R.C.C WATER TANKS

Storage Reservoirs and Over Head Tanks are used to store water,liquid petroleum & similar liquids. The force analysis of thereservoirs or tanks is about the same irrespective of the chemicalnature of the product.In general there are three kinds of water tanks1.Tanks resting on ground2.Underground Tanks3.Elevated tanksThe tanks may have circular or rectangular section.Tanks resting on ground & Underground may have flat bottomslab, while Elevated ones may have flat or Conical bottom.

WATER TANKBASED ONPLACEMENT OFTANK1. RESTING ON GROUND2. UNDER GROUND3. ELEVATEDBASED ON SHAPE OFTANK1. CIRCULAR2. RECTANGULAR3. SPHERICAL4. INTZ5. CONICAL BOTTOM2

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RECTANGULARTANKCIRCULARTANKINTZE TANKPRESTRESSED TANKSFor smallercapacities we go forrectangular tanksFor biggercapacities we gofor circular tanksIntze tank isconstructed toreduce theproject costbecause lowerdome in thisconstructionresists horizontalthrustFor biggertanks,prestressing isthe superiorchoice resultingin a saving ofup to 20%.5

Rectangular tanks are used when the storagecapacity is small.Rectangular tanks should be preferably squarein plan from point of view of economy.It is also desirable that longer side should not begreater than twice the smaller side.Moments are caused in two directions of the walli.e., both in horizontal as well as in vertical direction.Exact analysis is difficult and are designed byapproximate methods.When the length of the wall is more in comparisonto its height, the moments will be mainly in thevertical direction, i.e., the panel bends as verticalcantilever6

1.Concrete should be impervious-Concrete should be rich in cement content i.e. water-cementratio should be low. The quantity of cement in mix should notbe less than 3KN/m³, again to keep shrinkage low the quantityof cement should not exceed 5.3KN/m³.Generally M30 grade ofconcrete is used.To avoid leakage problems, Limit State Method of Designshould be avoided. IS:456-2000 is silent about permissiblestresses in direct tension, hence from IS:3370, it is obvious thatguidelines of previous version of IS:456 should be used which isbased on Working Stress Method.

REMEDY2. CRACK DEVELOPMENT1.2.3.Due to dimensionalmovements in concreteon account oftemperature & moisturechanges.Due to differentialexpansion of thickmembers due to Heat ofHydration.Cracks caused byUnequal Settlement Proper curingProvision of jointsReinforcements in each of 2directions at right angles.Expansion & Shrinkage ofconcrete should beconsidered during its design.Subdivide the tank in smallercompartments withprovision of necessary joints.

CONCRETEPERMISSIBLESTRESS INDIRECTTENSION(N/mm²)PERMISSIBLESTRESS INTENSION DUETO BENDING(N/mm²)PERMISSIBLESTRESS INSHEAR(N/mm²)M 151.11.51.5M 201.21.71.7M 251.31.81.9M 301.52.02.2M 351.62.22.5GRADE OF

TYPE OF STRESS IN STEELREINFORCEMENTPERMISSIBLE STRESSES IN N/mm2Plain round mild steelbarsHigh yield strengthdeformedbars(HYSD)1)Tensile stresses in the members underdirect tension(?s)2) Tensile stress in members inbending(?st)On liquid retaining face of members115150115150On face of away from liquid for membersless than 225mmOn face away from liquid for members225mm or more in thickness3) Tensile stresses in shearreinforcement(?sv)115150125190115125150175For members less than225mm in thicknessFor members 225mm or more in thickness

For thickness up to 100mm, minimum % ofreinforcement should be 0.3%For thickness from 100mm to 450mm, it may bereduced linearly to 0.2%i.e. P(min) 0.3 up to 100mm thick sectionsP(min) 0.3- 0.1*[(t-100)/(450-100)] where ‘t’ liesb/w 100mm to 450mmMinimum reinforcement should be ensured in bothdirections.If thickness of section is more than 225mm, layer ofbars are required near both face.

