary conditions. c)   The differential conditions prevailing through the large thickness of massive concrete Use of 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 inters position 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 sect ions 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. The coefficient of expansion due to temperature change is taken as 11 x 10 -6 /° C and coefficient of shrinkage may be taken as 450 x 10 -6 for initial shrinkage and 200 x 10 -6 for drying shrinkage.  5.   GENERAL DESIGN REQUIREMENTS (I.S.I)   5.1   Plain Concrete Structures : Plain concrete member of reinforced concrete liquid retaining structure may be designed against structural failure by allowing tension in plain concrete as per the permissible limit s for tension in  bending. This will automatically take care of failure due to cracking. However, nominal reinforcement shall be provided, for plain concrete structural members. 5.2   Permissible Stresses in Concrete: a)   For resistance to cracking : For calculations relating to the resistance of members to cracking, the permissible stresses in tension (direct and due to bending) and shear shall confirm to the values specified in Table 1.The permissible tensile stresses due to bending apply to the face of the member in contact with the liquid. In members less than 225mm ∅   thick and in contact with liquid on one side these permissible stresses in bending apply also to the face remote from the liquid.  b)   For strength calculations : In strength calculations the permissible concrete stresses shall be in accordance with Table 1. Where the calculated shear stress in concrete alone exceeds the permissible value, reinforcement acting in conjunct ion with diagonal compression in the concrete shall be provided to take the whole of the shear. 5.3   Permissible Stresses in Steel: a)   For resistance to cracking . When steel and concrete are assumed to act together for checking the tensile stress in concrete for avoidance of crack, the tensile stress in steel will be limited by the requirement that the permissible tensile stress in the concrete is not exceeded so the tensile stress in steel shall be equal to the product of modular rat io of steel and concrete, and the corresponding allowable tensile stress in concrete.  b)   For strength calculations : In strength calculations the permissible stress shall be as follows:    Tensile stress in member in direct tension 1000 kg/cm 2 .    Tensile stress in member in bending on liquid retaining face of members or face away from liquid for members less than 225mm thick 1000 kg/cm 2 .
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