Design of water tanks

A) Introduction:Two numbers of circular thickeners have been provided as per the process requirement. The wall of the thickener is designed as circular wall as per the coefficents in IS 3370-part IV. 1

The wall is designed as hinged at base for hoop forces and span B.M in the wall . On inner face of the wall steel is provided for 100% fixity condition. B) Geo-technical Investigation & Soil Data Adopted in Design:One number of Bore hole BH10 has been done at thickener location. Average EGL at this location is 595.5 . Refusal strata is met at depth of 1.5 M , N value =112 ( 594 M level). Proposed FGL in this area is 593.0 M . The founadtion is provided 500 mm below FGL on the refulsal strata. The bearing capacity as per the soil report is as follows :1.5 M below EGL

25 T/Sqm

2.5 M below EGL

30T/Sqm

For hard Rock Bearing pressure of 60 T/sqm is adopted.

Note:- The bearing capacities in the table are Net safe bearing Capacities of the soil and gross bearing capacity at founding level are adopted as follows:-

Soil weight T/Cum

Gross Bearing Capacity = Net Bearing Capacity + 1.6 x Depth of Foundation.

( Soil load from Existing Ground level to Finished ground level or above due to plinth filling will be treated as load , with soil weight of 2.0 T/Cum) For the Back fill soil following soil properties are adopted:Soil Properties adopted in the design:- ( Back Fill material)

j

Angle of Internal Friction

30

Degree

( For cantilever walls)

Coefficient Active pressure

0.33

Ka

( For propped / circular Walls)

Pressure at Rest

0.5

Kr

( For Computation of Loads)

Weight of Soil

20

Kn/M3

Submerged Soil Weight

10

Kn/M3

(For stability against Uplift)

Weight of Soil

16

Kn/M3

(For stability against Uplift)

Submerged Soil Weight

8

Kn/M3

Subsoil Water level

Not Present

Force resistance from soil due to friction is computed as per the following equation. F= 2 / 3 ( tan ( )) x W W= Vertical Load on the Slab / Footing 2/3xtan ( )

=0.384

C) Liquid Retaining Structures:All water retaining structure are designed as per the provisions in IS 3370-Part II - 2009 with controlled crack width of 0.1mm as per the specifications. The water level for the design shall be adopted as follows :For strength calculations water shall be taken up to top of the wall assuming all outlets are blocked . For serviceability limit state condition water level is adopted at working top liquid level or the overflow level as appropriate ( refer cl.4.2 of IS 3370-II -2009) Strength Calculations will be carried out using limit state design as per IS456-2000 with load factor of 1.5 . For empty condition soil level will be adopted as applicable.

Non Liquid Retaining Structures and Buildings :All non liquid retaining structures like foundations and columns of

2

elevated tanks and Buildings will be designed as per IS 456-2000. D) Design Idealization , Load Conditions and Load combinations:I) Design Idealization :Rectangular tank's walls will be designed as rectangular plates for 3

the boundary conditions as applicable using MOODY'S charts. Permissible stresses will be adopted as per the material retained. The wall will be treated as fixed at base & common edges with adjacent wall panels as applicable. Walls with aspect ratio greater than 3 , will be designed as one way as per boundary conditions . For walls of rectangular tanks the direct horizontal tension and bending action will be adopted as per IS 3370-partII

Circular tanks Clarifiers , thickeners etc. will be designed for HOOP forces and Bending Moment . The foundation of the wall will provide nominal fixity to the wall , for foundation design 50% fixity is adopted. In the wall vertical span steel and hoop steel is provided for the hinged base condition . However on the inner face of the wall vertical steel at base is provided for fixed base condition. E)

F) Specific Design Requirement as per tender Specifications. E1) Materials for Construction:I) Plain Concrete as Mud mat :PCC as mud mat below RCC , 100mm Thick.

M10

1000 gauge polythene sheet will be provided as sliding layer below base slab and raft slab of water retaining structures. II) Concrete Grades for RCC Structures :Liquid Retaining Structure

M30

Buildings / non Liquid retaining Structures

M25

All structural concrete will be with aggregates 40mm down for Footings and base slab and with 20mm down aggregates for all water retaining and other structures. For screed concrete 12.5mm 4

down size aggregates will be used.

III) Steel as Reinforcement for the Concrete:For Water retaining structures :

CRS Fe 500Grade.

