Diseño de Tanques de Agua Cap 1

C H A P T E R 1Tank History, Typical

Configurations,Locating, Sizing,

and Selecting

Ira M. Gabin, P.E.Dixon Engineering

Richard A. Horn, P.E.CB&I

The rapid development and expansion of public water supply systemsat the beginning of the 20th century led to the establishment of publichealth standards for drinking water systems. An area of major con-cern for these systems was the storage facilities. Early steel reservoirsand standpipes were of riveted construction. Modern welded-steelreservoirs can be built to very large capacities with either domed orcolumn-supported roofs.

In the 1970s, it became common for smaller-capacity reservoirsand standpipes to use bolted construction technology, originally de-veloped for industrial and agricultural uses. Prefabricated panels andbolted connections reduced erection costs and made these structurespopular in rural areas. The advent of factory-applied ceramic coat-ings reduced future maintenance costs, adding to the tanks’ attrac-tiveness to water supply systems with limited financial resources.Bolted tanks with diameters greater than 30 ft (9 m) are often builtwith low-maintenance aluminum geodesic domed roofs, a technology

1

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Source: Steel Water Storage Tanks: Design, Construction, Maintenance, and Repair

2 C h a p t e r O n e

FIGURE 1-1 Geodesic dome on bolted-steel reservoir.

commonly found on wastewater plant storage tanks as well. Figure1-1 shows a geodesic dome on a bolted reservoir.

The earliest elevated storage tanks were constructed of wood inthe manner of water refilling stations for steam-powered trains. Somewere built on stone or brick columns. Limitations as to size and dura-bility, as well as public health concerns, led to steel becoming thematerial of choice for elevated tanks. Most steel elevated tanks con-structed before 1950 were riveted, their legs consisting of opposedchannels connected by latticework bracing. Roofs on most small tanksand many larger ones were the familiar cone or “witch’s hat” design(Fig. 1-2). Some larger elevated tanks had hemispherical or ellipsoidalroof designs.

Welded construction became the industry norm by the early 1950sand remains the standard for most elevated tanks. Legged tanks con-tinued to be built in great numbers; however, the lattice legs replacedtubular sections. Many larger-capacity legged tanks were of the radialarm design shown in Fig. 1-3. These have been phased out in favor ofthe toroelliptical legged tank style.

Early prototypes of single-pedestal tanks were developed in the1940s and became a common alternative to legged tanks by the 1950s.The more efficient shape of these structures provided the advantageof lower maintenance costs. In the 1960s, the fluted-column single-pedestal design was introduced, which provided a usable area in thecolumn for pumping equipment, storage, offices, and other municipaluses.

Legged tanks continue to be built primarily in sizes up to 1 milliongallons (mil gal) (3.8 million liters [ML]) as a lower-cost alternativeto single-pedestal or fluted-column tanks. Single-pedestal tanks arewidely specified from very small to large capacities. Larger capacities



Tank History, Typical Configurations, Locating, Sizing, and Selecting

3T a n k H i s t o r y , C o n f i g u r a t i o n s , L o c a t i n g , S i z i n g , S e l e c t i n g

FIGURE 1-2 Witch’shat roof design.

FIGURE 1-3 Legged tank with radial arm design.





(0.75 to 2 mil gal [2.8 to 7.6 ML] or more) are generally single-pedestalor fluted-column tanks. Some fluted-column tanks have even largercapacities.

In the late 1980s, composite-tank technology combined a concretepedestal with the steel-bowl geometry of the fluted-column tank. Thisaddressed one of the concerns of the fluted-column design—the largesteel surface area and resulting higher repainting costs. Built generallyto hold 0.75 to 2 mil gal (2.8 to 7.6 ML) of water, composite tanks arenow in use throughout the United States and Canada.

Other materials and technologies are available for specialized ap-plications. However, the steel, glass-lined steel, concrete, and com-posite tanks discussed in this chapter comprise the large majority oftanks currently in use and being specified for new construction.

ReservoirsA reservoir is a ground-supported, flat-bottom cylindrical tank with ashell height less than or equal to its diameter. Reservoirs are one of themost common types of water storage structure. They are used as a partof the distribution system as well as to hold treated water for pumpinginto the distribution system. Of the three types of steel water tanks, areservoir, because of its low height, is generally the most economicalto fabricate, erect, and maintain. See Figs. 1-4 and 1-5 for a photo anda cross-sectional view of a welded-steel reservoir; see Figs. 1-6 and 1-7for a photo and a cross-sectional view of a bolted-steel reservoir. Table1-1 gives typical sizes of welded-steel reservoirs, and Table 1-2 givescapacities of glass-coated, bolted-steel reservoirs and standpipes.

Storage reservoirs for potable water are covered by roof structures,which may be either column supported or self-supporting. Standardtank accessories may include shell and roof manholes, screened roofvents, inside or outside ladders, and connections for pipes as required.

StandpipesStandpipes are ground-supported, flat-bottom cylindrical storagetanks that are taller than their diameter. They are usually built wherethere is little elevated terrain and where extra height is needed to cre-ate pressure for water distribution. See Figs. 1-8 and 1-9 for a photoand a cross-sectional view of a welded-steel standpipe and Figs. 1-10and 1-11 for a photo and a cross-sectional view of a bolted-steel stand-pipe. Table 1-3 gives capacities and sizes of typical welded-steel stand-pipes.

Standpipe systems are often designed so that the water in the tank,until it reaches a certain low level, maintains the system pressure.When that low level is reached, pumps come on, valving is changed,





FIGURE 1-4 Welded-steel reservoir. (Photo: Gay Porter DeNileon, AWWA)

Capacity level

Roof manholesRoof vent

Approved ladder, cage, platform, or safety devices complying with Occupational Safety and Health Act

Roof rafters

Columnbases

Shell manholes(two required)

Overflowpipe

Splashpad

Inlet–outlet

(optional)

Base elbow or valve pit

Weir box(optional)

Column support

Tank bottomcrowned at center

Sand pad

Compacted backfill

Crushed rock or gravel Concrete foundation

12 in. (0.3 m)

3 ⁄4 in. (19 mm)

FIGURE 1-5 Cross-sectional view of welded-steel reservoir. (Source: AWWAManual M42, Steel Water-Storage Tanks)





FIGURE 1-6 Bolted-steel reservoir, glass fused to steel.

Approved ladder, cage, and platform

complying with Occupational Safety

and Health Act

Roof manway Gravity

ventilatorInternal

overflow

funnel

Overflowpipe

Splash

pad

Gradelevel

Inlet–outlet

24-in. (0.6-m) round

access door

Floor sloped toward

outlet pipe

FIGURE 1-7 Cross-sectional view of bolted-steel reservoir. (Source: AWWAManual M42, Steel Water-Storage Tanks)




Capacity Range of Sizes Available

Diameter Height to Diameter Height to(US gal) (m3) (ft [in.]) TCL (ft [in.]) (m) TCL (m)

