Water Retaining Structures Design

Design & Detailing of Water Retaining Structures

& Precast Water Tank Floor

System

ESE ESE Tay Tay Ah Ah ChingChing27 Feb 0227 Feb 02

ContentsIntroductionIntroductionDesign & Detailing ConsiderationsDesign & Detailing ConsiderationsTypes of Water Tanks, Usages & Design Types of Water Tanks, Usages & Design GuidelinesGuidelinesErection of Erection of Precast Precast Ring TankRing TankDesign of Water TankDesign of Water Tank

Ultimate Limit State Design Ultimate Limit State Design –– CP 65CP 65Serviceability Limit State Design Serviceability Limit State Design –– BS 8007BS 8007

Precast Precast Water Tank Floor SystemWater Tank Floor System

Water Retaining Structures

Water Retaining Structures

Swimming poolSwimming poolBasement wall / slabBasement wall / slabRetaining WallsRetaining WallsWWater ater TanksTanks

Wet riser tanks (suction tanks)Wet riser tanks (suction tanks)Roof tanksRoof tanks

Design & Detailing Considerations

Design Considerations

Design for Strength according to CP 65Design for Strength according to CP 65Special considerations for crack control Special considerations for crack control according to BS8007according to BS8007

Max crack widthMax crack widthMax crack spacingMax crack spacingMin steel contentMin steel content

Detailing Considerations

Detailing follows usual rules for normal Detailing follows usual rules for normal structures.structures.Bars to be continuous, and sudden changes Bars to be continuous, and sudden changes in reinforcement ratios to be avoided.in reinforcement ratios to be avoided.No construction joint below specified highest No construction joint below specified highest water level shall be allowed (for water tanks). water level shall be allowed (for water tanks). The entire concrete tank below the water level The entire concrete tank below the water level shall be cast in one operation with partially shall be cast in one operation with partially hanged internal formwork (for water tanks).hanged internal formwork (for water tanks).WaterWater--stop to be used where construction stop to be used where construction joints are unavoidable. Bars to be joints are unavoidable. Bars to be continuous across the joints.continuous across the joints.


The tank roof and the portion above the water The tank roof and the portion above the water level shall be cast in second operation.level shall be cast in second operation.After passing the water test, the internal After passing the water test, the internal surface of the concrete tank shall be coated surface of the concrete tank shall be coated with approved type ofwith approved type of cementitiouscementitiouswaterproofing coating before laying of tiles.waterproofing coating before laying of tiles.For the slab of water tank, ratio of length to For the slab of water tank, ratio of length to width width 1.5 and the shorter span of the slab 1.5 and the shorter span of the slab shall be 3.6m maximum.shall be 3.6m maximum.


The minimum area for main and distribution steel The minimum area for main and distribution steel is 0.35% for walls and slabs for deformed grade is 0.35% for walls and slabs for deformed grade 460 reinforcement and 0.64% for plain grade 250 460 reinforcement and 0.64% for plain grade 250 bars. bars. The maximum spacing of bars shall be 300mm or The maximum spacing of bars shall be 300mm or the thickness of the section, whichever is the the thickness of the section, whichever is the lesser. lesser. Nominal cover of concrete for reinforcement Nominal cover of concrete for reinforcement should be not less than 40mm.should be not less than 40mm.


Waterstop

Cover to links >40mm

Construction Joint

Types of Water Tanks

Types of Water TanksCISCIS

RectangularRectangularPrecast Precast

Circular TankCircular TankRing TankRing Tank

UsageUsageRoof tanksRoof tanks

Wet riser tanks for fireWet riser tanks for fire--fighting (>60m above fighting (>60m above GL, 22 sty & above)GL, 22 sty & above)Water tanks for domestic supplyWater tanks for domestic supply

Suction tanks (reserve tanks to pump water to Suction tanks (reserve tanks to pump water to roof tanks)roof tanks)

Suction Tanks

Reserve tank to pump water to roof Reserve tank to pump water to roof tankstanksCapacity depends on water demand Capacity depends on water demand for the block:for the block:

< 22 stories < 22 stories no fireno fire--fighting requirementfighting requirement> 22 stories > 22 stories firefire--fighting requirementfighting requirement

Precast Precast circular tanks or rectangular circular tanks or rectangular inin--situ situ tanks.tanks.

