# 03ASCEGuidelinefortheDesignofBuriedSteelPipe2001.pdf

G01 G02G03G04G05G06G07G08G09G0AG06G0BG04G0CG06G09G04G0DG02G0CG0CG06G08G09G07G04G01 A public-private partnership to reduce risk to utility and transportation systems from natural hazards G01 G01 G01 G0EG0FG06G10G04G0CG06G09G04G0DG01G0BG11G05G01G12G13G04G01G14G04G0DG06G15G09G01G01 G11G0BG01G16G0FG05G06G04G10G01G17G12G04G04G0CG01G18G06G19G04G01 G01 G1AG0FG0CG1BG01G1CG1DG1DG1EG01 G02G03G04G05G06G07G08G09G0AG06G0BG04G0CG06G09G04G0DG02G0CG0CG06G08G09G07G04G01 A public-private partnership to reduce risk to utility and transportation systems from natural hazards G01 G01 G01 G01 G0EG0FG06G10G04G0CG06G09G04G01G0BG11G05G01G12G13G04G01G14G04G0DG06G15G09G01G01 G11G0BG01G16G0FG05G06G04G10G01G17G12G04G04G0CG01G18G06G19G04G01 G01 G1AG0FG0CG1BG01G1CG1DG1DG1EG01 G01 G01 G02G02G02G03G04G05G06G07G08G09G04G0AG0BG08G0CG06G0BG08G0AG06G0DG04G0BG0BG08G04G0AG09G06G03G0EG07G0F This report was written under contract to the American Lifelines Alliance, a public-private partnership between the Federal Emergency Management Agency FEMA and the American Society of Civil Engineers ASCE. This report was reviewed by a team representing practicing engineers and academics. Guidelines for the Design of Buried Steel Pipe July 2001 i Acknowledgments The following people with their affiliations contributed to this report. G. A. Antaki, Co-chairman WSRC, Aiken, SC J. D. Hart, Co-chairman SSD, Inc., Reno, NV T. M. Adams Stevenson and Associates, Cleveland, OH C. Chern Bechtel, San Francisco, CA C. C. Costantino City College of New York, New York, NY R. W. Gailing Southern California Gas Co., Los Angeles, CA E. C. Goodling Parsons Energy however, because of the buoyancy of the soil in water, the actual total pressure load is psfft ft lb ft ft lb P v 12941010033.01104.62 33 G05G06G06G07G08G05 3.5 Example 3 A 30-inch diameter pipe is jacked 10 feet underground into undisturbed medium clay with a total unit weight of 120 pounds per cubic foot. The cohesion coefficient c is estimated to be 500 psf. Check the vertical earth load pressure using Equations 3-1 and 3.3 3 lb 120 10ft 1,200psf ft v P G03G04 G01G01 G05G06 G07G08 Guidelines for the Design of Buried Steel Pipe July 2001 Page 12 22 2 lb lb 10 ft 12 in lb 1200 2500 2800 0 30 in 1 ftft ft ft vu P G03G04G03G04 G01G09 G01G09G0A G05G06G05G06 G07G08G07G08 Since the vertical earth load pressure must be greater than or equal to zero, there is no vertical earth load on the pipe. 3.6 Figure Figure 3.1-1 Soil Prism Above Pipe Guidelines for the Design of Buried Steel Pipe July 2001 Page 13 4.0 Surface Live Loads 4.1 Applied Loads In addition to supporting dead loads imposed by earth cover, buried pipes can also be exposed to superimposed concentrated or distributed live loads. Large concentrated loads, such as those caused by truck-wheel loads, railway car, locomotive loads, and aircraft loads at airports are of most practical interest. Depending on the requirements of the design specification, the live-load effect may be based on AASHTO HS-20 truck loads, Cooper E-80 railroad loads or a 180 kip airplane gear assembly load, as indicated in Table 4.1-1. The values of the live load pressure P P are given in psi and include an impact factor F’ 1.5 to account for bumps and irregularities in the travel surface. Other impact factors are listed in Table 4.1-2. Note Live-load depends on the depth of cover over the pipe and becomes negligible for HS-20 loads when the earth cover exceeds 8 feet; for E-80 loads when the earth cover exceeds 30 feet; and for airport loads when the earth cover exceeds 24 feet. Live load transferred to pipe, lb/in 2 Live load transferred to pipe, lb/in 2 Height of cover, ft Highway H20* Railway E80† Airport‡ Height of cover, ft Highway H20* Railway E80† Airport‡ 1 12.50 -- -- 14 § 4.17 3.06 2 5.56 26.39 13.14 16 § 3.47 2.29 3 4.17 23.61 12.28 18 § 2.78 1.91 4 2.78 18.40 11.27 20 § 2.08 1.53 5 1.74 16.67 10.09 22 § 1.91 1.14 6 1.39 15.63 8.79 24 § 1.74 1.05 7 1.22 12.15 7.85 26 § 1.39 § 8 0.69 11.11 6.93 28 § 1.04 § 10 § 7.64 6.09 30 § 0.69 § 12 § 5.56 4.76 35 § § § 40 § § § Notes * Simulates a 20-ton truck traffic load, with impact † Simulates an 80,000 lb/ft railway load, with impact ‡ 180,000-pound dual-tandem gear assembly, 26-inch spacing between tires and 66-inch center-to center spacing between fore and aft tires under a rigid pavement 12 inches thick, with impact § Negligible influence of live load on buried pipe Table 4.1-1 Live Loads Guidelines for the Design of Buried Steel Pipe July 2001 Page 14 Installation Surface Condition Height of cover, ft Highways Railways Runways Taxiways, aprons, hardstands, run-up pads 0 to 1 1.50 1.75 1.00 1.50 1 to 2 1.35 1.50 1.00 1.35 2 to 3 1.15 1.50 1.00 1.35 Over 3 1.00 1.35* 1.00 1.15† Notes * Refer to data available from American Railway