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327

the performance of buried pipes. It is interesting to note that for the 24- and the 30-in tests, wall crushing starts at deflections in the range of 3.5 to 4.0 percent. For the 18-in tests, wall buckling occurred first and took place at about 6.0 percent deflection. Overall performance. The pipe performed well for a pipe of that level of stiffness and wall area. The resulting deflections were reasonable and about what would be expected. The seam integrity was good. No seams opened or failed during the tests, even at extreme heights of cover. Live load tests. In tests with simulated H-20 live load, the pipe did not perform well in tests with a minimum cover (before loading) of 1 ft. However, tests showed the pipe would perform well with a cover of 2 ft. The actual minimum cover at which the pipe will perform well is between 1 and 2 ft. Additional tests would be required to determine the actual critical minimum cover. Results show the performance of the pipe could be enhanced if the ring stiffness and the local longitudinal stiffness were increased. Load deflection tests. The 18- and 30-in pipes demonstrated a capacity for a height of cover (before wall crushing or severe deformation) of 52 to 64 ft in soil at 95 percent of standard Proctor density, and 30 ft in soil at 90 percent standard density. The 24-in-diameter test pipes were thinner than intended and, therefore, more flexible than would be permitted in practice. The performance limits for the 24-in pipes tested ranged from 24 to 27 ft of cover. AISI Handbook

Design information for corrugated steel products is available in the Handbook of Steel Drainage and Highway Construction Products, which is published by the American Iron and Steel Institute (AISI). Also, many manufacturers publish design information for their products. Such information should be secured and considered by the designer. For corrugated steel pipes with circular sections, standard analysis and design procedures which have been discussed in this book apply and may be used by the design engineer. See Table 6.4. Example 6.2—Corrugated steel A 48-in-diameter (3 in by 1 in) corrugated steel pipe is to be placed in an embankment with 60 ft of soil cover. The soil in the pipe zone is to be coarse sand with some fines and is to be compacted to 90 percent Proctor density. What thickness is required so that the pipe deflection does not exceed 5 percent?

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Steel and Ductile Iron Flexible Pipe Products

329

Use Spangler's equation. Ay D

=

Q.lyH EI/r3 + 0.061£'

H = 60 ft

D = 48 in

Let

y = 120 lb/ft3 E = 30 X 106 lb/in2 E' = 1000 lb/in

(from Table 3.4)

Solve for EI/r3. - 0.061 E'

(0.1) (120) (60) (1/1441 -0.061(1000) 0.05 = 100 - 61 = 39

or I==

39r3 E

=

39 (24)3 30 X 106

= 0.018 in4/in = 0.22 in4/ft From Table 6.4, the uncoated thickness should be 0.1345 in. Now assume the yield stress vy for the steel is 33,000 lb/in2. What wall area is required for ring compression design with a safety factor of 2? CTy

Design compression stress fc — — — 16,500 lb/in Vertical soil pressure Pv = (120) (60) = 7200 lb/ft2 or

330

Chapter Six

fc =

PDL 2A

PD 2A/L

Solve for AIL. A L

=

PD= (50) (48) = Q 2/c 2(16,500)

.n2/in

= (0.073 in2/in) (12 in/ft) = 0.88 in2/ft From Table 6.4, the uncoated thickness is 0.0598 in. Thus, the deflection design controls, and the thickness found in the beginning of the example is the required thickness.

Steel pressure pipes are used in many varied and diverse applications in industrial, agricultural, and municipal markets. The discussion here will be limited to steel pipe used primarily in the municipal water market (see Table 6.5). However, principles used are applicable to all steel pressure pipe. AWWA M11, Steel Pipe—A Guide for Design and Installation

This manual gives procedures for determining the required thickness for steel pressure pipe. The internal pressure used in design should be that to which the pipe may be subjected during its lifetime. The thickness selected should be that which satisfies the most severe requirement. The minimum thickness of a cylinder should be selected to limit TABLE 6.5

Selected Standards for Steel Pressure Pipes in Water Service

AWWA C200 AWWA C203 AWWA C205 AWWA C206 AWWA C207 AWWA C208 AWWA C209 AWWA C210 AWWA C213 AWWA C214 AWWA C602 AWWA Mil

Steel water pipe 6 in and larger Coal-tar protective coatings and linings for steel water pipelines— enamel and tape applied hot Cement-mortar protective lining and coating for steel water pipe—4 in and larger—shop-applied Field welding of steel water pipe Steel pipe flanges for waterworks service—sizes 4 through 144 in Dimensions for fabricated steel water pipe fittings Cold-applied tape coatings for special sections, connections, and fittings for steel water pipelines Coal-tar epoxy coating system for the interior and exterior of steel water pipe Fusion-bonded epoxy coating for the interior and exterior of steel water pipelines Tape coating systems for the exterior of steel water pipelines Cement-mortar lining of water pipelines in place—4 in (100 mm) and larger Steel pipe design and installation