50,000 189 19 [3] 24 [0] 5.9 7.3

60,000 227 21 [0] 24 [0] 6.4 7.3

75,000 284 23 [6] 24 [0] 7.2 7.3

100,000 379 23 [6] 32 [0] 7.2 9.8

27 [0] 24 [0] 8.2 7.3

125,000 473 26 [0] 32 [0] 7.9 9.8

30 [3] 24 [0] 9.2 7.3

150,000 568 28 [6] 32 [0] 8.7 9.8

33 [0] 24 [0] 10.0 7.3

200,000 757 33 [0] 32 [0] 10.0 9.8

38 [3] 24 [0] 11.7 7.3

250,000 946 37 [0] 32 [0] 11.3 9.8

42 [9] 24 [0] 13.0 7.3

300,000 1,136 40 [6] 32 [0] 12.3 9.8

46 [9] 24 [0] 14.3 7.3

400,000 1,515 46 [6] 32 [0] 14.2 9.8

54 [0] 24 [0] 16.5 7.3

500,000 1,893 46 [6] 40 [0] 14.2 12.2

52 [0] 32 [0] 15.9 9.8

60 [6] 24 [0] 18.4 7.3

600,000 2,271 51 [0] 40 [0] 15.6 12.2

57 [0] 32 [0] 17.4 9.8

750,000 2,839 57 [0] 40 [0] 17.4 12.2

64 [0] 32 [0] 19.5 9.8

1,000,000 3,785 66 [0] 40 [0] 20.1 12.2

74 [0] 32 [0] 22.6 9.8

1,500,000 5,678 80 [6] 40 [0] 24.5 12.2

90 [6] 32 [0] 27.6 9.8

2,000,000 7,571 93 [0] 40 [0] 28.4 12.2

104 [6] 32 [0] 31.9 9.8

3,000,000 11,356 114 [0] 40 [0] 34.7 12.2

127 [6] 32 [0] 38.9 9.8

4,000,000 15,142 131 [6] 40 [0] 40.1 12.2

147 [6] 32 [0] 44.9 9.8

5,000,000 18,927 147 [0] 40 [0] 44.8 12.2

165 [0] 32 [0] 50.3 9.8

7,500,000 28,391 180 [0] 40 [0] 54.9 12.2

201 [6] 32 [0] 61.4 9.8

10,000,000 37,854 233 [0] 32 [0] 71.0 9.8

208 [0] 40 [0] 63.5 12.2

Source: AWWA Manual M42, Steel Water-Storage Tanks.Note: TCL = top capacity level.

TABLE 1-1 Capacities and Sizes of Typical Welded-Steel Water-StorageReservoirs

7




Nom

inalH

eig

ht

(ft)

∗

15

19

24

28

33

38

43

47

52

57

61

66

70

75

79

84

89

93

98

102

107

112

116

121

Nom

inal

Dia

mete

r(f

t)∗

Capacit

yin

Thousands

ofG

allons†

14

16

22

27

32

37

44

49

54

59

65

70

75

80

86

91

96

10

11

07

11

21

17

12

21

28

13

31

39

17

24

31

39

47

54

63

70

78

86

93

10

11

08

11

61

23

13

11

39

14

61

54

16

11

69

17

71

84

19

21

99

20

33

43

53

64

74

86

96

10

61

17

12

21

37

14

81

58

16

81

79

18

91

99

21

02

20

23

02

41

25

12

61

27

2

25

54

71

88

10

51

22

14

21

59

17

61

93

21

02

27

24

42

61

27

82

96

31

33

30

34

73

64

31

81

10

71

32

15

81

83

21

22

38

26

32

89

32

03

40

36

53

91

41

64

42

36

11

41

49

18

52

20

25

62

92

32

73

63

39

84

34

46

95

05

42

15

11

99

24

62

94

34

13

88

43

64

83

53

15

78

50

21

82

86

35

54

23

49

15

59

62

86

96

62

32

63

26

42

85

30

63

27

34

83

6

70

42

15

53

68

58

16

94

8

81

56

77

44

92

11

,09

9

90

69

19

06

1,1

22

1,3

37

10

18

74

1,1

47

1,4

20

12

01

,24

71

,63

7

∗To

conv

ertf

eett

om

eter

s,m

ulti

ply

by0.

3048

.†

Cap

acit

yin

thou

sand

sof

gallo

ns.T

oco

nver

tgal

lons

tocu

bic

met

ers,

mul

tipl

yby

0.00

3785

4.So

urce

:AW

WA

Man

ualM

42,S

teel

Wat

er-S

tora

geTa

nks.

TA

BLE

1-2

Capaci

ties

ofG

lass-C

oate

d,B

olted-S

teelR

eserv

oirs

and

Sta

ndpip

es

8





FIGURE 1-8Welded-steelstandpipe withdecorativepilasters.

and water is pumped from the lower portion of the standpipe into thesystem.

As with reservoirs, steel standpipes are covered with a roof struc-ture and may be provided with ornamental trim. Standard accessoriesmay include shell and roof manholes, roof vent(s), a fixed outsideladder, and connections or pipes as required. Inside ladders are notrecommended in locations where freezing weather can be expected.

Roof Designs for Reservoirs and StandpipesThe emphasis on making steel water reservoirs and standpipes at-tractive as well as functional has led to the development of a widevariety of roof designs. Alternative roof styles for welded tanks in-clude conical, toriconical, umbrella, dome, and ellipsoidal designs.Some are column supported; others are self-supporting. Bolted-steeltanks are usually provided with conical roofs or may be furnishedwith an aluminum geodesic dome. Column-supported roof struc-tures are not usually used on steel standpipes taller than 50 ft (15 m).Whichever design is selected, it is particularly important to designany rafters, trusses, columns, stiffeners, and connections to minimizepotential corrosion sites. All interfaces and connections of such mem-bers should be analyzed for their corrosion potential, and protective





Approved ladder, cage, platform, or safety devices complying with Occupational Safety and Health Act

Painter’s trolley rail

Weir box(optional)

Capacity level

Concretefoundation

Sand pad

Crushed rock or gravel

Compacted backfillor undisturbed soil

Shell manholes(two required)

Tank bottomcrowned at center

Inlet–outlet(optional)

Base elbow orvalve pit

Splashpad

Roof ventRoof plate

Roof manholes

Overflow pipe

FIGURE 1-9 Cross-sectional view of typical welded-steel standpipe.(Source: AWWA Manual M42, Steel Water-Storage Tanks)

coatings should be applied to all surfaces deemed necessary from acost/benefit standpoint.

Column- and Rafter-Supported Cone RoofsThe column- and rafter-supported roof (Fig. 1-12) is generally the mosteconomical for a reservoir. The roof has a minimum slope for adequatedrainage and provides easy access to the manhole for interior inspec-tion. Column loads are spread to a safe limit by column bases, andconcrete footings under the columns are not usually required.

A modification of this design incorporates a transition from theshell plate to the roof plate that is a smooth curve rather than a sharpbreak. This transition, or knuckle plate, is a dished or rolled section thatusually requires a stiffener at the rafter attachment point (Fig. 1-13).




FIGURE 1-10 Bolted-steel standpipe.

Approved ladder, cage, and platform complying with Occupational Safety and Health Act

Roof accessGravityventilator

Internaloverflowfunnel

Overflowpipe

Splash

pad

Gradelevel

Inlet–outlet(optional)

24-in. (0.6-m) roundaccess door

Floor sloped towardoutlet pipe

Topelbow

Roof walkway and guard rail

FIGURE 1-11 Cross-sectional view of bolted-steel standpipe. (Source: AWWAManual M42, Steel Water-Storage Tanks)






Diameter Height to Diameter Height to

(US gal) (m3) (ft [in.]) TCL (ft [in.]) (m) TCL (m)

50,000 189 14 [9] 40 [0] 4.5 12.2

60,000 227 16 [2] 40 [0] 4.9 12.2

75,000 284 18 [0] 40 [0] 5.5 12.2

100,000 379 19 [0] 48 [0] 5.8 14.6

125,000 473 21 [3] 48 [0] 6.5 14.6

150,000 568 23 [3] 48 [0] 7.1 14.6

200,000 757 24 [10] 56 [0] 7.6 17.1

250,000 946 27 [9] 56 [0] 8.5 17.1

300,000 1,136 28 [5] 64 [0] 8.7 19.5

400,000 1,514 32 [10] 64 [0] 10.0 19.5

500,000 1,893 34 [7] 72 [0] 10.5 21.9

600,000 2,271 37 [10] 72 [0] 11.5 21.9

750,000 2,839 42 [6] 72 [0] 12.9 21.9

1,000,000 3,785 46 [4] 80 [0] 14.1 24.4

1,500,000 5,678 56 [9] 80 [0] 17.3 24.4

2,000,000 7,571 65 [6] 80 [0] 20.0 24.4

2,500,000 9,464 69 [10] 88 [0] 21.3 26.8

3,000,000 11,356 76 [6] 88 [0] 23.3 26.8

4,000,000 15,142 84 [6] 96 [0] 25.8 29.3

5,000,000 18,927 94 [6] 96 [0] 28.8 29.3

Source: AWWA Manual M42, Steel Water-Storage Tanks.Note: TCL = top capacity level.

