Unit II – Water Tank

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Subject code : CE 2401Name of the subject : DESIGN OF REINFORCEDCONCRETE & BRICK MASONRY STRUCTURESDate of deliverance :ChapterReference details :VSA Educational and CharitableDesignof RCTrust’sElements,Group of Institutions, Salem – 636 010Mr. N . krishnarajuDepartment of Civil EngineeringUnit II – water tankUnderground rectangular tanks – Domes – Overhead circular and rectangular tanks – Design of staging andfoundations-DesignDesign as per BIS Codal Provisions.Aim:underwater tank, dome & over head circular and rectangular tankAs per the IS Code requires undergroundwith staging and foundation to be designed to ensure stability and satisfies the BIS codal provisions.WATER TANKSIn general there are three kinds of water tanks-tanks resting on ground, underground tanks andelevated tanks.The tanks resting on ground like clear water reservoirs, settling tanks, aeration tanks etc. aresupported on the ground directly. The walls of these tanks are subjected to pressure and the base issubjected to weight of water and pressure of soil. The tanks may be covered on top.The tanks like purification tanks, Imhoff tanks, septic tanks, and gas holders are built underground.The walls of these tanks are subjected to water pressure from inside and the earth pressure from outside.The base is subjected to weight of water and soil pressure. These tanks may be coveredred at the top.Elevated tanks are supported on staging which mayay consist of masonry walls, R.C.C. tower orR.C.C. columns braced together.gether. The walls are ssubjected to water pressure.re. The base has to carry the loadof water and tank load. The staging has to carry load of water and tank. The staging is also designed forwind forces.From design point of view the tanks may be classified as per their shape-rectanrectangular tanks, circulartanks, intze type tanks. Spherical tanks conical bottom tanks and suspended bottom tanks.Design requirement of concreteIn water retaining structures a dense impermeable concrete is required thereforre, proportion of fineand course aggregates to cementent should be such as to give high quality concrete.Concrete mix weaker than M200 is not used. The minimum quantity of cementent in the concrete mix3shall be not less than 300 kg/m .The design of the concrete mixx shall be such that the resultant concrete is sufffficiently impervious.Efficient compaction preferably by vibrationvibris essential. The permeability of thee thoroughly cocompactedconcrete is dependent on water cementent ratio. Increase in water cement ratio increaseses permeability, whileconcrete with low water cement ratio is difficult to compact.pact. Other causes of leakage in concrete aredefects such as segregation and honey combing. All joints should be madeade waterwater-tight as these arepotential sources of leakage.Prepared by Mr.R.YUVARAJA,, Assistant Professor / CivilPage 1

Subject code : CE 2401Name of the subject : DESIGN OF REINFORCEDCONCRETE & BRICK MASONRY STRUCTURESDate of deliverance :ChapterReference details :VSA Educational and CharitableDesignof RCTrust’sElements,Group of Institutions, Salem – 636 010Mr. N . krishnarajuDepartment of Civil EngineeringDesign of liquid retaining structures is different from ordinary R.C.C, structures as it requires thatconcrete should not crack and hence tensile stresses in concrete should be within permissibleissible limits.liA reinforced concrete membermber of liquid retainingtaining structures is designed on the usual principlesignoring tensile resistance of concrete in bending. Additionally it should be ensured that tensile stress onthe liquid retaining face of the equivalealent concrete section does not exceed the permissibleissible tensile strengthof concrete as given in table1. For calculation purposes the cover is also taken into concrete area.Cracking may be caused due to restraint to shrinkage, expansion and contraction of concrete ddue totemperature or shrinkage and swellingelling due to moisture effects. Such restraint mayay be caused by –(i)The interaction between reinforcement and concrete during shrinkage due to drying.(ii)The boundary conditions.(iii) The differential conditionsconditiprevailing through the large thickness of massive concrete.Use of small size bars placed properly, leads to closer cracks but of smalleraller width. The risk ofcracking due to temperatureperature and shrinkage effects may be minimized by limitingiting the changes in moisturecontent and temperature to which the structure as a whole is subjected. The risk of cracking can alsobe minimized by reducing the restraint on the free expansion of the structure with long walls or slabfoundedd at or below ground level, restraint can be minimized by the provision of a sliding layer. This canbe provided by founding the structure on a flat layer of concrete with interposition of some material tobreak the bond and facilitate movement.ent.In case length of structure is large it should be subdivided into suitable lengths separatedseparby movement joints, specially where sections are changed the movement jointsints should be provided.Where structures have to store hot liquids, stresses caused by difference in tetemperature betweeninside and outside of the reservoirservoir should be taken into account.accouThe coefficient of expansion due to temperature change may be taken as 11 x 10-6/ C-6-6and coefficient of shrinkage may be taken as 450 x 10 for initial shrinkage and 200 x 10 for dryingshrinkage.3. Joints in Liquid Retaining Structurectures. Joints are classified as given below.(a)Movement Joints. There are three types of movement joints.(i)Contraction Joint. It is a movement joint with deliberate discontinuityity without initial gapbetween the concrete on either side of the joint. The purpose of this joint is to accommmodate contraction ofthe concrete. The jointint is shown in Fig. 1(a).1(Prepared by Mr.R.YUVARAJA,, Assistant Professor / CivilPage 2