Steel and Ductile Iron Flexible Pipe Products

331

the circumferential tension stress to a certain level. The maximum pressure in the pipe must be used in the design calculations. Surge or water hammer pressures and pressures created by the pumping operations must also be considered. With pressure determined, the wall thickness is found by using Eq. (4.2): 2crmax where

t = minimum specified wall thickness, in PI = internal pressure, lb/in2 D = outside diameter of pipe steel cylinder (not including coatings), in o"max — allowable stress, lb/in2

For steel pipe, a design stress equal to 50 percent of the specified minimum yield strength is often accepted for steel water pipe. This design (working) stress is determined with relation to the steel's yield strength rather than its ultimate strength. For some applications, other safety factors may apply. For example, the Bureau of Reclamation in its design criteria for penstocks has adopted a safety factor of 3 based on the ultimate tensile strength or a safety factor 1.33 based on the minimum yield strength. Table 6.6 is reprinted from AWWA Mil. It lists grades of steel referenced in AWWA C200, Standard for Steel Water Pipe 6 Inches and Larger, and gives design stresses to be used as a basis for working pressure. Also given are the yield stresses and the ultimate stresses for the various grades of steel. The designer can easily calculate working pressure, via Eq. (4.2), corresponding to 50 percent of the specified minimum yield strength for several types of steel commonly used. A required thickness may not be available from a manufacturer. It is, therefore, recommended that the pipe manufacturers be consulted before final selection of diameter and wall thicknesses. For transient pressures, the hoop stress may be allowed to rise, within limits, above 50 percent of yield for transient loads. When ultimate tensile strength is considered, a safety factor well over 2 is realized. The stress of transitory surge pressures together with static pressure may be taken at 75 percent of the yield point stress, but should not exceed the mill test pressure. The designer should, however, never overlook the effect of water hammer or surge pressures in design. Internal pressure, external pressure, special physical loading, type of lining and coating, and other practical requirements govern wall

332

Chapter Six

TABLE 6.6

Grades of Steel Used in AWWA C200

Specifications for fabricated pipe ASTMA36 ASTMA283GRC GRD ASTMA570GR30 GR33 GR36 GR40 GR45 GR50 ASTMA572GR42 GR50 GR60 Specifications for manufactured pipe ASTMA53, A 135, and A 139 GRA GRB ASTMA139 GRC GRD GRE

Design stress 50% of yield point, lb/in2

Minimum yield point, lb/in2

Minimum ultimate tensile strength, lb/in2

18,000 15,000 16,500 15,000 16,500 18,000 20,000 22,500 25,000 21,000 25,000 30,000 Design stress 50% of yield point, lb/in2

36,000 30,000 33,000 30,000 33,000 36,000 40,000 45,000 50,000 42,000 50,000 60,000 Minimum yield point, lb/in2

58,000 55,000 60,000 49,000 52,000 53,000 55,000 60,000 65,000 60,000 65,000 75,000 Minimum ultimate tensile strength, lb/in2

15,000 17,500 21,000 23,000 26,000

30,000 35,000 42,000 46,000 52,000

48,000 60,000 60,000 60,000 66,000

thickness. Good practice with regard to internal pressure is to use a working tensile stress of 50 percent of the yield point stress under the influence of maximum design pressure. Select linings, coatings, and cathodic protection, as necessary, to provide the required level of corrosion protection. The wall thickness selected must resist external loadings imposed on the pipe. Such loadings may take the form of outside pressure, either atmospheric or hydrostatic, both of which are uniform and act radially as collapsing forces. Buried pipe must be designed to resist earth pressure in the trench or fill condition. These considerations are discussed in Chaps. 2 and 3. For external pressure or internal vacuum, buckling should be considered. The following formula from Chap. 3 applies:

E 4 (1 - v2) \R

(3.14)

where R = radius to neutral axis of shell (for thin pipes, difference between inside diameter, outside diameter, and neutralaxis diameter is negligible), in