TABLE 1-3 Capacities and Sizes of Typical Welded-Steel Standpipes

Self-Supporting Dome Roof and Umbrella RoofSteel self-supporting roofs are constructed of plates that are buttwelded, lap welded, or lap bolted. They are supported directly onthe top angle and shell plate. This type of roof is used where an un-cluttered interior and smooth exterior appearance are desired. Dome-roof sections are pressed to form a spherical shape. Umbrella roofs areformed to a radius in one direction only, forming chords like the clothbetween the spines of an umbrella (Fig. 1-14).

Structural stiffeners may be used internally on large-diameterroofs to avoid excessive plate thickness on welded or bolted tanks.Sometimes steel trusses may be used to support the roof, but these





Capacitylevel

Vent

Columnbase

Girders arerequired whenmore than onecolumn is used

12 in. (0.3 m)

3⁄4 in. (19 mm)

3 ⁄16-in. (4.7-mm) lap-welded roof plate

Channelrafters

1 ⁄4-in. (6.4-mm)

lap-welded

bottom plate

One or moresupportingcolumns

Top angle

Butt-weldedtank shell

FIGURE 1-12 Tank with column- and rafter-supported cone roof. (Source:AWWA Manual M42, Steel Water-Storage Tanks)

should be avoided if possible, because they may create corrosionproblems. In addition, the trusses should be kept above the waterline to prevent damage by ice and accelerated rates of corrosion.

A modification of the self-supporting dome is the toriconical roof.This consists of a rolled or pressed knuckle and a higher-pitched self-supporting center.

Aluminum dome roofs are sometimes erected on bolted-steel orwelded-steel tanks. These aluminum domes are usually constructed

Knuckle plate 12 in. (0.3 m)

¾ in. (19 mm)

Radius


Channelrafter

Columnbase

Capacity

level

One or moresupportingcolumns

3/16 in. (4.7-mm)

lap-welded roof plate

¼-in. (6.4-mm)lap-weldedbottom plate

FIGURE 1-13 Column- and rafter-supported roof with knuckle. (Source: AWWAManual M42, Steel Water-Storage Tanks)





Capacity level

Vent

3 ⁄16-in. (4.7-mm) minimum thicknesslap- or butt-weldedroof plate

1 ⁄4-in. (6.4-mm) lap-weldedbottom plate


Topangle

Spherical radius

= 1.2 D. max.

0.80 D. min.

Cap plate

FIGURE 1-14 Self-supporting dome roof or umbrella roof. (Source: AWWAManual M42, Steel Water-Storage Tanks)

of triangulated space truss (geodesic) panels. The dead weight of thesedomes is usually 3 lb/ft2 (143 N/m2) or less, compared with 3.8 lb/ft2

(181 N/m2) for a bolted-steel roof and 7.6 lb/ft2 (364 N/m2) for awelded-steel roof.

Self-Supporting Ellipsoidal RoofThe self-supporting ellipsoidal roof is not a true ellipse, but it is formedwith two radii yielding major- and minor-axis proportions of approxi-mately 2:1. The transition from shell to roof is a smooth unbroken curve(Fig. 1-15). This roof design is suitable for large- and small-diameterreservoirs and standpipes. On tanks 50 ft (15 m) in diameter or less,the roof is usually free of internal structural members. Larger-diametertanks usually have radial and circumferential stiffening members orrafters, which may be subject to corrosion problems if they are notproperly designed or maintained.

Self-Supporting Cone RoofAn inexpensive and very functional type of roof for small-diameterreservoirs and standpipes is the self-supporting cone roof without in-ternal structural members. This roof is usually too steep to walk on.Access to manholes and vents by a roof ladder or steps and handrailshould be provided. All means of access should be designed individ-ually and installed to comply with current standards.





Capacity level

Vent1⁄4-in. (6.4-mm) minimum thicknessand butt welded in areafilled with water

Area abovecapacity levelmay be lap welded

1 ⁄4-in. (6.4-mm) lap-weldedbottom plate


Knu

ckle

FIGURE 1-15 Self-supporting ellipsoidal roof. (Source: AWWA Manual M42,Steel Water-Storage Tanks)

Elevated TanksAn elevated steel water tank has two primary components: the tank it-self and its supporting structure. Such tanks are ordinarily used wherethere is insufficient elevated terrain to ensure distribution of water atsuitable pressure by gravity. These tanks are of welded construction.

Elevated tanks can be categorized into several different types. Thevarious diameters and head ranges for the tanks described in the re-maining figures and tables in this chapter are only representative andmay vary with individual fabricators. Specific diameter/head rangecombinations should be determined by the tank fabricator within thelimits indicated in the tables. Height should be specified by the pur-chaser as the dimension between the top of the foundation and the topcapacity level of the tank. Further dimensions, which are a function ofthe fabricator’s standard, should not be specified. To minimize cost,desired operating ranges should be specified to fall within standardavailable tank dimensions. However, individual operating needs maydictate nonstandard operating ranges.

Multiple-Column Elevated Tanks

Small-Capacity Elevated (Double-Ellipsoidal) TanksThe small-capacity multiple-column elevated (or double-ellipsoidal)tank has a cylindrical sidewall, an ellipsoidal bottom and roof, and a





FIGURE 1-16Double-ellipsoidaltank. (Photo: GayPorter DeNileon,AWWA)

top capacity level (TCL) in the roof several feet or meters above thetop of the cylindrical shell. Although in the past they were constructedin capacities up to 1 mil gal (3.8 ML), today, double-ellipsoidal tanksare typically constructed only in capacities of 200,000 gal (760,000 L)or less. See Figs. 1-16 and 1-17 for a photo and a cross-sectional viewof a small-capacity elevated (double-ellipsoidal) tank. Table 1-4 givescapacities and sizes of typical double-ellipsoidal elevated tanks.

Medium-Capacity Elevated TanksFor medium-capacity multiple-column elevated tanks, the toroellip-soidal design provides a lower initial cost by using the strength of steelmost efficiently. The features used (torus bottom and ellipsoidal roof)cause the central riser to support, as well as contain, a considerableportion of the stored water, while the major portion of the steel bottomacts as a membrane in tension. These tanks usually have a capacitybetween 200,000 gal (760,000 L) and 500,000 gal (1.9 ML). See Figs.1-18 and 1-19 for a photo and a cross-sectional view of a medium-capacity elevated tank. Table 1-5 gives capacities and sizes of typicalmedium-capacity elevated tanks.





Diameter

Headrange

As r

equired

Purc

haser

to s

pe

cify

6 in. (0.15 m) min.

Balcony orstiffening girder

FIGURE 1-17 Cross-sectional view of double-ellipsoidal tank. (Source: AWWAManual M42, Steel Water-Storage Tanks)

Large-Capacity Multiple-Column Elevated TanksLarge-capacity elevated tanks (>500,000 gal [>1,893 m3]) provide eco-nomical service for communities that need to store a substantial vol-ume of water. Lower operating and pumping costs are ensured be-cause of the low head range, which achieves minimum variation of wa-ter pressure throughout the system. See Figs. 1-20 and 1-21 for a photoand a cross-sectional view of a large-capacity elevated tank. Table 1-6gives capacities and sizes of typical large-capacity elevated tanks.

Pedestal Elevated Tanks

Small-Capacity Single-Pedestal TanksThe single-pedestal spherical tank is widely favored for smaller-capacity tanks when appearance is a concern. The gracefully flaredbase contains sufficient space for pumping units and other operatingequipment, a feature common to all pedestal-type vessels.