( Steel with corrosion resistant Characteristics)

E2) Minimum Thickness of Reinforced Concrete:Following minimum thicknesses of the members will be provided Walls Liquid retaining Structures

250.00

mm

Bottom Slab liquid retaining structures

250.00

mm

Walls Foundations ( at base slab & wall junction)

250.00

mm

Roof slab of Liquid retaining structures

200.00

mm

Launder base slab & wall

150

mm

Shell roof

100

mm

Floors , roof slab , walkways , canopy slabs

125

mm

Walls of cable & Pipe trenches

200

mm

Under ground Pits etc.

200

mm

Footings at edge

200

mm

Footing at column face

300

mm

Width

300

mm

Depth

300

mm

Width

230

mm

Depth

300

mm

100

mm

Columns:-

Beams:-

Parapets , Chajjas etc.

Preacst Trench covers

75

mm

Free face

30

mm

Face in contact with Soil

40

mm

E3) Minimum Cover to any Reinforcement:Slabs:-

5 Beams:-

Top & Bottom

40

mm

Side / face in Contact with Soil

40

mm

Super Structure 50

mm

Sub. Structure 50

mm

Bottom , Top

50

mm

Sides

50

mm

Face in Contact with Soil

40

mm

Free face

30

mm

Face in contact with liquid

50

mm

Face in contact with Soil

50

mm

Free face

50

mm

10

mm

Main bar

10

mm

Dist. Steel

8

mm

Main bar

12

mm

Links

8

mm

Columns & Pedestals

Foundations :-

Retaining Walls , Basement & Pit Walls:-

Liquid retaining Structures

E4) Minimum Bar Diameter of Reinforcement bars :Major Foundations Block Foundations :-

Columns & Pedestals

Beams:-

Slabs & Base Slabs :-

Walls & Wall Foundations :-

Main bar

12

mm

Stirrups

8

mm

Main bar

10

mm

Dist. Steel

10

mm

Main bar

10

mm

Dist. Steel

8

mm

8

mm

Main bar

10

mm

Dist. Steel

8

mm

Other Minor elements shell roof :-

6

F) Design of The wall of the Thickener :-

7

Typical Wall Section

F1) Design of main wall:Wall top Level=

597.5 M

Base slab level=

593 M

Wall Height =

H=

4.5 M

Wall Th. provided at base t=

0.25 M

Wall Th. provided at span t=

0.25 M

Tank Diameter D=

19.5 M

HxH/Dxt is adopted =

4.15

Design of wall For Bending Moment :Base B.M Tank Full case :-

Coefficient as per IS 3370=

=9.81x4.5

Water pressure at base Triang.

0.026 44.145 Kn/Sqm

Refer table 10 of IS 3370-Part IV for Coefficient B.M= 0.026x44.145x4.5x4.5

23.24234 Kn-M

Ultimate B.M = Mu=1.5 M D ( Cms )=

25

34.86 Kn-M

d=

19.5 Cms

Mu/(bx d x d) Refer Sp16 , Table 4

0.916858

Pt ( required )

0.23 %

Ast=

4.485 Sqcm

Provided on Inner Face Base B.M for Tank Empty Case:-

Y10-175 C/C Not critical

Refer table 13 of IS 3370-Part IV for Coefficient Maximum Span B.M in wall for Hinged Case=0.0118xwxHxHxH D ( Cms )=

25

10.54845 Kn-M

d=

19.5 Cms

Mu/(bx d x d) Refer Sp16 , Table 4

0.416112

Pt ( required )

0.105 %

Ast= Provided on outer face

2.0475 Sqcm Y10-175 C/C

Check for shear force in the wall at base :- Adopted as for fixed base

8

Refer table 11 of IS 3370-Part IV Coefficient for shear Shear force = V

0.23 45.69008 Kn

(Note : Shear will be lesser at critical section " d " from base slab face.) Ultimate shear=1.5 x V D ( Cms )=

25

d= Vu/(b x d)

Refer Sp16 , Table 61

Pt ( required ) Ast= Provided on Inner Face

68.53511 Kn 19.5 Cms 0.351462 0.23 % 4.485 Sqcm Y10-175 C/C

Mimimum Steel Required :Mimimum Pt

0.35 %

Surface zone =25/2=

12.5 Cms

Ast= Provided

4.375 Sqcm Y10-175 C/C B.F

9

Check for Crack width:-

10

As crack width is