There are several analysis methods like Bending Theory, Plate Loadtheory, Finite Element analysis etc. But we commonly use“Approximate Method of Analysis” for water tanks. In this method, itis assumed thatIn case of Circular tanks, bottom 1/3rdOr 1m (whichever is greater)& for Rectangular tanks,Bottom 1/4th or 1m(whichever is greater) ispredominantly underCantilever action.Rest of the Wall is resisting theWater Pressure by forcesDeveloped in Horizontaldirection.

In this method, the bottom height of wallBP is designed as a Cantilever Fixed at B &subjected to Triangular load given byArea DBC of Pressure Triangle.Load at B wHReinforcement for Cantilever Action is providedUp to height ‘h’ from inner face of wall.The advantage of this method over other sophisticated methods is that1.It is a simple method2.It is more practical & gives a feel of the Structural Behaviour3.“Confusion driven disasters “ caused due to mistakes ofDraughtsmen in understanding sign conventions & analysis stepsof other complex methods can be avoided.

INTRODUCTION Rectangular Tanks are provided whencapacity of liquid to be stored is small. For small capacities, circular tanks areuneconomical on account of curvedshuttering thus Rectangular Tanks prove tobe economical in this case. Unlike Circular tanks, rectangular tanksoccupy entire available area, so it is easy todivide the tank in compartments & design.

DESIGN OF RECTANGULARTANKS 1.2.The components of aRectangular Tank are1.Side walls2.Base Slab3.Roof SlabThe design of walls byApproximate method isbroadly classified intotwo categoriesTanks having ratio L/B 2Tanks having ratio L/B 2

In this case, Tank is designed as a Horizontal slab all around (b/wcorners) & subjected to triangular load due to hydrostatic pressure from0 at top to H/4 or 1m above the base (whichever is more).From bottom junction to Height of H/4 or 1m (whichever is more), thewall is treated as a vertical cantilever fixed at base.Maximum Pressure(p) w(H-h) at D.Maximum Cantilever Moment [(1/2)(wH)(h)](h/3) at (h/3) from baseThe pressure(p) is resisted by the closed frame action of tank.The fixed end moments at A are (pB²/12) & (pL²/12) . Using MomentDistribution method, they can be Balanced.

PULL ON LONG WALLS.PULL ON SHORT WALLS Since the short walls span b/w Longwalls, above D, the water pressureon short walls gets transferred toLong walls as Tension.Considering 1 m high strip of shortwall at height ‘h’Total Tension w(H-h)*1*BPull on each Long wall (Tι) Tι (1/2)w(H-h)B In the similar fashion, Longwalls support the short walls &water pressure gets transferredto short walls as Tension(pull).Considering 1 m High strip ofLong walls at height ‘h’Total Tension w(H-h)*1*LPull on each short wall (Tв)Tв (1/2)w(H-h).L

In this case, the Long walls are treated as vertical cantilever fixedat base & Short walls are treated as Horizontal slabs(bendinghorizontally) b/w Long walls. Maximum B.M in long walls (1/2)wH*H*(H/3) wH³/6 For short walls, Maximum BM at level P may be taken under BM at ends of span [w(H-h)B²]/12 BM at centre of span [w(H-h)B²]/16PULL ON LONG & SHORT WALLS Pull on Long wall (Tι) 0.5w(H-h).B Pull on Short Wall (Tв) w(H-h)*1 Due to monolithic construction, it isAssumed that water pressure on about 1mOf long wall adjacent to corner causesTension in short wall.

