design of underground water tank

Design of Underground Water tank

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The design of underground water tank is carried out as per IS 3370 part 1 and part 2 code. The design is by assuming no water table and uncracked design.

Below here are the various Design constants and codal provision for the Structural design of tank as per IS 3370 code.

  1. Exposure condition – The exposure condition as per IS 3370 part 1 -2009, clause 4 shall be considered as “severe” for the structure retaining liquid. The clear cover which we use in the designs shall be for the severe condition.
  2. As per table 1 of IS 3370 part 1, Minimum grade of concrete to be used for water retaining structures shall be M30.
  3. One more important clause is as per 7.2 a i.e. No relief shall be considered due to soil pressure on wall.
  4. Stability factor due to uplift condition shall be 1.2 .
  5. For Load combinations “Water Load” shall be considered as “Dead Load”

Permissible stresses in design for uncracked section are in the below table as per IS 3370 part 2 in N/ sq.mm

S.no Grade of concrete Direct Tension Bending Tension
1 M25 1.3 1.8
2 M30 1.5 2.0
3 M35 1.6 2.2
4 M40 1.8 2.4
5 M45 2.0 2.6
6 M50 2.1 2.8

Permissible Stress in Reinforcement Steel (N/ sqmm)

S.no Type of Stress Mild Steel High Strength Steel
1 Tensile stress under Tension, bending and shear 115 130

Design of Underground water tank in a step by step procedure for the following Parameters.

Input details :-
  1. Length of tank – 4m
  2. Width of tank  – 4m
  3. Depth of tank –  4m
  4. Grade of concrete – M30
  5. Grade of Steel – Fe500
  6. Concrete density– 25 Kn/ cu.m
  7. Soil density – 18 Kn/cum
  8. Water density – 10 Kn/ cum
  9. Clear cover for severe condition as per IS 456 is 45 mm +5mm =50 mm additional protection.
  10. Dia of the Bar – 20 mm
  11. Bending stress permissible – 2 N/ sqmm
  12. Permissible stress in reinforced steel – 130 N/ sqmm
  13. Permissible stress in tension – 1.5 N/sqmm.

With the above constants we shall calculate the design parameters required. The wall condition considered is that the wall is fixed at bottom and free at the top. The wall behaves as Cantilever wall. Below is the sketch indicating the two critical loading conditions.

Case 1 – Water inside the tank and no soil outside

Case 2 – Soil outside and no water inside.

Pressure diagram for water tank

MOMENT CALCULATIONS

From the above Pressure diagram

A ) Moment @ water inside = 80 * 1.33 =  106.4 Kn-m

B )  Moment @ soil outside = 43.2 *1.33 = 57.45 Kn-m

Hence, we shall check the thickness for a moment of 106.4 Kn-m

CALCULATIONS FOR THICKNESS

Thickness required for the wall for moment of 106.4 Kn-m is

Provide the thickness of wall as 570 mm. Here we need to note one thing that the thickness required is at the bottom of wall. The wall thickness can be varied i.e 570 mm at the bottom and whatever required at the top for the moment at top most fibre.

REINFORCEMENT CALCULATIONS

Permissible stresses for Un cracked section are as below

  • Direct tension – 1.5 N/sq.mm
  • Tension due to bending – 2.0 N/sq.mm
  • Permissible stress in steel in tension – 130 N/sq.mm
  • m = 9.33
  • k  =  0.42
  • j =   0.86

The above design constants modular ratio and lever arm details are calculated from stress diagram.

Effective depth water face = 570 – cover(50) – 20 – 10 mm (cg of bar) = 490 mm

Effective depth soil face = 570 mm – cover (50) –  20 mm (cg of bar) = 500 mm

VERTICAL REINFORCEMENT ON WATER FACE

Vertical Reinforcement required on water face

Hence Provide Steel of 20 Dia @ 160 mm c/c = 1000/160 *314 = 1962.5 sq.mm

MINIMUM REINFORCEMENT ON WATER FACE

As per IS 3370 Min steel required for a cross section is 0.35 %

Steel required is  0.35 /100 * 1000 * 250 mm = 875 sq.mm

Here 250 mm is considered because of the surface zone concept in IS 3370 part 2.

Provide Horizontal steel for minimum steel per face of atleast 875 sq mm

Provide 12 dia @ 110 c/c = 1000 / 100 * 113 = 1130 sq.mm

VERTICAL REINFORCEMENT ON SOIL FACE

Reinforcement required

Hence Provide Steel of 12 Dia @ 100 mm c/c = 1000/100 *113 = 1130 sq.mm

MINIMUM REINFORCEMENT ON SOIL FACE

As per IS 3370 Min steel required for a cross section is 0.35 %

Steel required is  0.35 /100 * 1000 *250 mm = 875 sq.mm Per face

Provide Horizontal steel for minimum steel per face of at least 997.5 sq.mm

Provide 12 dia @ 100 c/c = 1000 / 100 * 113 = 1130 sq.mm

REINFORCEMENT OF BOTTOM SLAB FOR UNDERGROUND WATER TANK

The bottom is considered as resting on ground and hence the water pressure counteracts the upward soil pressure. The bottom slab is designed as grade slab for Min steel as per IS 3370 assuming depth as 200 mm and reinforcement is provided for D/2 depth as per the surface zone concept of 3370 for grounded slabs.

DESIGN CHECK OF MINIMUM REINFORCEMENTS FOR UNDERGROUND WATER TANK

As per IS 3370 Min steel required for a cross section is 0.35 %

Steel required is  0.35 /100 * 1000 *100 mm = 350 sq.mm Per face

Provide 10 dia @ 200 c/c = 1000 /200 * 78.5 = 392.5 sq.mm on both top and bottom faces.

Important  Notes

  1.  The stresses considered above for water face is also 130 N/ sq.mm conservative, but for respective project the stresses as per IS 456 can also be considered as per respective projects.
  2. Preferably and as far as possible keep the spacing of reinforcement bars same for ease of construction.
  3. Minimum reinforcement can be reduced further for high strength steel. Please refer to clause 8 of IS 3370 part 2 – 2009, page 5.
  4. In case there are Long and short walls then additional steel is required for the tension generated in short walls, which shall be considered in the design for calculation of horizontal reinforcement. This can seen clearly seen and understood if modelled in staad just to check the distance up to which we need that additional steel requirement.
  5. The design performed assuming that no water table is encountered, Otherwise, buoyant check needs to be performed for factor of safety of 1.2 .
  6. If there is outside water then the pressure diagram varies for dry and submerged soil which is the below fig.
  7. The Tank size shall be determined as per the required volume.
Pressure diagram with water table
Pressure diagram with water table
Sample detailing of reinforcement for underground water tank is indicated in the below sketch for better understanding
Detailing of underground water tank

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