Steel and Ductile Iron Flexible Pipe Products

t Pcr E v

333

= wall thickness, in = collapsing pressure, lb/in2 = modulus of elasticity (30,000,000 for steel) = Poisson's ratio (usually taken as 0.30 for steel)

Substituting the above values of E and v gives Pc = 528 X 10 6 f-^y

(6.3)

For convenience to the reader, the more exact approach to buckling is repeated here from Chap. 3 as follows: \1/2

where qa = allowable buckling pressure, lb/in2 FS = design factor 2.5 for(h/D)>2 3.0 for(h/D)<2 h = height of ground surface above top of pipe, in D = diameter of pipe, in Rw = water buoyancy factor = 1 - (0.33hw/h) Q
JD

—

(1 + v) [ (2h + D)2 + D2 (1 - 2v) ]

The B' has some dependence on Poisson's ratio for the soil. However, this effect is small, as is shown in Fig. 3.22. The above equation simplifies when the value for Poisson's ratio is taken as V2. This equation is conservative and should be used for the calculation of B'. D —

1.5(2/1 +Z>) 2

Minimum plate or sheet thicknesses for handling are based on two formulas adopted by many specifying agencies: OQQ 288

U

P t° 54-in ID

(6.4)

334

Chapter Six

t = —TTr —

PiPe sizes greater than 54-in ID

(6.5)

In no case shall the shell thickness be less than 14 gage (0.0747 in). Example 6.3—A 108-in transmission A 108-in-diameter water transmission line is to be installed. Steel has been selected as the piping material. The joint is to be a bell-and-spigot type of joint welded both inside and out as shown: -Weld T\\\W\\\\\\\\\\\\\W

.g^\\\\\\\\\\\\W^\\\\\\\\\W Weld'

The wall thickness is to be 0.5 in. Because of the large diameter, the pipe will be very flexible and will be braced with internal bracing (stills) when manufactured. These stills will remain in the pipe sections until the pipes have been Installed and pipe zone soil has been placed and compacted to the specified density. The stills will be removed after backfilling is complete. The pipeline will then be lined with a Portland cement type of mortar before the line is placed in service. Design parameters: Wall thickness 0.5 in Yield stress 36,000 lb/in2 Ultimate strength 60,000 lb/in2 Modulus 29 X 106 lb/in2 Poisson's ratio 0.3 Thermal coefficient of expansion 6.5 X 10~6 (1/°F) Ductile-brittle transition temperature 70°F Surge pressure allowance 40 lb/in2 Cover depth 6 ft Pipe zone soil Crushed stone Pipe zone density 90 percent standard Proctor Water temperature 34°F Evaluate the proposed steel pipe for this application. Are there any special precautions which should be taken or special construction methods which should be followed? 1. Check pipe stiffness PS and evaluate possible ring deflection.

F El PS = f- = 6.7 ^ Ay

r*

Steel and Ductile Iron Flexible Pipe Products

=

335

6.7 (29 X 106) (0.5)3 (12) (54)3

- 12.85 lb/in2 This pipe is quite flexible. However, the pipe is going to be held in the undeflected state until pipe zone soil is compacted and the overburden is placed. The resulting deflection after the stills are removed will be quite low. 2. Check the pressure design. First, find the hoop stress for design pressure plus surge.

Second, find the hoop stress for design pressure only.

The yield stress is 36,000 lb/in2. The safety factor is greater than 2; therefore, pressure design is all right. 3. Consider longitudinal stresses. AWWA C206 indicates that temperature considerations should be made in design. AWWA C206 and AWWA Mil suggest the use of either closure welds or expansion joints to alleviate stresses due to temperature change. Longitudinal stresses will also be produced by the Poisson effect. Temperature stresses and Poisson stresses, along with bending stresses due to nonparallel loading in the bell-spigot connection, may be large enough to cause failure. Assume the pipe is placed and tack-welded during the day. It is July and August, and the pipe temperature during tack welding is between 80 and 130°F. The tack welds hold firm, and the welding process is completed by a welding crew who are following behind the pipe-laying crew. No closure welds or expansion joints are being used. After the line is completed, it is put in service with water at 120 lb/in2 and 34°F. (See Chap. 4, the steel pipe longitudinal stresses section.) First, find the longitudinal stress due to the Poisson effect. vp = vvh

but

vh = 12,960 lb/in2

a, = (0.3) (12,960) = 3888 lb/in2 Second, find the longitudinal stress due to temperature change. vT = EOL (AT) = (29 X 106) (6.5 X 10~6) (AT)