Types of Water Tanks

Distribution System

Suction tank

Pump

Outlets to DUs

1st storey from pump

Building

(Domestic)(Fire-fighting)

Roof Tanks

CIS Suction TanksDetailing

CIS Suction TanksDetailing

Construction of CIS Tanks (Roof)



1 – 3 months to construct the CIS water tank

Need to improve the site productivity

Solution – “To precast the water tank”

Roof Water Tanks

Before 1985, roof tanks were made of stainless steel or mild steel

RC water tanks were introduced in HDB in 1985 to replace SS & MS water tanks

< 20 sty precast circular water tanks> 20 sty in-situ rectangular tanks

The precast segmental ring water tanks have replaced the in-situ RC tanks for domestic use since Jul 1999.

Precast Water Tanks

Precast Water Tanks

Precast Ring Water Tanks

There are basically two types of components, the base and the body segments. A few of the body segments will be stacked on top of each other to form the height of the tank required. The maximum height of the tank is 5.13m.

600

130

3700

1550

3830

1550

35203640

TYPE A(3 rings)

600

1303700

1550

5130 1300

1550

35203640

TYPE B (4 rings)

3700

130

1100

1300

2530

3520

3640

TYPE C(2 rings)

StandardisationItems Type A

(3 Rings) Type B

(4 Rings) Type C

(2 Rings) Remarks

1) Sizing

- Total height (including 130mm thick cover)

- External diameter (including 60mm thick protrusion at the joints)

- Weight of base (including tiles) - Weight of body

segments (including tiles)

- Weight of cover - Weight of empty

tank - Weight of water tank

filled with water

3830mm 3640mm 7t 6.2t 3.7t 23.1t 48t

5130mm 3640mm 7t 6.2t (1550mm) 5.3t (1300mm) 3.7t 28.4t 65t

2530mm 3640mm 9.1t 5.3t 3.7t 18.1t 33t

Annex 1 and 2 Type C is used in buildings with 21 storeys where no fire fighting water is required. Type A can also be used if there is no height constraint. But total volume of the tank shall be considered when calculating the no.of tanks required.

Standardisation

2) Min clear height from soffit of water tank to lowest point of main roof

2300mm 2300mm 2300mm

3) Min clearance between tank & tank and tank & wall

900mm 900mm 900mm Annex 4

4) Min clearance between piping connection & main water tank suporting beam and /or secondary cross beams

100mm 100mm 100mm Annex 4

5 Range of Storey heights of building

21-30 22-30 21 Beyond 30 storeys, this tank could be used subject to discussion between ARCH, SE and ME due to different requirement for fire fighting water.

3.7 ton each

100

1350 Vol for fire

fighting = 13.4m3

Vol for DU = 22.3m3

370

6.2 ton

TYPE B7 ton each

100

370

Vol for DU =

15.7m3

Vol for fire

fighting = 9.2m3

TYPE A9.1 ton

850

6.2 ton

100

370

TYPE C

Vol for DU =

14.06m3

5.3 ton

Standardisation

Standardisation

Design GuidelinesConcrete water tanks shall preferably rest directly Concrete water tanks shall preferably rest directly on column, whenever possible. on column, whenever possible. For case where it is not possible to place the For case where it is not possible to place the water tanks directly on columns, deep and broad water tanks directly on columns, deep and broad transfer beams shall be designed to carry the transfer beams shall be designed to carry the water tank loads to the nearest supports.water tank loads to the nearest supports.No roof shall be provided over theNo roof shall be provided over the precastprecastconcrete water tanks. concrete water tanks. A working platform shall be provided at one side A working platform shall be provided at one side of the water tank, where there is a catof the water tank, where there is a cat--ladder.ladder.