ChapterReference details :VSA Educational and CharitableDesignof RCTrust’sElements,Group of Institutions, Salem – 636 010Mr. N . krishnarajuDepartment of Civil EngineeringSubject code : CE 2401Name of the subject : DESIGN OF REINFORCEDCONCRETE & BRICK MASONRY STRUCTURESDate of deliverance :(a)(b)Fig 1.A contraction joint may be eithereith complete contraction jointint or partial contractioncjoint. Acomplete contraction joint is one in which both steel and concrete are interrupted and a partialcontraction joint is one in which only tthe concrete is interrupted, the reinforcing steell running through asshown in Fig. 1(b).(ii)Expansion Joint. It is a joint with complete discontinuity in both reireinforcing steel andconcrete and it is to accommodate eithher expansion or contraction of the structure. A typical expansionjoint is shown in Fig. 2.Fig. 2This type of joint requires the provision of an initial gap between the adjoining parts of a structurewhich by closing or opening accommodmmodates the expansion or contractiontion of the structure.(iii)Sliding Joint. It is a jointjoi with complete discontinuity in both reinforceorcement and concreteand with special provision to facilitate movement in plane of the joint. A typical jointint is shown in Fig. 3.This type of joint is providedovided between wall and floor in someso cylindrical tank designs.(b)Construction Joint. Thishis type of joint is provided for convenience in construction.Arrangement is made to achieveve subsequent continuity without relative movement.mov ent. One applicationPrepared by Mr.R.YUVARAJA,, Assistant Professor / CivilPage 3

Subject code : CE 2401Name of the subject : DESIGN OF REINFORCEDCONCRETE & BRICK MASONRY STRUCTURESDate of deliverance :ChapterReference details :VSA Educational and CharitableDesignof RCTrust’sElements,Group of Institutions, Salem – 636 010Mr. N . krishnarajuDepartment of Civil Engineeringof these joints is between successive liffts in a reservoir wall. A typical jointint is shown in Fig. 4.RCPARED SILDING SURFACEOR RUBBER PAOFig.33Fig.4The number of joints should be as smmall as possible and these joints should be kept from possibility ofpercolation of water.(c)Temporary Open Joints.Joints A gap is sometimes left temporarily betwetween the concrete ofadjoining parts of a structure whichFig. 5 Temporary open jointsafter a suitable interval and before the structurestructis put to use, is filled with mortar or concconcrete completely asin Fig. 5(a) or as shown in Fig. 5 (b) and (c) with suitable jointing materials. In the firsst case width of thegap should be sufficient to allow the sides to be prepared before filling.Prepared by Mr.R.YUVARAJA,, Assistant Professor / CivilPage 4