= (188.5) (AT)

336

Chapter Six

Assume AT = 70°F. Then <JT = 13,195 lb/in2 Third, what is the total longitudinal stress? 07, = cr (Poisson) + a (temperature) = 3888 4- 13,195 = 17,083 lb/in2 Fourth, the nonparallel loading in the bell and spigot will produce a bending moment and will effectively magnify the stress found above. What is that magnification factor? MC o A- stress = 05 = — Bending —

where M = moment = vjjAt = crL (bt) (t) t = thickness A = area = bt

" 12

Therefore, _

"B~

(qL) (bt) (t) ft/2)

b?nz

= 6crL Then, the bending stress is 6 times the longitudinal stress. However, the maximum stress is the sum of the bending stress and the longitudinal stress. °inax

=

°~B + °L

=

7°L

The magnification factor is 7. Therefore, amax = (7)(17,083) = 119,581 lb/in2. The pipe will fail before this stress is reached. In fact, it did. This pipeline was actually designed and constructed as described in this example. The designer failed to consider longitudinal stresses and did not allow for closure or expansion joints. There were three separate failures caused by longitudinal stresses. Each time a repair was made, the line was returned to service. After the third failure, a general repair was ordered. Every other joint was cut to relieve the built-in stresses. As the joints were cut, there were snap-back openings of as much as 1 in. The temperature of the pipe during the repair was 55° F, which is 21° higher than the service tempera-

Steel and Ductile Iron Flexible Pipe Products

337

ture, so there will still be some stress at 34°F. Had the steel been more ductile, it might have been able to relieve itself by simply stretching. For the steel selected, the ductile-brittle transition temperature was 70°F. Therefore, the steel behaved in a brittle manner and failed. Ductile Iron Pipe Ductile iron pipe has essentially replaced gray cast iron pipe. Ductile iron (DI) is, as its name implies, more ductile than gray cast iron, but still retains somewhat brittle properties. It is very popular among public works people who repair and maintain water systems. Many perceive this pipe to be able to withstand abuse during handling and repair operations. The corrosion rate for ductile iron is essentially the same as for gray cast iron. However, since the wall is usually thinner, corrosion is more critical. Design procedures call for a corrosion allowance called a service factor. When pipe is installed in highly corrosive soil, steps should be taken to protect it. Ductile iron pipe usually has a cement-mortar lining. This lining improves the hydraulic efficiency and also provides some corrosion protection. Other linings and coatings are available. See Table 6.7. Example 6.4—A 30-in DI pipe Calculate the thickness for 30-in ductile iron (DI) pipe laid on a flat-bottom trench with backfill tamped to the centerline of the pipe, laying condition type 2 (Fig. 6.46), under 10 ft of cover for a working pressure of 200 lb/in2. (See ductile iron section in Chap. 4 for design procedure for pressure pipe. Also see AWWA C150. Certain tables from AWWA C150 have been reproduced here for the reader's convenience. This example is taken from AWWA C150.) 1. Design for trench load. First, earth load (Table 6.8) Pe = 8.3 lb/in2 may be obtained from Fig. 2.19. Truck load (Table 6.8) Pt = 0.7 lb/in2, and trench load Pv = Pe + Pt = 9.0 lb/in2. Second, select Table 6.13 for diameter-thickness ratios for laying condition type 2. Third, entering the Pv of 9.0 lb/in2 in Table 6.13, we see that the TABLE 6.7

Selected Standards for Ductile Iron Pipe

AWWA C104 AWWA C105 AWWA C110 AWWA Clll AWWA C115 AWWA C150 AWWA C151 AWWA C600 ASTM E 8 ASTM A 539

Cement mortar lining for ductile iron Polyethylene encasement for ductile iron Ductile iron and gray iron fittings Rubber-gasket joints for ductile iron Flanged ductile iron Thickness design of ductile iron pipe Ductile iron pipe in metal- and sand-lined molds Installation of ductile iron water mains and their appurtenances Materials properties test Physical properties

338

Chapter Six

Type 2

Type 1

Types

Type 4

Type5

Figure 6.46 Standard pipe-laying conditions. (Reprinted, by permission, from ANSI/AWWA C-150/A21.50-96, American Water Works Association, 1996.)

bending stress design requires a D/t of 128. From Table 6.12, diameter D of 30-in-OD pipe is 32.00 in. Net thickness t for bending stress is

Fourth, also from Table 6.13, the deflection design requires D/tl of 108. Minimum thickness 11 for deflection design is

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