Design Guidelines

There must be sufficient clearance (>1500mm) There must be sufficient clearance (>1500mm) for thefor the pipingspipings at the underside of the beam (to at the underside of the beam (to highlight to Architect early if there is a need to highlight to Architect early if there is a need to change the level of the water tank floor).change the level of the water tank floor).The sanitary pipe position shall be fixed as The sanitary pipe position shall be fixed as shown in the construction manual. shown in the construction manual.

Section Through Tank

Typical Water Tank Layout

slabslabslab slabslab

100mm100mm

900mm

slabBooster pump room

2100mm

900mm

Erection of Precast Ring Tank


Installation of base tank


Placing of 1T10 around groove


1T10 and waterprooing strip in place


Installation 2nd ring segment

Vertical T20 bars to provide lateral restraint


Installation of 3rd & 4th ring segments



Installation of tank cover


Preparation of non-shrink grout for grouting groove


Pressure grouting of groove


Oozing out of grout indicates complete grouting


All grout outlet holes shall be plugged

Design of Water Tanks

Design Methods

Design Of Water TanksDesign Of Water TanksUltimate limit state (ULS) Ultimate limit state (ULS) –– CP65CP65Serviceability limit state (SLS)Serviceability limit state (SLS) –– BS8007BS8007

Ultimate Limit State Design (CP 65)Partial Safety FactorPartial Safety Factor

γγf = 1.4 for load combinations (1) & (2)f = 1.4 for load combinations (1) & (2)γγf = 1.2 for load combinations (3)f = 1.2 for load combinations (3)

BendingBendingk = M / k = M / ffcucu*b*d*b*d22

Z =Z = d(0.5+(0.25 d(0.5+(0.25 –– k/0.9)k/0.9)1/21/2) < 0.95d) < 0.95dAAss = M / 0.87f= M / 0.87fyy

**z z

ShearShearvv = V / b*d < = V / b*d < vvcc

TensionTensionFFtt = = kkRR** γγ * h * r* h * rAAss > > FFtt / 0.87f/ 0.87fy

1.2 (DL+Water Load+ WL)

1.4 DL + 1.4 Water Load

1.0DL + 1.4Water Load

Base slab

Wall

y * For design of circular tanks, coefficient for bending moment, shear force & tensile force refer to BS 5337.

Serviceability Limit State Design(BS 8007)

Partial Safety FactorPartial Safety Factorγγf = 1.0 for all load combinations (1), (2) & (3)f = 1.0 for all load combinations (1), (2) & (3)

Triangular Stress BlockTriangular Stress BlockPlane sections remain plane after bendingPlane sections remain plane after bendingStresses in the steel and concrete are proportional to Stresses in the steel and concrete are proportional to the strainsthe strainsThe concrete is cracked up to the neutral axis, and The concrete is cracked up to the neutral axis, and no tensile stress exists in the concrete below itno tensile stress exists in the concrete below it

Serviceability Limit State DesignCracks cannot be avoided in RC structuresCracks cannot be avoided in RC structuresLimit crack widthsLimit crack widths

0.2 mm for severe or very severe exposure0.2 mm for severe or very severe exposure0.1 mm for critical aesthetic appearance0.1 mm for critical aesthetic appearance

Allowable steel stresses (deformed bars)Allowable steel stresses (deformed bars)100 N/mm100 N/mm22 for design crack width of 0.1mmfor design crack width of 0.1mm130 N/mm130 N/mm22 for design crack width of 0.2mmfor design crack width of 0.2mm

Crack width calculationCrack width calculationMinimum reinforcementMinimum reinforcementSpacing of reinforcementSpacing of reinforcementCheck reinforcement provided against BS 8007 Check reinforcement provided against BS 8007