Subject code : CE 2401Name of the subject : DESIGN OF REINFORCEDCONCRETE & BRICK MASONRY STRUCTURESDate of deliverance :ChapterReference details :VSA Educational and CharitableDesignof RCTrust’sElements,Group of Institutions, Salem – 636 010Mr. N . krishnarajuDepartment of Civil EngineeringSpacing of Joints. Unless alternative effective means are taken to avoid cracks by allowing for theadditional stresses that may be induced by temperature or shrinkage changes or by unequal settlesettlement,movementent joints should be provided at the following spacings:spacings:(a)In reinforced concrete floors, movement joints should be spaced at not more than 7.5 mapart in two directions at right angles. The wall and floor joints should be in line except where slidingjoints occur at the base of the wall in which correspondencecis not so important.(b)For floors with only nominalnopercentage of reinforcement (smaller than the minimumspecified) the concrete flooroor should be cast in panels with sides not more than 4.5 m. (c)In concretewalls, the movement joints should normally be placed at a maximum spacing of 7.5 m. inreinforced walls and 6m. in unreinforced walls. The maximum length desirablerable between verticalmovement joints will depend upon the tensile strength of the walls, and may be increased by suitablereinforcement. When a sliding layer is placed at the foundation of a wall, the length of the wall thatcan be kept free of cracks depends on the capacity of wall section to resist the frictionriction induced at theplane of sliding. Approximately the wall has to stand the effect of a force at the place of sliding equal toweight of half the lengthefficient of friction.of wall multiplied by the co-efficient(d)Amongst the movemeent joints in floors and walls as mentionedentioned above expansionjoints should normally be provided at a spacing of not more than 30 m. betweensuccessiveve expansion joints or between the end of the structure and the next expansion joint; all otherjoints being of the construction type.(e)When, however, the temmperaturere changes to be accommodated are abnorabnormal or occur morefrequently than usual as in the case of storage of warm liquids or in uninsulated roof slabs, a smallerspacing than 30 m should be adopted, that is greater proportion of movementent joints should be of theexpansion type). Whenhen the range of temperature is small,sfor example,ple, in certain cocovered structures, orwhere restraint is small, for example,ple, in certain elevated structures none of the movemment joints providedin small structures up to 45m. length need be of the expansion type. Where slidinging joints are providedbetween the walls and either the floor or roof, the provision of movement joints in each element can beconsidered independently.4.General Design for Requirements (I.S.I)1.Plain Concrete Structures. Plain concrete member of reinforced concrete liquidretaining structures may be designed against structural failure by allowing tension in plain concrete asper the permissible limitsits for tension in bending.beThis will automaticallyatically take care of failure due tocracking. However, nominal reinforcemment shall be provided, for plain concrete structural members.Prepared by Mr.R.YUVARAJA,, Assistant Professor / CivilPage 5

Subject code : CE 2401Name of the subject : DESIGN OF REINFORCEDCONCRETE & BRICK MASONRY STRUCTURESDate of deliverance :ChapterReference details :VSA Educational and CharitableDesignof RCTrust’sElements,Group of Institutions, Salem – 636 010Mr. N . krishnarajuDepartment of Civil Engineering2. Permissible Stresses in Concrete.(a)For resistance to cracking.cking. For calculations relating to the resistaance of members tocracking, the permissible stresses in tension (direct and due to bending) and shear shall confirm to thevalues specified in Table 1. The permmissible tensile stresses due to bending apply to the face of themember in contact with the liquid. In membersmless than 225 mm. thick and in contact with liquid on oneside these permissibleissible stresses in bending apply also to the face remoterefrom the liquid.uid.(b)For strength calculations.calculatiIn strength calculations the permissibleissible concrete stressesshall be in accordance with Table 1. Where the calculated shear stress in concrete alone exceeds thepermissible value, reinforcementent acting in conjunction with diagonal compressionpression in the concrete shall beprovidedded to take the whole of the shear.Table 1Permissibleble concrete stresses in calculations relating to resistancere istance to crackingGrade of concreteM 150M 200M 250M 300M 350M 4003.Permissibleissible stress in kg/cm2TensionDue toDirectBending111512171318152016221724Shear ( Q/bjdQ/bjd)151719222527Permissible Stresses in Steel(a)For resistance to cracking.cking. When steel and concrete are assumeded to act together forchecking the tensile stress in concrete for avoidance of crack, the tensile stress in steeel will be limited bythe requirement that the permissible tensile stress in the concrete is not exceeded so the tensile stress insteel shall be equal to the product of modularmratio of steel and concrete, and the corresponding allowabletensile stress in concrete.(b) For strength calculationss. In strength calculations the permissible stress shall be as follows:(i)(ii)Tensile stress in memberber in direct tensionTensile stress in membber in bending on liquid retainingface of membersmbers or face away ffrom liquid for members less than225 mm thick.Prepared by Mr.R.YUVARAJA,, Assistant Professor / Civil1000 kg/cm21000 kg/cmkg/c 2Page 6