Diameter Head Diameter Head

(US gal) (m3) (ft) Range (ft) (m) Range (m)

25,000 95 18–20 12.5–15.5 5.5–6.1 3.3–4.7

30,000 114 18–20 15.0–16.5 5.5–6.1 4.6–5.0

40,000 151 22–23 15.0–17.0 5.7–7.0 4.6–5.2

50,000 189 22–24 18.0–20.0 6.7–7.3 5.5–6.1

60,000 227 22–25 19.0–23.0 6.7–7.6 5.3–7.0

75,000 284 26–30 16.0–24.0 7.9–9.1 4.9–7.3

100,000 379 23–30 20.0–25.0 3.5–9.1 6.1–7.6

125,000 473 30–32 23.0–28.0 9.1–9.7 7.0–8.5

150,000 568 32–34 24.5–29.5 9.7–10.4 7.5–9.0

200,000 757 36–38 28.0–29.5 11.0–11.6 8.5–9.0

Source: AWWA Manual M42, Steel Water-Storage Tanks.

TABLE 1-4 Capacities and Sizes of Typical Double-Ellipsoidal Elevated Tanks

FIGURE 1-18Medium-capacitywelded-steelelevated tank.(Photo: Gay PorterDeNileon, AWWA)





Balcony orstiffening girder

6 in. min.

HeadRange

Purc

haser

to s

pe

cify

As r

equired

FIGURE 1-19 Cross-sectional view of medium-capacity, torus-bottom welded-steel elevated tank. (Source: AWWA Manual M42, Steel Water-Storage Tanks)

Ladders to the container and roof are inside to protect againstunauthorized access. These tanks are usually constructed in capacitiesof 200,000 gal (760,000 L) or less. See Figs. 1-22 and 1-23 for a photo anda cross-sectional view of a small-capacity single-pedestal tank. Table1-7 gives capacities and sizes of typical small-capacity single-pedestaltanks.

Small-capacity elevated tanks are also constructed as various com-binations of cones and cylinders. An alternative design is shown inFig. 1-24.

Large-Capacity Single-Pedestal TanksThe tubular supporting pedestal gives the large-capacity single-pedestal tank a distinctively contemporary look. Large capacities (0.2to 2 mil gal [0.76 to 7.6 ML]) are provided by this low-head-rangespheroidal tank design. See Figs. 1-25 and 1-26 for a photo and across-sectional view of a large-capacity single-pedestal tank. Table 1-8






Diameter Height to Diameter Height to

(US gal) (m3) (ft) TCL (ft [in.]) (m) TCL (m)

200,000 757 36–38 28 [30] 11.0–11.6 8.5–9.1

250,000 946 38–40 28 [33] 11.6–12.2 8.5–10.1

300,000 1,136 43–45 28 [31] 13.1–13.7 8.5–9.4

400,000 1,514 46–50 30 [36] 14.0–15.2 9.1–11.0

500,000 1,893 50–56 29 [38] 15.2–17.1 8.8–11.5

600,000 2,271 51–0 40 [0] 15.6 12.2

57–0 32 [0] 17.4 9.8

750,000 2,839 56–65 34 [45] 17.1–19.8 10.4–13.7

1,000,000 3,785 64–65 45 [46] 19.5–19.8 13.7–14.0


TABLE 1-5 Capacities and Sizes of Typical Medium-Capacity Elevated Tanks

FIGURE 1-20 Large-capacity elevated tank. (Photo courtesy of LandmarkStructures, Inc.)





Purc

haser

to s

pecify

As r

equired

Headrange

Diameter

6 in. (0.15 m) min.

FIGURE 1-21 Cross-sectional view of large-capacity, multicolumn elevatedtank. (Source: AWWA Manual M42, Steel Water-Storage Tanks)




500,000 1,893 60–65 24–25 18.3–19.8 7.3–7.9

600,000 2,271 65–70 24–25 19.8–21.3 7.3–7.9

750,000 2,839 70–76 25–30 21.3–23.2 7.6–9.1

1,000,000 3,785 75–87 25–35 22.9–25.5 7.6–10.7

1,500,000 5,678 91–98 30–35 27.7–29.9 9.1–10.7

2,000,000 7,571 105–106 34–36 32.0–32.3 10.4–11.0

2,500,000 9,464 108–117 39–41 32.9–35.7 11.9–12.5

3,000,000 11,356 119–127 35–40 36.3–38.7 10.7–12.2


TABLE 1-6 Capacities and Sizes of Typical Large-Capacity Welded-SteelElevated Tanks





FIGURE 1-22Sphericalsingle-pedestaltanks give pleasantsilhouette. (Photo:Walter Baas,AWWA)

gives capacities and sizes of typical large-capacity single-pedestaltanks.

Modified Single-Pedestal TanksThe attractive modified single-pedestal tank has a central support col-umn (usually fluted to give structural rigidity) that encloses the riserpipe, overflow pipe, and access ladder to the tank roof. The supportcolumn may be constructed of steel or concrete. The space within thecolumn can provide multistory usable floor space for pumping, stor-age, and office facilities. Although available in all capacities, thesetanks are not usually constructed in capacities less than 500,000 gal(1.9 ML). See Figs. 1-27 and 1-28 for a photo and a cross-sectional viewof a modified single-pedestal tank. Table 1-9 gives capacities and sizesof typical modified single-pedestal tanks.

Composite Elevated TanksComposite elevated tanks are of an attractive design that uses thebest design features of steel and concrete. Concrete, which is excellentfor compression loads, is used as the support column for the steelbowl. The concrete has the advantage of requiring either no painting




Head

range

As r

equired

Purc

haser

to s

pecify

6 in. (0.15 m) min.

Diameter

FIGURE 1-23 Cross-sectional view of small-capacity spherical single-pedestaltank. (Source: AWWA Manual M42, Steel Water-Storage Tanks)




25,000 95 19–20 15–17 5.8–6.1 4.6–5.2

30,000 114 20–21 15–18 6.1–6.4 4.6–5.5

40,000 151 21–23 19–22 6.4–7.0 5.8–6.7

50,000 189 23–24 19–23 7.0–7.3 5.8–7.0

60,000 227 24–26 22–24 7.3–7.9 6.7–7.3

75,000 284 25–28 23–27 7.9–8.5 7.0–8.2

100,000 379 29–30 25–30 8.8–9.1 7.6–9.1

125,000 473 31–33 27–32 9.4–10.0 8.2–9.7

150,000 568 33–34 30–34 10.1–10.4 9.1–10.4

200,000 757 36–38 36–38 11.0–11.6 11.0–11.6


TABLE 1-7 Capacities and Sizes of Typical Small-Capacity Single-PedestalTanks

23





FIGURE 1-24 Alternative single-pedestal tank design.

or a low-cost exterior coating for aesthetic purposes. The steel bowlconstruction is similar to that found on the fluted-column tanks; thebowl can be built with either a cone or a domed roof. The most commondesigns use a domed concrete floor with a steel liner. Commonly builtto store between 750,000 gal and 2 mil gal (2.8 and 7.6 ML), these tanksprovide many of the benefits of a fluted-column tank with significantlyless area that requires painting, thereby reducing maintenance costs.The diameter of the concrete column is generally somewhat smaller(30 to 60 ft [9 to 18 m]) than for a fluted-column tank, so the area in thecolumn for other uses is reduced. See Figs. 1-29 and 1-30 for a photoand cross-sectional view of a composite elevated tank.

Locating, Sizing, and Selecting a Water TankLocating, sizing, and selecting a water-storage tank involve the eval-uation of several design considerations and require an awareness ofzoning and other regulations. The purpose of this section is to dis-cuss these considerations and to provide the reader with a checklistto work through in the effort to arrive at a reasonable solution.