Serviceability Limit State Design

h

b

d

A s

x

z

2x/3

F cc

F st

ε cc

ε st

f cc

Section StressStrain

NeutralAxis

For equilibrium of forces :

F cc = F st

0.5bxf cc = A sf st

E c = f cc/ε cc ; E s = f st/ε st

ε st = (d-x)ε cc/x 0.5bxE cε cc = A sα sE c(d-x)ε cc/xα e = E s/E c A c(x/2) = α e A s(d-x)A c = bx

A c

α eA s

Transformed Section

x

d-x

For equilibrium of forces :

0.5bx 2 = α eA sd - α eA sx

0.5bx 2 + α eA sx - α eA sd = 0

x = {-α eA s +/- [(α eA s) 2 + 2bα eA sd] 1/2}/b

Moment of resistance :

M = F ccz = F stz

M = 0.5bxf cc(d-x/3) = A sf st(d-x/3)

Crack Width Calculation - Flexural3a crε m

1 + 2(a cr - c min)/(h-x)

Maximum surface crack width at any point, w max =

ε m = ε 1 - ε 2

a cr = the distance from the point considered to a point of zero concrete strain(surface of the nearest longitudinal bar)

ε m = the average concrete strain,allowing for the stiffening effect of the concrete in the tension zone

c min = the minimum cover to the tension steel

h = the overall depth of the member

x = the neutral axis depth

ε 2 =b t(h-x)(a`-x)

3E sA s(d-x)

b t = the width of the section at the centroid of the tension steel

ε 1 = the apparent strain

For a limiting design surface crack width of 0.2 mm,


ε 2 =1.5b t(h-x)(a`-x)

3E sA s(d-x)

Due to stiffening effect of the concrete

a` = the distance from the compression face to the point considered

Crack Width Calculation - flexuralx

a`h

b t

ε mε 2a cr

c min

ε m = ε 1 - ε 2


ε 2 =b t(h-x)(a`-x)

3E sA s(d-x)



ε 2 =1.5b t(h-x)(a`-x)

3E sA s(d-x)


a` = the distance from the compression face to the point considered

Crack Width Calculation - flexural

For a limiting design surface crack width of 0.2 mm at the soffit,

x

a`h

b t

ε mε 2a cr

c min

ε m = ε 1 - ε 2


3a crε m

1 + 2(a cr - c min)/(h-x)

Maximum surface crack width at any point, w max =

ε m < 0 indicates that the section is uncracked

x

a`

h

b t

ε mε 2a cr

c min

a mc min

ε 2 =b t(h-x)(a`-x)

3E sA s(d-x)ε 2,h =

b t(h-x) 2

3E sA s(d-x)Constant valuew max = 3a mε m Maximum crack width occurs at midway between the bars

w min = 3c minε m Minimum crack width occurs immediately below the bar

Crack Width Calculation - TensionMaximum surface crack width, w max = 3a crε m

= ε 1 - ε 2

a cr = the distance from the point considered to a point of zero concrete strain(surface of the nearest longitudinal bar)

ε m = the average concrete strain,allowing for the stiffening effect of the concrete in the tension zone

ε 2 =2b th

3E sA s

ε 1 = the apparent strain = F t/(E sA s)



ε 2 =b th

E sA s F tF t

h = the overall depth of the member


ε m < 0 indicates that the section is uncracked

Crack Control (Immature conc.)

Due to effects of :Due to effects of :Drying shrinkageDrying shrinkageThermal contractionThermal contraction

Minimum reinforcement for crack distributionMinimum reinforcement for crack distributionCritical steel ratio,Critical steel ratio, ρρcritcrit = 0.0035= 0.0035

ρρprovprov >= >= ρρcrit crit where where ρρprov prov = = AAss/A/Acc

Crack Control (Immature conc.)