Subject code : CE 2401Name of the subject : DESIGN OF REINFORCEDCONCRETE & BRICK MASONRY STRUCTURESDate of deliverance :(iii)(iv)ChapterReference details :VSA Educational and CharitableDesignof RCTrust’sElements,Group of Institutions, Salem – 636 010Mr. N . krishnarajuDepartment of Civil EngineeringOn face away from liquid for members 225 mm. or more inthickness.Tensile stress in shearr reinforcement,rFor members less than225 mm thicknessFor membersmbers 225 mm or more in thicknessCompressivepressive stress in columnscolusubjected to direct load.1250 kg/cmkg/c 21000 kg/cmkg/c 21250 kg/m21250 kg/cm2Note 1. Stress limitations foror liquid retaining faces shall also apply to:(a)(b)Other faces within 225 mm of the liquid retaining face.Outside or external faces of structures away from the liquid but placed in water loggedsoils upto the level of highest subsoil water level.Note 2. The permissible stress of 1000 kg/cm2 in (i), (ii) and (iii) may be incrincreased to21125 kg/cm in case of deformed bars and in case of plain mild steel bars when the crossreinforcement is spot welded to the mainain reinreinforcement.4.Stresses due to drying Shrinkageinkage or Temperature Change.(i)Stresses due to drying shrinkage or temperature change may be ignored providedthat –(a)the permissible stresses specified above in (ii) and (iii) are not otherwise exceeded.(b)adequate precautionstions are taken to avoid cracking of concrete during theconstruction period and until the reservoir is put into use.(c)recommendation regarding joints given in article 8.3 and for suitable sliding layerbeneath the reservoir aree compliedcowith, or the reservoir is to be used only for the storageof water or aqueous liquidsliquat or near ambient temperature and the circucircumstances aresuch that the concreteoncrete will never dry out.o(ii)Shrinkage stresses mayay however be required to be calculated in special cases, when ashrinkage co-efficientefficient of 300 x 110-6 may be assumed.(iii) When the shrinkage stressessare allowed, the permissible stresses, tensile stresses toconcrete (direct and bending) as given in Table 1. may be increased by 33 per cent.5.Floors(i))articleProvision of movement joints. Movement joints should be provided as discussed in3.(ii)Floors of tanks resting on ground. If the tank is resting directly over ground, floor maybe constructed of concrete with nominnal percentage of reinforcement provided that it is certain that theground will carry the load without appreciablereciable subsidence in any part and that the concrete floor is cast inpanels with sides not more than 4.5 m. with contraction or expansion joints between. In such cases aPrepared by Mr.R.YUVARAJA,, Assistant Professor / CivilPage 7

Subject code : CE 2401Name of the subject : DESIGN OF REINFORCEDCONCRETE & BRICK MASONRY STRUCTURESDate of deliverance :ChapterReference details :VSA Educational and CharitableDesignof RCTrust’sElements,Group of Institutions, Salem – 636 010Mr. N . krishnarajuDepartment of Civil Engineeringscreed or concrete layer less than 75 mm thick shall first be placed on the ground and covered with asliding layer of bitumen paper or other suitable material to destroy the bond betwween the screed andfloor concrete.In normal circumstances the screed layer shall be of grade not weaker than M 100, whereinjurious soils or aggressive water are expected,ethe screed layer shall be of grade not weaker than M 150and if necessary a sulphate resisting or other special cement should beused.(iii)Floor of tankss resting on supports(a)If the tank is supported on walls or oother similarilar supports the floor slab shall be designed asfloor in buildings for bending momentsnts due to water load and self weight.(b)When the floor is rigidly connected to the walls (as is generally the case) the bendingmoments at the junction between thee walls and floors shall be taken into account in the design of floortogether with any direct forces transferred to the floor from the walls or from the floorloor to the wall due tosuspension of the floor from the wall. If the walls are non-monolithic with the floor sllab, such as in cases,where movement joints have been providedprovibetween the floor slabs and walls, the floor shall bedesigned (only for the vertical loads on the floor.(c)In continuous T-beamss and L-beams with ribs on the side remote from the liquid, thetension in concrete on the liquid side at the face of the supports shall not exceed the permissibleperstressesfor controlling cracks in concrete. The width of the slab shall be determinedined in usual manner forcalculation of the resistance to crackingg of T-beam,TL-beam sections at supports.(d)The floor slab may be suitably tied to the walls by rods properly embeddedbedded in both the slaband the walls. In such cases no separaterate beam (curved or straight) is necessary under the wall, providedthe wall of the tank itself is designed to act as a beam over the supports under it.(e)Sometimes it may be economicaleconoto provide the floors of circular tanks, in the shape ofdome. In such cases the dome shall be designed for the vertical loads of the liquid over it and the ratioof its rise to its diameter shall be so adjusted that the stresses in the dome are, as far as possible, whollycompressive. The dome shall be supported at its bottom on the ring beam which shall be designed forresultant circumferential tension in addition to vertical loads.6.Walls(i)Provision of Joints(a)Sliding joints at the baase of the wall. Where it is desired to allow the walls to expand orcontract separately from the floor, or to prevent moments at the base of the wall owing to fixity to thefloor, sliding joints may be employed.(b)The spacing of ver

of water and tank load. The staging has wind forces. From design point of view the tanks tanks, intze type tanks. Spherical tanks Design requirement of concrete In water retaining structures a dense and course aggregates to cement should be such as to give high quality concrete. Concrete mix weaker than M200 shall be not less than 300 kg/m3.

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