FIGURE 1-25 Large-capacity single-pedestal elevated tank. (Photo courtesy ofTnemec/STI/SPFA)

Locating a Water TankGenerally, locating tanks depends on where people are living now andwhere future neighborhoods will be built within the area served bythe water system. In addition, numerous other conditions can signif-icantly influence the choice of a suitable site and therefore the overallcost of the tank project. Answers to the following basic questions mustbe determined and considered when selecting a location for a newwater-storage tank.

Hydraulics� What are the maximum and minimum pressures that youwant to provide the end users?� Is it better to pump or use gravity flow to provide the neededpressure?� What are the local utility costs of pumping during daily andpeak demand periods?

Proximity to Users� Where is the growth in the community taking place now andprojected to be in the future?� Is land available in the area of future growth?





Access

tube

6 in. (0.15 m) min.

Head

range

Diameter

As r

equired

Purc

haser

to s

pecify

FIGURE 1-26 Cross-sectional view of large-capacity single-pedestal elevatedtank. (Source: AWWA Manual 42, Steel Water-Storage Tanks)




200,000 757 40–42 27–30 12.2–12.8 8.2–9.1

250,000 946 43–47 25–32 13.1–14.3 7.6–9.7

300,000 1,136 46–48 30–33 14.0–14.6 9.1–10.1

400,000 1,514 50–53 30–40 15.2–16.1 9.1–12.2

500,000 1,893 55–60 30–40 16.3–18.3 9.1–12.2

750,000 2,839 64–66 38–42 19.5–20.1 11.6–12.3

1,000,000 3,785 74–78 35–40 22.5–23.8 10.7–12.2

1,250,000 4,732 76–80 40–45 22.9–24.4 12.2–13.7

1,500,000 5,678 85–90 45–50 25.9–27.4 13.7–15.2

2,000,000 7,571 90–95 50–55 27.4–29.0 15.2–16.3


TABLE 1-8 Capacities and Sizes of Typical Large-Capacity Single-PedestalTanks





FIGURE 1-27 Folded-plate design of a modified single-pedestal tank support.(Photo courtesy of Tnemec/STI/SPFA)

Acquiring Land� What is the cost of the tank site being considered? Is the landeven available?� What is the cost of connecting water mains and permanentelectrical power at each site being considered?

Zoning� Is a zoning map available, and are the potential sites zonedto allow a tank project?

Federal Aviation Administration (FAA)� Would the FAA allow a tank at the required height to be builton the potential site?� Are obstruction lights or FAA painting required on the tankat the potential site?

Size of SiteIs the site large enough for� Erection equipment, steel storage, staging operations, ground

assembly, and crane operations with a safe and adequate dis-tance for items that may be dropped from the tank duringerection?





Fluted

column

Diameter

Head

range

As r

equired

Purc

haser

to s

pecify

FIGURE 1-28 Cross-sectional view of a modified single-pedestal tank.




250,000 946 41–43 29–31 12.5–13.1 8.8–9.4

300,000 1,136 43–45 29–31 13.1–13.7 8.8–9.4

500,000 1,893 49–64 30–39 14.9–19.5 9.1–11.9

750,000 2,839 63–65 37–40 19.2–19.8 11.3–12.2

1,000,000 3,785 73–78 35–42 22.2–23.8 10.7–12.8

1,250,000 4,732 76–80 40–45 22.9–24.4 12.2–13.7

1,500,000 5,678 85–87 39–46 25.9–26.5 11.9–14.0

2,000,000 7,571 97–102 38–46 29.6–31.1 11.6–14.0

2,500,000 9,464 107–110 43–45 32.6–33.5 13.1–13.7

3,000,000 11,356 109–120 40–45 33.3–36.6 12.2–13.7


TABLE 1-9 Capacities and Sizes of Typical Modified Single-Pedestal Tanks





FIGURE 1-29 Composite elevated tank.� Maintenance of the tank and piping after completion?� Abrasive blasting and painting of the tank now and in thefuture?

TopographyDoes the site have—or can it be made to have—good drainageto ease construction operations and minimize standing wateraround the completed tank?

Access to Site� Is the site accessible on public roads by concrete and largesemitrailer tractor rigs?� Is there an access road or temporary easement to the site?Will permission be given to build a road? Who will pay for theroad? Will it be a permanent or temporary road? If temporary,will it be necessary to remove it at the end of the project?

Soil Conditions� Is the soil bearing strength at the bottom of the tank founda-tion adequate to support the tank without requiring expen-sive deep foundations?





Upper roofcone

Concretesupport dome

Low water line

Steel bottomplate

High waterline

Lower cone

Concrete column

Access tube

Note: Not to scale.

FIGURE 1-30 Cross-sectional view of composite elevated tank.

� Where is the water table? Will the foundation need to be de-watered during construction?� Is the earth firm enough to support construction equipmentduring normal weather conditions or will gravel, crane mats,and other earth-stabilizing methods be required?

Hazards and Construction� Are there power lines or other obstructions above or besidethe site or proposed access road that would interfere with thesafety of site traffic, construction, painting, or maintenanceoperations? Will a power line be closer than 40 ft (12.2 m)from the tank?� Are there underground obstructions such as gas lines, sewers,or buried electrical or telephone lines? Were there mines orburial grounds on this site?





� If pile driving is required, will it disturb or cause failure of ordamage to neighboring foundations or other structures?� Will pile driving, excavation, steel erection, or abrasive blast-ing cause noise unacceptable to a neighbor such as a school,hospital, or nursing home?� Will the tank be in an area frequented by small children orvandals and, if so, could this be mitigated by site fencing?

Environmental Assessment� Has an environmental assessment been completed on the site?� What agencies, forms, and permits may be required, and howlong will approvals take?

NIMBY (Not in My Backyard)� Will the tank obstruct the view of historical landmarks orother items of concern to the citizens?� How sensitive are the neighbors to having a tank in closeproximity?

Determining answers to these questions can help you to betteranalyze and compare costs of alternate sites, so you can select themost desirable location for your new tank. Additionally, you will wantto understand and consider the following criteria during your siteselection.

HydraulicsOther issues that affect site selection include the required pressureat hydrants and residences, the required site elevation, compatibilitywith the distribution system, the geographic size and location of thedemand area, and the tank’s proximity to the water supply. Rules ofthumb for required water pressure are shown in Table 1-10. Check thelocal standards or codes for more specific requirements.

One hundred sixty-two US and Canadian water utilities re-sponded to an AWWA network modeling survey that requested theactual minimum and maximum distribution system pressures that

Pressure

Location (psi/kPa) Comments

At hydrants

during fire flow

conditions

35/241 20-psi (0.138-kPa) minimum at

other fire hydrants not directly

serving the fire

Residential 50–75/ Higher pressures may need to

0.345–0.517 use a pressure-reducing valve

TABLE 1-10 Required Water Pressure





0

20

40

60

80

100

0

20

40

60

80

100

Minimum pressure Maximum pressure

Min. psi Max. psi

Perc

ent

Perc

ent62%

4%

10%

28%

34%

23%

1%

55%

10%

12%

25%

15%

14%

18%

6%

> 60 psi

50–59 psi

40–49 psi

30–39 psi

20–29 psi

< 20 psi

> 170 psi

150–169 psi

130–149 psi

110–129 psi

90–109 psi

70–89 psi

< 70 psi

FIGURE 1-31 Pressure ranges for utilities.

they provided. Figure 1-31 shows the percentage of utilities in eachpressure range. If the pressures provided are more than 75 psi (0.517kPa), it may be necessary to provide a pressure-reducing valve to pre-vent home appliances from being overpressurized.

The required pressure can be provided through pumping, gravityflow, or a combination of the two. How pressure is provided dependson the sites available and the type of tank to be used.

Pumping with Ground Storage TanksPumping will be required if a ground storage tank is used where thetopography is relatively flat throughout the service area and a higher-elevation site is unavailable.

Gravity Flow with Ground Storage and Elevated TanksThe required pressure can also be obtained by building a ground stor-age tank on a hill or at higher elevation above the demand area so thatgravity flow provides the pressure, much like a water cooler.

