Design of circular water tanks
Concrete liquid retaining structures must be impervious. Hence, their design is based on no in cracking in concrete. Circular pre-stressed liquid retaining structures, are stressed to avoid tension in concrete.
Pre-stressed concrete liquid retaining structures require low maintenance and resist seismic forces satisfactory.
Circular pre-stressed concrete tanks are used in water treatment, water distribution, storm water management, large industrial tanks, bulk storage tanks and for storing liquefied natural gas (LNG).
The construction of the circular tanks is in the following sequence. First, the concrete core is cast and cured. The surface is prepared by sand or hydro blasting. Next, the circumferential pre-stressing is applied by strand wrapping machine. Shotcrete is applied to provide a coat of concrete over the pre-stressing strands.
IS:3370-1967 (1-4) Code of Practice for Concrete Structures for the Storage of Liquids provides guidelines for the analysis and design of liquid storage tanks. The four sections of the code are titled as follows:
Part 1: General Requirement.
Part 2: Reinforced Concrete Structures.
Part 3: Pre-stressed Concrete Structures.
Part 4: Design Tables.
In IS:3370-1967 (3), the design requirements for pre-stressed tanks are mentioned. A few of them are:
1. The computed stress in the concrete and steel, during transfer, handling and construction, and under working loads, should be within the permissible values as specified in IS:1343-1980.
2. The liquid retaining face should be checked against cracking with a load factor of 1.2.
3. The ultimate load at failure should not be less than twice the working load.
4. When the tank is full, there should be compression in the concrete at all points of at least 0.7 N/mm2. When the tank is empty, there should not be tensile stress greater than 1.0 N/mm2. Thus, the tank should be analyzed both for the full and empty conditions.
5. There should be provisions to allow for elastic distortion of the structure during pre-stressing. Any restraint that may lead to the reduction of the pre-stressing force should be considered.
6. The cover requirement is as follows. The minimum cover to the pre-stressing wires should be 35 mm on the liquid face. For faces away from the liquid, the cover requirements are as per IS:1343-1980.
The general equations from Eq 1 to Eq 3, would serve well for the design of circular pre-stressed liquid retaining structure.
Determine the area of steel required per meter height of a circular pre-stressed water tank with an inside diameter of 18 m and a height of 6 m water pressure. Compute the thickness of concrete required.
f i = 1034 MPa, f e = 827 MPa, f ct = -5.17, MPa and n = 10.
Design for the following two cases: