Design & Detailing of Water Retaining Structures & Pre Cast Water Tank Floor System

Design & Detailing of Water Retaining Structures & Precast Water Tank Floor System ESE Tay Ah Ching 27 Feb 02

Contents Introduction Design & Detailing Considerations Types of Water Tanks, Usages & Design Guidelines Erection of Precast Ring Tank Design of Water Tank Ultimate Limit State Design – CP 65 Serviceability Limit State Design – BS 8007 Precast Water Tank Floor System

Water Retaining Structures

Water Retaining Structures

Swimming pool Basement wall / slab Retaining Walls Water Tanks Wet riser tanks (suction tanks) Roof tanks

Design & Detailing Considerations

Design Considerations

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

Max crack width Max crack spacing Min steel content

Detailing Considerations

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


The tank roof and the portion above the water level shall be cast in second operation. After passing the water test, the internal surface of the concrete tank shall be coated with approved type of cementitious waterproofing coating before laying of tiles. For the slab of water tank, ratio of length to width 7 1.5 and the shorter span of the slab shall be 3.6m maximum.


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


Cover to links >40mm

Construction Joint

Waterstop

Types of Water Tanks

Types of Water Tanks

CIS

Precast

Rectangular Circular Tank Ring Tank

Usage

Roof tanks Wet riser tanks for fire-fighting (>60m above GL, 22 sty & above) Water tanks for domestic supply Suction tanks (reserve tanks to pump water to roof tanks)

Suction Tanks Reserve tank to pump water to roof tanks Capacity depends on water demand for the block:

< 22 stories D no fire-fighting requirement > 22 stories D fire-fighting requirement

Precast circular tanks or rectangular in-situ tanks.

Types of Water Tanks Distribution System Roof Tanks

Building

Outlets to DUs (Fire-fighting)

Suction tank

Pump

(Domestic)

1st storey from pump

CIS Suction Tanks Detailing

CIS Suction Tanks Detailing

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 ¾ < 20 sty Ä precast circular water tanks ¾ > 20 sty Ä 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.

3700 130 1550 3830 1550

600 3520 3640 TYPE A (3 rings)

3700 130 1550

5130

1300

1550

600 3520 3640 TYPE B (4 rings)

3700 130 1300 2530 1100

3520 3640 TYPE C (2 rings)

Standardisation Items

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)

3830mm

5130mm

2530mm

3640mm

3640mm

3640mm

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

7t

7t

9.1t

6.2t

6.2t (1550mm) 5.3t (1300mm)

5.3t

3.7t 23.1t

3.7t 28.4t

3.7t 18.1t

48t

65t

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

Standardisation

370

6.2 ton

6.2 ton

370

5.3 ton Vol for DU = 15.7m3

370

Vol for DU = 14.06m3

1350 850

Vol for fire fighting = 9.2m3

100 TYPE C

Vol for DU = 22.3m3

9.1 ton

Vol for fire fighting = 13.4m3

100 TYPE A

7 ton each

100 TYPE B

Standardisation

Design Guidelines

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

Design Guidelines

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

Section Through Tank

Typical Water Tank Layout slab

slab

slab

Booster pump room

slab

slab

100mm 100mm

900mm

slab

2100mm

900mm

Erection of Precast Ring Tank


Installation of base tank


Placing of 1T10 around groove


1T10 and waterprooing strip in place


Vertical T20 bars to provide lateral restraint

Installation 2nd ring segment


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 Tanks Ultimate limit state (ULS) – CP65 Serviceability limit state (SLS) – BS8007

Ultimate Limit State Design (CP 65)

1.4 DL + 1.4 Water Load

Partial Safety Factor

1.0DL + 1.4Water Load

γf = 1.4 for load combinations (1) & (2) 1.2 (DL+Water Load+ WL) γf = 1.2 for load combinations (3)

Bending

k = M / fcu*b*d2 Z = d(0.5+(0.25 – k/0.9)1/2) < 0.95d As = M / 0.87fy*z

Shear

Wall

v = V / b*d < vc

Tension

Ft = kR* γ * h * r As > Ft / 0.87fy

Base slab

* 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 Factor

γf = 1.0 for all load combinations (1), (2) & (3)

Triangular Stress Block

Plane sections remain plane after bending Stresses in the steel and concrete are proportional to the strains The concrete is cracked up to the neutral axis, and no tensile stress exists in the concrete below it

Serviceability Limit State Design

Cracks cannot be avoided in RC structures Limit crack widths

Allowable steel stresses (deformed bars)

0.2 mm for severe or very severe exposure 0.1 mm for critical aesthetic appearance 100 N/mm2 for design crack width of 0.1mm 130 N/mm2 for design crack width of 0.2mm