An elevated tank provides the required pressure by raising thewater storage height up to an elevation above the demand area sothat gravity can provide the pressure. Costs can be lessened if theelevated tank is also constructed on a hill site or at higher elevation.This not only lessens the necessary height of the tank but also canreduce its cost.





Gravity Flow and Pumping with a StandpipeWater could also be stored in a standpipe (a tall cylindrical tank) wherethe topography is relatively flat throughout the service area and a hillor higher-elevation site is unavailable. In a full standpipe, the upper-most one-third of water stored provides effective pressure for gravityflow. If the tank is two-thirds full, the upper half of the water wouldprovide emergency pressure. In a tank only one-third full, the waterprovides little or no pressure (i.e., ineffective pressure) and wouldhave to be pumped to be used.

Much like with an elevated tank, costs can be saved if the stand-pipe is constructed on a higher-elevation site or hill. This not onlylessens the necessary height of the tank but also can reduce its cost.

Gravity Flow Height CalculationsFollowing is an example of how to calculate the minimum height atwhich to store water to provide an assumed minimum pressure forresidential use through gravity flow. (Check your local standards orcodes.)

Height (for 50 psi [345 kPa] minimum)

= 50 psi [345 kPa]

(62.4 lb/ft3/144 sq in./sq ft)

= 115.4 ft [35.1 m] (≈ 115 ft [≈ 35 m])

orHeight (for 50 psi [345 kPa] minimum)

= 50 psi [345 kPa](62.4 lb/ft3/144 sq in./sq ft)

= 115.4 ft [35.1 m]

To this calculated height, add the additional height required tomeet the friction loss of the water in the distribution piping.

Alternatively, one can use a conversion chart to find the requiredheight at which to store the water to provide the pressure needed.Figure 1-32 shows how various types of tanks provide this pressureusing gravity flow.

Pumping Versus Gravity FlowPumping If a site with an increased elevation of at least 115 ft (35 m)above the service area cannot be found, the only option with a groundstorage tank is to use pumping to provide the required pressure. If youare going to pump, you should be aware that water demand variesthroughout the day. As such, you will have to use a variable-speedpump.

A typical water usage graph (Fig. 1-33) shows the filling of a tankduring the night and early morning hours when demand is low. The





115 ft (3

5 m

)

Elevated tank Standpipe

Reservoir

FIGURE 1-32 Providing pressure using gravity flow.

tank is emptied during the day; water demand peaks sometime be-tween 5 p.m. and 9 p.m.

Electric utilities charge more for electricity during their peak de-mand period (see sample rates in Fig. 1-34). By overlaying the sampleelectric rates on the water usage graph (Fig. 1-35), one can see that thepeak demands for electricity and water occur about the same time ofday. Using these data, one can make the following calculations:� Peak demand (5 p.m. to 9 p.m.) pumping costs: $0.1175/kWh

average utility cost to pump half of the daily water demandto end users.

Constant pumping rate

Filling tank

Usage rate

Peak demand

Midnight 6:00 A.M. Noon Midnight6:00 P.M.

Emptying tank

Time

FIGURE 1-33 Typical water usage.





$0.14

$0.12

$0.10

$0.08

$0.06

$0.04

$0.02

$0.00

Ave

rag

e c

ost

pe

r kilo

wa

tt-h

ou

r

Midnight 3:00 A.M. 3:00 P.M. 9:00 P.M. Midnight6:00 P.M.Noon9:00 A.M.6:00 A.M.

Time

FIGURE 1-34 Sample electric rates.

� Nonpeak demand pumping costs: $0.1080/kWh average util-ity cost to pump the other half of the daily water demand toend users.� Tank filling costs: $0.0675/kWh average daily utility cost tofill the tank by pumping.

In this case, utility costs during peak demand are almost 75 percentmore than the cost of the average rate used to fill the tank, while evennonpeak costs are about 60 percent more. These calculations shouldbe modified for your system using your local daily water usage andutility rates. Regardless of the local factors, pumping during peak

$0.14

$0.12

$0.10

$0.08

$0.06

$0.04

$0.02

$0.00

Ave

rage c

ost per

kilo

watt

-hour

Midnight 6:00 A.M. 6:00 P.M.Noon Midnight

Time

Constant pumping rate

Peak demand

Usage rate

Emptyingtank

Filling tank

FIGURE 1-35 Higher rates during peak demand.





electricity rates to meet peak water demand is usually more expensivethan gravity flow and can become quite costly over time.

Additionally, if you lack sites with hills or higher elevations andchoose to pump to meet the pressure and water demand, incorporatethe following initial and lifetime costs into your present-value analysisas follows:� The additional daily costs of pumping over and above gravity

flow (peak and nonpeak)� The added cost of a variable-speed pump (usually required;larger than the constant-speed pump used at night for agravity-flow tank)� Cost of a backup pump or pumps� Cost for additional piping and controls for the backuppump(s)� Cost of backup generator� Expense of enlarging the pump building to house the addi-tional pumps and piping� Cost to maintain and replace all of these as needed.

Often, when these additional costs are considered, it is most likelythat the extra initial costs to provide gravity flow may actually be amore cost-effective solution over time.

Gravity flow One can save these peak-demand electricity costs by peakshaving. To peak shave, start by locating a ground storage tank on theside of a hill, or build an elevated tank or standpipe. A smaller pumpcan then be used to pump the water up into the tank during the nightand early morning at a constant rate when electricity rates are muchlower. Then, during the demand period, water can be provided at theneeded pressure by using gravity flow. This avoids the much higherelectricity rates during this time period and allows use of a smaller,less costly pump.

Because of these advantages, gravity flow is the preferred methodof providing water pressure. If possible, place the tank on a hill orelevate it to take advantage of this method.

The ideal location: For any type of storage tank, the ideal locationis on a hill that is in the middle of the demand area and is owned bythe community.

Proximity to UsersWhen choosing a site for a new water-storage tank, the prospectivetank owner should consider the growth in residential demand (single-family, multifamily, and high-rise structures) and commercial demand(industry, schools, and hospitals). A new residential development on





the north side of the service area and a new tank on the south sidewould result in very little water pressure for residents of the newdevelopment. The ideal situation is to construct a new water-storagetank in the service area before the area experiences population growthand buildup. This way, you have a better chance to get the right pieceof land at the right time and at the right price.

Acquiring LandWhen acquiring land, the prospective tank owner must consider theavailability and suitability of the land for a tank project; the costs forthe land, required support utilities, and the length of connections tothe existing distribution system; and the surrounding conditions.

NIMBY (not in my back yard!) One of the biggest issues that a waterutility can face when attempting to locate a new water-storage tank isthe public concern of NIMBY! Despite these concerns, even the mostappearance-conscious communities can agree to a mutually beneficialsolution to this stumbling block. The following are some successfulapproaches to be used in overcoming public concern:

� Encourage community involvement: When choosing the style ofthe tank, let the citizens express their concerns and provide in-put. In some communities, citizen groups have used conteststo select the color scheme of the tank exterior or the letteringand logo design.� Educate the citizenry: Explain the reasons the new tank isneeded and the beneficial effect it will have on them person-ally (for example, improved water pressure and fire protec-tion). Demonstrate how improved fire flow will affect insur-ance rates, assure them of the safety record of water-storagetanks, and explain the anticipated maintenance cycle.� Help the public visualize the completed tank: Using an artist’s con-ception, computerized renderings, and a digital photographof the site, compile an image that shows the community whatthe finished water-storage tank will look like.

Zoning RegulationsOnce a site has been located, check on the zoning of the selected siteto ensure that it is currently zoned for this use or can be rezoned.Obtaining proper zoning for a water tank is typically more difficult ina residential area than in an industrial area or in an area near publicfacilities such as schools, government property, and airports.

Often, schools are built in the areas of population growth, andthe school yard may make a good site for a tank. There are manyaesthetically pleasing tank styles that limit access.