Engineering Association AREA † Refer to data available from Federal Aviation Administration FAA Table 4.1-2. Impact Factor FG09 versus Height of Cover For live-loads other than the AASHTO truck, the Cooper rail and the 180 kips aircraft gear assembly loads, the pressure P p applied to the buried pipe by a concentrated surface load P s , without impact, as shown in Figure 4.1-1, can be calculated using Boussinesq’s equation 5.2 2 2 12 3 G01 G01 G02 G03 G04 G04 G05 G06 G07 G08 G09 G0A G0B G0C G0D G0E C d C P P S P G01 4-1 where P p pressure transmitted to the pipe P s concentrated load at the surface, above pipe C depth of soil cover above pipe d offset distance from pipe to line of application of surface load The pressure P p must be increased for the fluctuating nature of surface line loads by multiplying by the impact factor FG02 given in Table 4.1-2. When a surcharge load is distributed over the ground surface area near a pipeline, it is possible that the external surcharge may cause lateral or vertical displacement of the soil surrounding the buried pipeline. In this case, additional ination, such as a specialized geotechnical investigation, may be needed to determine if the pipeline could be subjected to soil displacement. A detailed investigation may be in order if the distributed surcharge load over an area larger than 10 square feet exceeds the values tabulated below for the weight of material placed or height of soil fill added over the pipeline. 500 psf or 5 feet of fill – for pre-1941 pipelines 1,000 psf or 10 feet of fill – for pipelines with 12-inch diameters or larger 1,500 psf or 15 feet of fill – for pipelines smaller than 12 inches in diameter Guidelines for the Design of Buried Steel Pipe July 2001 Page 15 4.2 Ovality and Stress 4.2.1 Ovality A buried pipe tends to ovalize under the effects of earth and live loads, as illustrated in Figure 4.2-1. The modified Iowa deflection ula may be used to calculate the pipe ovality under earth and live loads G01G02 1 3 0.061 eq DKPy D EI E R G03 G01 G04G05 G06G07G08G09 G0AG0B 4-2 where D pipe outside diameter, inches G04y vertical deflection of pipe, inches D l deflection lag factor 1.0-1.5 K bedding constant 0.1 P pressure on pipe due to soil load P V plus live load P P , psi R pipe radius, inches EI eq equivalent pipe wall stiffness per inch of pipe length, in./lb. E modulus of soil reaction, psi The pipe wall stiffness, EI eq , is the sum of the stiffness of the bare pipe, lining subscript L and coating subscript C. G01G02 LL CC eq EI EI E I E IG01G09 G09 4-3 where I 3 12 t t wall thickness of pipe, lining, or coating The modulus of soil reaction E is a measure of the stiffness of the embedment material surrounding the pipe. E is actually a hybrid modulus, being the product of the modulus of the passive resistance of the soil and the radius of the pipe. Values of E’ vary from close to zero for dumped, loose, fine-grained soil to 3000 psi for highly compacted, coarse-grained soil. Recent studies show that the confined compression modulus can be used in place of E . 4.2.2 Through-Wall Bending Under the effect of earth and surface loads, the through-wall bending stress in the buried pipe, distributed as shown in Figure 4.2-2, is estimated according to 4-4 Guidelines for the Design of Buried Steel Pipe July 2001 Page 16 4 bw yt E DD G01 G01G02G03G02G03 G04 G05G06G05G06 G07G08G07G08 4-4 where G0B bw through-wall bending stress G04y/D pipe ovality D outside diameter of pipe t pipe wall thickness E modulus of elasticity of pipe 4.2.3 Crushing of Side Walls The burial depth should be sufficient that the pressure P on the pipe due to the earth and surface load is less than that causing the crushing of the side wall see Figure 4.2-3 . For buried pressure-steel piping and pipelines, with D/t typically smaller than 100, and a yield stress larger than 30,000 psi, crushing of the sidewall is quite unlikely. 4.2.4 Ring Buckling If the soil and surface loads are excessive, the pipe cross-section could buckle as shown in Figure 4.2-4. Appendix A uates ring buckling, which depends on limiting the total vertical pressure load on pipe to 3 32 1 D EI EBR FS eq W where FS factor of safety 2.5 for C/D 2 3.0 for C/D 750 meters per second, stiff soil is 200 meters per second – 750 meters per second, and soft soil N1s Case a with Weff N1W Check Case a with r a Weff W r G02 1.5 N1s Case b r 1.5 N1s Case a with r Rgcg and Weff N1N2W r G02 1.5 N1s Case c r 1.5 N1s Case a with r Rgcg and Weff N1N2W a distance to nearest charge ft N1 number of charges in a row N2 number of rows of charges s in-line spacing of charges ft W individual charge weight lb r G02 sWeff W/s r s Case a with Weff N1W Guidelines for the Design of Buried Steel Pipe July 2001 Page 55 13.0 Fluid Transients 13.1 Applied Loads Rapid changes in the flow rates of liquid