Tank wall section subjected to combined effect of BM &Tension Distance of main reinforcement from centralaxis xThe effect of Horizontal tensile forces is toreduce the net moment in the walls.Thus final horizontal design moment M-TxThe BM reduces towards top above edge, hencespacing may be increased towards topNear corners, BM is on inner face & near centre itis on outer side.Area of steel for net BM (A )A (M-Tx)/(Lever arm * safe stress in steel)Area of steel for pull (A) T / safe stress in steelTotal Area of steel A A

DESIGN OF CIRCULAR TANKSDesign of circular tanks issimplest in nature. For same capacity, its constructionrequires less concrete thanrectangular tanks, thus it iseconomical for large capacitystorage. On account of circular shape, itcan be made water tight easily asthere are no sharp corners.For purpose of design, circular tankscan be divided into followingcategories 1. Tanks with sliding or flexiblejoint b/w floor & wall. 2. Circular tank with rigid jointb/w floor & wall.

CIRCULAR TANK WITH RIGID BASE Since the wall in this case is fixed with floorslab, circumferential elongation of wall isnot possible at its Junction with floor slab& hence Hoop stress is 0 there & the entireLiquid pressure is resisted by Cantileveraction.At a certain point D at a height h, abovebase slab , full Hoop Stress develops &cantilever effect is 0.At midway b/w D & base slab, liquidpressure is resisted partly by Hoop action &partly by Cantilever actionFig a shows likely deformation of wallFig b shows Load distribution on wallFig c shows B.M Diagram

DESIGN CONSTANTS Depth of Neutral axis ndn m (cbc) /[ m (cbc) (st)]Lever arm jdj 1-(n/3)Moment of Resistance MM kbd²where k 0.5 (cbc)jkEquivalent concrete section Aс mAśţ Aģ (m-1)Aśţm modular ratio (Es/Ec)m also 280/3 (cbc)

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1)DESIGN OF ROOF SLAB:Reinforcement of slabSpecificationTop slab Long span Short spanSizeReinforcement details12m 5m8mm Ø @300mm c/c12mm Ø @1500mm c/c26

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SizeSpecificationLonger side wallReinforcement details12m 4m Long span8mm Ø @300mm c/c Short span1. ve bending moment2. -ve bending momentShorter side wall12mm Ø @90mm c/c20mm Ø @110mm c/c5m 4m Long spanI. At topII. At bottom12mm Ø @170mm c/c12mm Ø @80mm c/c Short spanI. At supportsII. At middle12mm Ø @70mm c/c10mm Ø @80mm c/c28

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3)DESIGN OF BASE SLAB:SizeSpecificationBase slab Short spanI. -ve bending momentII. ve bending moment Long spanI. -ve bending momentII. ve bending momentReinforcement details5m 4m16mm Ø @80mm c/c16mm Ø @110mm c/c16mm Ø @110mm c/c10mm Ø @220mm c/c30

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SpecificationBeam I Main reinforcementSize480mm 300mm3 bars of 20mm Ø & 3 bars of 32 mm Ø StirrupsBeam II Main reinforcement12mm Ø @50mm c/c ( near support)12mm Ø @80mm c/c ( near middle)480mm 300mm3 bars of 20mm Ø8mm Ø @300mm c/c StirrupsBeam III Main reinforcementI. Span ABII. Span BC StirrupsReinforcement details480mm 300mm3 bars of 12mm Ø2 bars of 12mm Ø8mm Ø @300mm c/c32

1)2)3)4)5)Storage of water in tanks has become a necessity fordrinking and washing purposes in the present daylife.For small capacities we go for rectangular watertanks & for large capacities we go for circular tanks.The designed RCC rectangular tank can store waterupto 240000 litersIn this design project we have analyzed the overhead rectangular RCC water tank, throughtheoretical design and STAAD Pro program.Although the stresses and bending moment arenearly equal but all the parameters used in thetheoretical design cannot be fully adopted in theprogram design.33

DESIGN OF CIRCULAR TANKS Design of circular tanks is simplest in nature. For same capacity, its construction requires less concrete than rectangular tanks, thus it is economical for large capacity storage. On account of circular shape, it can be made water tight easily as there are no sharp corners. For purpose of design, circular tanks