25-02-2013, 12:41 PM
WATER TANKS
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INTRODUCTION
(Figures 10.1 to 10.3). Tanks may be of rectangular, cylindrical, polygonal or of any other shape depending upon their capacities, economy and architectural requirements. Tanks may be :
(a) Underground water tanks,
(b) Surface tanks resting on ground, and
© Overhead water tanks.
The support for an overhead tank may be a group of columns with intermediate bracings. The bracings are provided for resisting horizontal forces as well as for reducing effective length of columns. For larger capacity tanks, staging may be in the form of single shaft circular or polygonal in plan and may be tapering. The foundation may be isolated footing, or annular footing with or without beam, or raft (Figure 10.3). For simplicity of analysis and design, only those types of tanks and staging have been described which may be analysed by membrane theory.
DESIGN AND DETAILING SPECIFICATIONS OF WATER TANKS
Specifications
Water tanks must be leak proof. The designed sections of its components must be crack resistant, and even if the cracks are formed they must be very fine and evenly distributed, so that the full section may be effective to prevent leak and to protect reinforcement from corroding.
To meet above requirements IS : 3370 (Part I to IV), Code of Practice for Concrete Structures for Storage of Liquids, prescribes the following:
(a) The elements shall be designed by Working Stress Method assuming full cross section (including cover) uncracked and effective by allowing limited tensile strength of concrete δct (Table 10.1).
Curtailment of Hoop Reinforcement
Theoretically hoop reinforcement requirement gradually reduces and approaches to zero at the top. Hence curtailment becomes necessary specially where the tank is deep. This is explained through Example 10.1.
Design of Base Slab
The pressure on the ground due to water, wt. of wall and self weight of base slab is almost uniformly distributed over the whole base area and it is equal and opposite to the uniformly distributed reaction. Hence, a minimum thickness of slab with minimum reinforcement is provided for base slab. To make the joint between wall and base slab flexible, such that the wall may deform at the junction without any restraint, the wall and the base are separated from each other and the joint is filled with bitumen (Figures 10.1 and 10.2).
THEORY OF STRUCTURES-II
Construction has been an activity which has witnessed many civilizations. Different construction materials and techniques have been tried in the past. In modern times, for construction of any type of structure generally the choice for material is confined to either concrete or steel. Concrete, though strong in compression, is extremely weak in tension. Steel, on the other hand, is very strong in tension as well as in compression. Therefore, the combination of concrete and steel has proved to be most suitable choice to withstand stresses.
The widespread use of reinforced concrete in a variety of structural members in different type of structures has compelled a proper understanding of the design and detailing procedures. All reinforced concrete structures need proper designing taking into account tensile and compressive stresses, shears, creep and thermal effect, etc. This course, entitled “Theory of Structures-II”, covers the key aspects of design and detailing of different reinforced concrete structures.