FAA ConsiderationsForms must be completed and filed with the Federal Aviation Ad-ministration (FAA) to establish whether a tank can be built on thechosen site at the required height. The FAA is concerned about anyobstruction to its airspace 200 ft (61 m) above ground level andany obstruction within an approach pattern to an airport runway.Lengths of approach pattern vary depending on the size of the air-port, the length of the runway, and the direction of the runway, asfollows:� Large airport: No obstruction that exceeds a 100:1 surface

within 20,000 ft (6.1 km) of an airport having at least onerunway >3,200 ft (>975 m).� Small airport: No obstruction that exceeds a 50:1 surface within10,000 ft (3 km) of an airport whose longest runway is <3,200ft (<975 m).� Heliport: No obstruction that exceeds a 25:1 surface within5,000 ft (1.5 km) of a heliport.

If the FAA determines that the tank will be in the approach pattern,the tank may have to be equipped with aviation lighting or paintedin a special aviation warning paint scheme. The most common of theaviation paint schemes is the red-and-white checked pattern foundon tanks near airports.

The following circulars, forms, and information regarding ob-struction evaluation and airport airspace analysis are available on theFAA Web site (https://oeaaa.faa.gov):� For information on proposed tank construction projects, con-

sult “Proposed Construction or Alteration of Objects that mayAffect the Navigable Airspace” (Advisory Circular 70/7460-2K).� Standards for marking and lighting tanks and other structuresare provided in “Obstruction Marking and Lighting” (Advi-sory Circular 70/7460-1K).� “Notice of Proposed Construction or Alteration” (Form 7460-1) should be completed by the tank owner before the site iseven purchased and certainly prior to construction. The formcan now be completed and submitted online at the FAA Website. Information required includes latitude, longitude, loca-tion marked on a US Geological Survey (USGS) map, elevationof site (mean sea level), and the greatest height of any part onthe tank, including handrails or antennas upon completion.Once the FAA has reviewed the information on the form, itwill make a determination on the proposed tank and location





and post it online. The determination may be one of the fol-lowing:� A tank can be built on this site at the height requested.� A tank cannot be built on the proposed site at all.� A tank can be built on this site but not at the height re-

quested.� A tank can be built on this site at the height requested,but will require an obstruction light and/or obstructionmarking.� A tank can be built on this site but not at the height re-quested and will require an obstruction light and/or ob-struction marking.� “Supplemental Notice of Actual Construction or Alteration”

(Form 7460-2) is usually completed by the tank contractor. Itmust be submitted 30 days prior to the start of tank erectionand requires information similar to that requested on Form7460-1, except for the following:� Indicate the start and completion dates of the construction.� Must indicate the greatest height of the tank or equipment

during construction. Often the tank contractor uses a der-rick with a boom to erect the tank. The height of the derrickand boom may actually exceed the maximum height of thetank on completion. The FAA will want to know this andmay actually require the tank contractor to install an ob-struction light at the tip of the boom to alert pilots to thetank’s location.

Size of SiteTank constructors recommend that the distance from the edge of thetank to the site boundary be a minimum of 50 to 75 ft (15.24 to 22.86 m).A tank may be constructed on a smaller site, but it will require extrahandling and planning to stage materials in a disciplined sequence.Eliminating space constraints enables the tank contractor to build thetank more efficiently and can reduce costs up to a point.

Take into consideration the space needed for the following:� Material storage during construction� Erection and painting operations� Support facilities such as pump houses, valve vaults, andparking areas� Future maintenance and repainting� Placement of tank at safe distance from private property andutilities.





TopographyThe tank site can have a major influence on the cost of constructionand on design details for the foundation. The site should allow gooddrainage away from the foundation(s), provide a level working sur-face for construction, and have some type of erosion protection. Stand-ing or ponding water on the site can add dewatering costs to the projectand may even require changes to the foundation design, leading toadded costs. Consider these added costs when evaluating sites.

Access to SiteAccess to the tank site is an important aspect of site selection. Devel-opers and residents often want the tank to be located in the back ofthe development, away from the streets or even in off-road remote lo-cations. This poses a problem getting the large trucks and equipmentrequired for construction to the tank site.

Other things that must be considered when assessing site ac-cess are the distance from paved roads, permanent versus temporaryroads, accessibility by large trucks, and securing temporary easementsfor site access during construction, if needed. The best site access isvia a permanent road up to the tank. The most economical meansof achieving this is to put the tank access road in with the originalsubdivision roads.

Soil ConditionsA full soil investigation should be conducted before the final site ischosen and certainly before it is purchased. The soil assessment willdetermine whether the soil is adequate to support the tank and itscontents and what type of foundation must be designed. Some sitesmay require deep foundations (piles or drilled piers) that could addsignificant costs to the design and construction of the tank.

The soil investigation will provide needed information about thefollowing issues:� Soil bearing capacity (how much of a load the can soil support)� Site classification for seismic design� Excessive or uneven settlement� Water table elevations� Rock elevations if present� Site history� Substrata conditions� Slope stability

The depth at which the required soil bearing is obtained to supportthe foundation along with the slope stability has implications for the





size of the site required. For example, with a 1:1 slope stability anda 50-ft (15.24-m)-diameter foundation with the required soil bearing15 ft (4.57 m) down, the minimum size of the hole for the foundationwould be 15 ft + 50 ft + 15 ft = 80 ft (4.57 m + 15.24 m + 4.57 m =24.38 m). To this, one would have to add room for digging equipmentand room to store the excavated material on site.

The results of the soils investigation can affect the design andcosts of both the foundation and tank to such an extent that one couldactually save money on the overall project by paying more for a sitewith better soil conditions. It is prudent to make the site purchase onlyafter you have received the results of the soil investigation.

Obstructions/HazardsObstructions that must be avoided include overhead power lines,underground utilities, and existing structures. OSHA (OccupationalSafety and Health Administration) and many tank contractors spec-ify safe minimum work distances required from power lines depend-ing on what voltage the lines carry. Construction hazards may in-clude abrasive blasting, painting, pile-driving vibration, noise, andfire.

Waves and energy produced by AM antennas comprise one ofthe least understood obstructions. AM antennas are typically the tall,slender, red-and-white antennas that do not have dishes or whip an-tennas hanging off of them; the entire structure acts as the broadcastantenna. On the electromagnetic spectrum, AM waves are the longestwaves generated and can be from 656 to 1,968 ft (200 to 600 m) long.These long waves carry energy. Metal objects used in tanks or tankconstruction such as rebar, steel plate, and even crane lines can actas receiving antennas that collect and store the AM wave energy. If agrounded worker touches these energized metal objects, the collectedenergy is released, possibly shocking the worker and making the worksite unsafe. Whether the AM antenna has any effect on your tank sitedepends on how far the antenna is from your tank, what power it isbroadcasting at, and whether it is a directional or nondirectional an-tenna. At the Federal Communications Commission (FCC) Web site(www.fcc.gov/mb/audio/amq.html), one can insert the latitude andlongitude of the centerline of the tank (also used in the FCC submit-tal) and use the “Stations within a Radius” input. The Web site willindicate if any AM antennas are present. If so, station details willindicate whether the transmission location is directional or nondirec-tional. Problems can be present for distances up to 0.6 mile (1 km) fornondirectional and 1.9 miles (3 km) for directional antennas. If youencounter an AM antenna that might be a problem and are seriouslyconsidering the site in question, you may want to hire a specialist tofurther examine the situation.





If the expense and risks of dealing with these obstructions andhazards adds enough costs to your project, you might be better offpaying more for a site that is free of such obstructions and hazards.

Environmental IssuesEnvironmental issues that come into play during tank construction in-clude the protection of vegetation, wildlife, wetlands, and floodplains;historical landmarks and burial sites; and local wind and snow. Moststates require that a permit request be submitted to the US Environ-mental Protection Association (USEPA) before construction to identifyany such environmental issues.