Crack spacingCrack spacingssmaxmax = (= (ffctct//ffbb)()(φφ/2/2ρρprovprov))

ffctct//ffbb = 1.6/2.4 = 0.67= 1.6/2.4 = 0.67ffctct -- the tensile strength of the concretethe tensile strength of the concreteffbb -- the average bond strength between concrete the average bond strength between concrete and steeland steel

φφ -- the size of the reinforcing barthe size of the reinforcing barSpacing of reinforcementSpacing of reinforcement

Bar spacing not exceed 300mm or thickness of the Bar spacing not exceed 300mm or thickness of the section, whichever is the lessersection, whichever is the lesserFor welded fabric reinforcement, bar spacing not exceed For welded fabric reinforcement, bar spacing not exceed 1.5 times the thickness of the section1.5 times the thickness of the section

Check Reinforcement Provided -Bending & Tensile Force (SLS)

Allowable steel stressesAllowable steel stressesfyfy = 100 N/mm= 100 N/mm2 2 for 0.1mm crack widthfor 0.1mm crack widthfyfy = 130 N/mm= 130 N/mm2 2 for 0.2mm crack widthfor 0.2mm crack width

Moment of resistanceMoment of resistanceM = M = FFststzz = = AAssffstst(d(d--x/3)x/3)ffstst = M/[A= M/[Ass(d(d--x/3)]x/3)]

Tensile force due to hydrostatic loadingTensile force due to hydrostatic loadingffstst = F= Ftt/A/Ass

ReferencesReinforced Concrete DesignReinforced Concrete Design

W. H. Mosley & J. H. W. H. Mosley & J. H. BungeyBungeyReinforced and Reinforced and Prestressed Prestressed ConcreteConcrete

F. K. Kong & R. H. EvansF. K. Kong & R. H. EvansReinforced Concrete Design to BS 8110Reinforced Concrete Design to BS 8110

A. H. AllenA. H. AllenDesign of Concrete Structures for Retaining Aqueous Design of Concrete Structures for Retaining Aqueous LiquidsLiquids

R. ChengR. ChengDesign of LiquidDesign of Liquid--Retaining Concrete StructuresRetaining Concrete Structures

R. D. AnchorR. D. Anchor

Precast Water Tank Floor System

Precast Water Tank Floor SystemPrecastPrecast all elements above the main roofall elements above the main roof

With effect from 15 Apr 1999With effect from 15 Apr 1999PrecastPrecast componentscomponents

Water tank beamWater tank beamPilot project Pilot project -- SembawangSembawang N5 C4 & C5N5 C4 & C5Tender date Tender date -- 26 Sep 199726 Sep 1997

Water tank floor slabWater tank floor slab70 mm 70 mm thkthk nonnon--prestressed prestressed plankplankNot for booster pump room floorNot for booster pump room floor

Precast Water Tank Floor SystemColumn stumpColumn stump

For normal For normal precast precast water tankswater tanksSplice sleeve connectionSplice sleeve connection1500 mm length1500 mm length

•• PCF1 (300x400) PCF1 (300x400) -- 4T25 (s)4T25 (s)•• PCF2 (300x500/600) PCF2 (300x500/600) -- 4T25 (s)4T25 (s)•• PCF3 (200x600) PCF3 (200x600) -- 3T28 (s)3T28 (s)•• PCF4 (300x500/600) PCF4 (300x500/600) -- 3T28 (s)3T28 (s)

Precast Water Tank Floor SystemScreen wallScreen wall

Pilot project Pilot project -- Sengkang Sengkang N3 C13N3 C13Design Instruction Sheet : Design Instruction Sheet : DR/PDR/P--SW1/311SW1/311100 mm100 mm thkthk wallwall150x300 stump at both ends of the wall150x300 stump at both ends of the wall2T16 splice sleeves2T16 splice sleeves

Precast Water Tank Floor System

Precast Water Tank Floor



Water Retaining Structures Design

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