Crack width calculation Minimum reinforcement Spacing of reinforcement Check reinforcement provided against BS 8007

Serviceability Limit State Design b

ε cc

f cc F cc

x d

h

2x/3

Neutral Axis

As

z

F st ε st

Section

Strain

Stress Ac

For equilibrium of forces : 2=α A d-α A x F cc = F 0.5bx st e s e s

0.5bxf2 0.5bx E c = f cc/ε cc ; E s = f st/ε st ε st = (d-x)ε Moment of resistance : cc/x α e = E s/E c A c = bx

+=αAeAsfsstx cc

- α eA sd = 0

x

x = {-α eA s +/- [(α eA s) 2 + 2bα eA sd] 1/2}/b d-x 0.5bxE ε = A α E (d-x)ε /x c

cc

s

s

c

cc

A (x/2) = α A (d-x) Mc= F ccz = Fe stzs M = 0.5bxf cc(d-x/3) = A sf st(d-x/3)

α eA s

Transformed Section

Crack Width Calculation - Flexural Maximum surface crack width at any point, w max =

3a crε m 1 + 2(a cr - c min)/(h-x)

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 εm=ε1-ε2 ε 1 = the apparent strain For a limiting design surface crack width of 0.2 mm, b t(h-x)(a`-x) ε2= 3E sA s(d-x) For a limiting design surface crack width of 0.1 mm, 1.5b t(h-x)(a`-x) ε2= 3E sA s(d-x)

Due to stiffening effect of the concrete

b t = the width of the section at the centroid of the tension steel a` = the distance from the compression face to the point considered

Crack Width Calculation - flexural x a` h

a cr

ε2

εm

c min bt εm=ε1-ε2 ε 1 = the apparent strain For a limiting design surface crack width of 0.2 mm, b t(h-x)(a`-x) ε2= 3E sA s(d-x) For a limiting design surface crack width of 0.1 mm, 1.5b t(h-x)(a`-x) ε2= 3E sA s(d-x) b t = the width of the section at the centroid of the tension steel a` = the distance from the compression face to the point considered

Crack Width Calculation - flexural Maximum surface crack width at any point, w max =

3a crε m

x

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

x a` h

εm=ε1-ε2

a` h

a cr

ε 1 = the apparent strain

ε2

εm

c ε m min < 0 indicates that the section is uncracked a cr c min

am

bt

ε2

εm

c min

bt w max = 3a mε m w min = 3c minε m

For a limiting design surface crack width of 0.2 mm at the soffit, 2 b tb(h-x)(a`-x) t(h-x) ε ε2,h2 = Maximum crack width occurs at Constant midwayvalue between the bars 3E 3E ssA A ss(d-x) (d-x)

Minimum crack width occurs immediately below the bar

Crack Width Calculation - Tension Maximum surface crack width, w max = 3a crε m 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 =ε1-ε2 ε 1 = the apparent strain = F t/(E sA s) For a limiting design surface crack width of 0.2 mm, ε2=

2b th 3E sA s

For a limiting design surface crack width of 0.1 mm, ε2=

b th E sA s

Ft

h = the overall depth of the member b t = the width of the section at the centroid of the tension steel ε m < 0 indicates that the section is uncracked

Ft

Crack Control (Immature conc.)

Due to effects of :

Drying shrinkage Thermal contraction

Minimum reinforcement for crack distribution

Critical steel ratio, ρcrit = 0.0035

ρprov >= ρcrit where ρprov = As/Ac

Crack Control (Immature conc.)

Crack spacing smax = (fct/fb)(φ/2ρprov) fct/fb = 1.6/2.4 = 0.67 fct - the tensile strength of the concrete fb - the average bond strength between concrete and steel φ - the size of the reinforcing bar Spacing of reinforcement Bar spacing not exceed 300mm or thickness of the section, whichever is the lesser For welded fabric reinforcement, bar spacing not exceed 1.5 times the thickness of the section

Check Reinforcement Provided Bending & Tensile Force (SLS)

Allowable steel stresses 2 fy = 100 N/mm for 0.1mm crack width 2 fy = 130 N/mm for 0.2mm crack width Moment of resistance M = Fstz = Asfst(d-x/3) fst = M/[As(d-x/3)] Tensile force due to hydrostatic loading fst = Ft/As

References

Reinforced Concrete Design W. H. Mosley & J. H. Bungey Reinforced and Prestressed Concrete F. K. Kong & R. H. Evans Reinforced Concrete Design to BS 8110 A. H. Allen Design of Concrete Structures for Retaining Aqueous Liquids R. Cheng Design of Liquid-Retaining Concrete Structures R. D. Anchor

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Design & Detailing of Water Retaining Structures & Pre Cast Water Tank Floor System

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