Sizing the Tank

DemandTank capacity One of the main purposes of a water-storage tank is toprovide storage to meet the water demands of the area it will service.As a rule of thumb, you can determine your new water-storage tankcapacity by making the following calculation:

Average daily usage (peak and nonpeak) + fire flow requirements+ added capacity to offset maintenance or pipe breaks+ additional capacity for future demand = tank capacity

Current average daily use This is the amount of water used on averagein a 24-hour period. Calculate this by determining the average waterusage currently per person and multiply this by the number of peoplethat the new area currently serves.� Peak demand: Peak demand typically occurs between 5:00 p.m.

and 9:00 p.m. and is usually half of the current average usage.� Off-peak demand: This comprises the other half of the averagecurrent daily usage.

Fire flow demand To the current average daily usage add an additionalone-half to one-third of the current average daily usage. This figurevaries depending on the local codes and standards. One should alsocheck the requirements of the Insurance Service Organization (ISO)(www.iso.com) and other local standards and codes.� Maintenance and piping breaks: As a contingency measure, con-

sider adding 10 percent, plus or minus, to provide extra stor-age if the service area distribution piping has leaks.� For future demand, project the future population for the ser-vice area and then multiply that by the current average dailywater use in gallons (liters) per person. An alternate method





0 to 2,000

2,000 to 5,000

5,000 to 10,000

10,000 to 20,000

20,000 to 52,000

Louisiana Florida

Idaho

Washington

Oregon

Nevada

California

NewMexico

Utah

Arizona

Nebraska

NorthDakota

Montana

Wyoming

Colorado

Oklahoma

Kansas

SouthDakota

Ark

ansa

s

Missouri

Georgia

Iowa

TennesseeNorth

Carolina

Wisconsin

os

en

niM

ta

India

na

Kentucky

Ohio

Virginia

Delaware

Massachusetts

Maine

New JerseyConnecticutRhode Island

Alaska

Hawaii

Puerto Rico

U.S. Virgin Islands

Mic

hig

an

Source: US Geological Survey Circular 1268

Water withdrawals in milion gallons per day

New Hampshire

Vermont

District of Columbia

Maryland

West

Virginia

South

Carolina

IllinoisOhio

Pennsylvania

New York

Mis

sssip

pi

Ala

bam

a

Texas

FIGURE 1-36 Average daily water usage per capita.

would be to check with the US Geological Survey to learn whatthe average daily usage is per person by state (Fig. 1-36).

Local standards and codes related to tank capacity should be con-sulted and complied with.

Turnover Tanks sized to meet peak demand must also have adequateturnover when demand for water is not at a peak. Unused water canbecome stagnant, generating unwanted tastes and odors. In cold cli-mates, lack of turnover can cause tank icing. Water turnover problemscan be solved by filling the tank to a lower capacity that matches thereduction in demand or by adding a recirculation system. Addition-ally, several mixing systems are available that can create a more uni-form residual chlorine content, reduce stagnation, and help preventthe generation of unwanted tastes and odors.

Volume/standard capacities For elevated tanks, the most economicalstorage is achieved by selecting a standard capacity and head rangeon the basis of the recommendations of the tank contractor. Typicalcapacity ranges of elevated tanks are given in Tables 1-8 and 1-9.

The largest-capacity elevated tank built to date is 4 mil gal (15,142m3). It may be possible to build larger capacity tanks, but they wouldbe the first of their kind.

Reservoirs and standpipes are more flexible in their height/diameter limitation. It was once thought that reservoirs could onlybe constructed in height increments of 8 or 10 ft (2.44 or 3.05 m). Steel





is now readily available in made-to-order heights (and widths) in ad-dition to these. An economical tank can be built to whatever diameterand height is required (Tables 1-1 and 1-3).

Diameter and Height SelectionFor a ground storage tank, three major factors influence the selectionof the most economical diameter and height. Of the following threefactors, soil bearing and earthquake design usually have the biggestinfluence.

Soil bearing The tank foundation and ultimately the soil must sup-port the weight of both the water and the tank. Each cubic foot of waterweighs 62.4 lb/ft3. The calculation of the weight of a 1-ft2 column ofwater from the bottom of the tank to its top capacity height can giveone an idea of the weight that must be supported. A sample calcula-tion for a 40-ft (12-m) column of water would be 40 ft × 62.4 lb/ft3 =2,496 lb/ft2 or about 2,500 lb/ft2. So, if a 2,500-lb/ft2 soil bearing is notavailable at the tank site, various foundation types could be evaluatedto support the column height of water needed. Deep foundations orlarge mats may increase costs to the extent that it may actually be moreeconomical to either change the height of the tank or evaluate othersites with higher soil bearing values.

Earthquake Typically, the taller and thinner the tank, the more thatearthquake may affect the design.

Wind The taller and wider a tank, the more wind may affect thedesign.

Here are some examples of diameters (D) and heights (H) ofground storage tanks with typical design conditions that might makethem more economical:� D = H: For decent soil bearing values of 4,000 psf/ft2, with

low earthquake factors and typical 90-mph wind design, atank in which diameter is equal to height may be the mosteconomical shape for small- and medium-size tanks.� D < H: For soil bearing values greater than 4,000 psf/ft2, withlow earthquake factors and typical 90-mph wind design, atank in which diameter is less than height may be the mosteconomical shape. In these tanks, there are fewer costs in thebottom and roof and more costs in the shell.� D > H: For soil bearing values less than 4,000 psf, with highearthquake factors and winds greater than 90 mph, a tankin which diameter is greater than height may be the mosteconomical shape. In these tanks, there are more costs in thebottom and roof and fewer costs in the shell.





It is important that you call your local tank contractor to helpdetermine the most economical diameter and height combination forthe design conditions at your site.

Selecting the Tank

Ground/Elevated StorageYour first decision is whether to build a ground storage tank or an el-evated tank. If higher-elevation sites are available that could providethe required pressure through gravity flow without the need for run-ning a lot of water main to reach the site, the more economical choicewould probably be a ground storage reservoir. If site elevations arenot available within the water system area or are not high enough toprovide the required pressure through gravity flow, an elevated tankor standpipe would probably be a better choice. Using a reservoir andpumping to meet the pressure and daily water demand will add dailypumping and peak demand charges throughout the life of your tank.See the previous section on hydraulics.

Again, it is important that you call your local tank contractor toprovide budget pricing for various tank options to evaluate the initialand lifetime costs of your new storage tank.

Aesthetics/AppearanceThe aesthetic appeal of a new water-storage tank is often one of themost talked-about elements of tank selection. The public may wanta tank that will blend into its surroundings, or be a highly visiblelandmark for the community, or match the system’s existing tanks.The tank owner and security personnel may want to place the tankon a more visible site that can be readily secured and monitored. Thisdecision must be handled on a case-by-case basis.

Ornamental TanksHighly stylized ornamental tanks can provide community or com-pany identity and advertisement, be more aesthetically pleasing, or re-solve NIMBY issues. Unique, decorative tanks have been constructedin many areas and, although more costly to construct, they are oftenlandmarks in which the community takes pride.

EconomicsAlthough the initial cost of constructing a tank has a significant eco-nomic impact, the tank’s operating cost, reliability, and maintenancerequirements must also be considered.

Special NeedsSometimes communities have special needs or desires; for example, acommunity may want to house the fire department in the base of the





tank. Multiuse tanks can be constructed to match the community’sneeds.

LiabilityTo limit liability, tank owners seek methods to control access. Somestyles of tanks—such as single-pedestal spheroid, fluted-pedestal, andcomposite elevated tanks—do not have exterior ladders, thereby effi-ciently limiting access. On legged or ground tanks, ladder guards canbe installed that limit access to the ladders.

Life-Cycle CostsAnticipated need for and scheduling of tank repainting and mainte-nance are important considerations. The style of tank, its surface area,and the type of surface all directly influence maintenance costs.




Documents

Diseño de Tanques de Agua Cap 1