Literature Study- Water Tanks

Literature Study

UNDERGROUND WATER TANK

Hariram S T

FILE NAME: F022R1.DOC FORM NO. 022 R1

ABSTRACT:

This paper deals with the design of underground water tanks, specifically with the comparison of the codal provisions of the Indian and British Standards. The Indian Standard for the design of water tanks which has been split into four parts is IS 3370 and the corresponding British Standard is BS 8007. The calculation of forces acting on the water tank and the calculation of moments and shears acting on the members by using design charts has also been discussed.


INPUT DATA REQUIRED FOR DESIGN:

1. Dimensions of tank2. SBC of soil3. Surcharge4. Unit weight of soil5. Angle of internal friction6. Level of ground water

CRITICAL CONDITIONS:

The two critical conditions for the design of walls are,1. Tank full with water and no earth pressure outside2. Tank empty with earth pressure outside

The beneficial effects of soil pressure on the wall in tank full condition are to be ignored. In case of internal partition walls, they should be designed for liquid loading on each side separately.

LOAD FACTORS:

As per BS 8007:1987, the partial safety factors for the limit state of collapse as given in BS 8110-1:1997 can be used. The factors given are 1.4 for DL+IL/WL+ (Earth and Water Pressure) and 1.2 for DL+IL+WL+ (Earth and Water Pressure). IS 456:2000, does not mention explicitly partial safety factors for liquid and earth pressure, but as per IS 3370(Part 2):2009, the recommendations of IS 456:2000 can be used for the limit state of collapse.

MATERIALS FOR CONSTRUCTION:

Minimum grade of concrete for RC structures is M30 (for reinforced concrete structures) and M25 for tanks of small capacity as per Indian Standards. As per British standards, the minimum grade of concrete is C35A which has 28 day strength of 35 N/mm2.

Lean concrete of grade M15 and of thickness 75 mm to be placed over which the base slab is to be cast. As per BS the grade of concrete for PCC is C20, while C25 grade is to be used where aggressive soils are encountered.

LIMIT STATES OF SERVICEABILITY:

Deflection:

The Span/Effective depth ratio for limiting the defection as given by both the Indian and British codes is the same.


Cracking:

Maximum permissible crack width as per IS is 0.2mm. As per BS, the crack width permitted is 0.1mm for aesthetic appearance and 0.2mm for severe and very severe conditions. The calculation of crack width for both mature and immature concrete as per both the standards is the same.

Floatation:

When the tank is subjected to ground water pressure, it must be designed to counter the upward pressure by its self weight. The methods that can be adopted to counter flotation are

1. By thickening the base slab to increase the self weight.2. By increasing the projection of the base slab beyond the perimeter of the wall so as to

mobilize enough soil weight to resist ground water pressure.3. Providing pressure release vessels discharging into the tank (when entry of ground water is

permitted).

BS 8007:1987 recommends a partial safety factor of 1.1 against flotation when the exact level of the ground water table is known. Whereas in the Indian Standards the factor against flotation is given as 1.2.

MINIMUM REINFORCEMENT:

As per IS 3370(part 2):2009, the minimum steel required is 0.35 % for high strength deformed bars and 0.64% for mild steel bars on each surface zone. This percentage can further be reduced to 0.24% for deformed bars and 0.40% for mild steel bars when the tank dimensions do not exceed


15m. The bar spacing should not exceed 300mm or the thickness of the member, whichever is less.

The British standard also specifies the same values with the addition of the maximum spacing for welded wire fabric to be 1.5 times the thickness of the section.

PERMISSIBLE STRESSES:

1. Resistance to cracking in Concrete:The compliance with the permissible stress values as given in IS 3370(part 2):2009, ensures that the cracks developed are not excessive.

2. Resistance to cracking in steel:The tensile stress in steel is to be taken as the product of the modular ratio of steel and concrete and the corresponding tensile stress in concrete. Crack width may be deemed satisfactory if steel stress under service conditions is less than 115 MPa for MS bars and 130 MPa for HYSD bars.

FORCES IN WATER TANKS:

The forces to be considered in design of water tanks are the triangular loading due to the water and soil on the walls, uniform loading on the base slab, surcharge weight and Internal vapor pressure (if any) for roof slabs.

In addition to the vertical, horizontal moments and shear caused by these forces, the walls also experience tension due to the reaction from the orthogonal walls. The base slab also experiences tension because of the horizontal reaction from the walls.

Forces on the walls

Forces on the walls

ka- Coefficient of active earth pressure

q – Surcharge pressure

h – Height of tank

γs- Density of soil

γw-Density of water


In calculating the amount of reinforcement, the amount of steel required for bending and tension are calculated separately and added together.

CALCULATION OF FORCES:

For the calculation of shear and moment acting on the walls and base slab, IS 3370(Part 4):1967 can be used. These charts give coefficients on the panels at different points for different end conditions and different loadings on the wall.Based on the coefficients for the vertical and horizontal moments and shear, the forces are calculated as :Horizontal Moment = Mx*w*a3

Vertical Moment = My*w*a3

Shear = coeff*w*a2

Similar to the Design Tables given by IS 3370 (Part 4):1967, Moody’s Charts can also be used for obtaining the coefficients for the calculation of moments and shear.

The Indian Design Charts does not provide for the end condition of fixed at top, so when the detailing has to be done as to provide fixity at top Moody’s charts can be used.


The origin for the panels as per IS 3370(Part 4)-1967 is taken at the centre, while in Moody’s charts it is taken at the corner. The vertical bending moment obtained

REFERENCES:

IS 3370(Part 1):2009 – Concrete Structures for Storage of Liquids-General Requirements IS 3370(Part 2):2009 – Concrete Structures for Storage of Liquids-Reinforced concrete

Structures IS 3370(Part 4):1967 – Concrete Structures for Storage of Liquids-Design Tables BS 8007:1987 – Design of concrete structures for retaining aqueous liquids R.D.Anchor. Design of Liquid Retaining Concrete Structures



Documents

Literature Study- Water Tanks