or two-phase piping systems liquid-gas or liquid- vapor can cause pressure transients, which in turn generate pressure pulses and transient forces in the piping system. The magnitude of these pressure pulses and force transients is often difficult to predict and quantify. Only the simplest cases can be calculated by hand, as is the case for a rapid valve closure in a liquid system. A valve closure is considered rapid if its closing time is 2 v c L L t c G01 13-1 where t c valve closing time, sec L v distance from the valve to an upstream pressure source such as a tank, inches c L sonic velocity in the liquid, inches per second 1 t D E K K c L G01G02 G03 G01 K bulk modulus of fluid G01G01G01G01G01G01G01G01G01G02 fluid density E pipe modulus of elasticity D pipe mean diameter t pipe wall thickness The pressure rise is f av dP g G08 G0B G01 13-2 where dP pressure rise in a liquid pipeline due to rapid valve closure, psi G08 f liquid density, pounds per cubic inch G07v change in liquid velocity from initial flow rate to zero closed valve, inches per second g gravitational coefficient 386 in/sec 2 This pressure rise first occurs at the closed valve, propagates upstream and reflects at the pressure source. For flow transients more complex than a rapid valve closure and for two-phase Guidelines for the Design of Buried Steel Pipe July 2001 Page 56 flow conditions, a detailed computational fluid dynamics analysis may be required to predict the pressure and force transient time history in the piping system. 13.2 uation 13.2.1 Pressure Transient The pressure rise due to a flow transient and its affects are the same in pipes above and below ground. The pressure rise could be large enough to burst the pipe. 13.2.2 Thrust Loads As a result of waterhammer, an unbalanced impulsive force, called a “thrust” load, is applied successively along each straight segment of buried pipe. This causes a pressure imbalance of dP between consecutive bends. The unbalanced impulsive load is G01G02 f FdPDMFAG01 13-4 where DMF dynamic magnification factor of impulsive load, maximum 2.0 dP pressure rise from waterhammer, psi A f pipe flow cross sectional area, square inches The thrust loads from pressure transients can cause large displacements in above-ground piping systems, which can bend or rupture the pipe, or fail pipe supports at welds or concrete anchor bolts. In contrast, buried welded steel pipes are continuously supported and therefore will not typically undergo large movements and bending loads due to waterhammer. Note Thrust forces from flow transients can open up joints in pipes connected by mechanical joints or bell-and-spigots. In this case, thrust blocks or thrust restraints are used to avoid opening the joints. Two s are used to analyze the effects of thrust loads the static and the dynamic . With the static , the thrust loads are calculated for each pipe segment, multiplied by the dynamic magnification factor and applied simultaneously to all pipe segments. In contrast, the dynamic analysis recognizes that the pressure wave travels in the pipeline at the speed of sound; therefore, the thrust force temporarily is applied to each segment by means of a time- history analysis. The time-history analysis requires a soil-pipe model and a series of thrust-force time tablesone for each straight pipe segment. 13.3 Example An 18-inch standard size 0.375-inch wall, flow area 233.7 in 2 ASTM A 106 Grade B carbon steel water pipe is buried 7-feet underground. The pipe is 1000 feet long with several bends. The water pressure is 150 psi and flows at 4 feet per second. The line should be designed for an Guidelines for the Design of Buried Steel Pipe July 2001 Page 57 accidental closure of an isolation valve, in 50 milliseconds. The velocity of sound is 4500 feet per second. The critical closing time is t C 21000/4500 0.44 sec. Since 50 msec 0.05 sec 0.44 sec, the accidental valve closure can be considered instantaneous, and the upstream pressure rise during the ensuing waterhammer event is dP 62.345004/[32.2144] 242 psi The hoop stress due to the waterhammer is 150 24218 9408 20.375 hw psiG02 G02 G01G01 The thrust load F is an impulse force axial to the pipe, applied successively along each straight segment, caused by the pressure imbalance of 242 psi between consecutive bends. Without more detailed analysis, the maximum value 2.0 of the dynamic magnification factor is applied to the thrust force to obtain the impulsive force. Therefore, the unbalanced impulsive force is F 2 dP A f 2 242233.7 113,110 lb The thrust force can then be applied to a pipe-soil model to obtain displacement and bending stresses, using either a conservative static approach or a time-history analysis as described in Section 13.2. Guidelines for the Design of Buried Steel Pipe July 2001 Page 58 14.0 In-Service Relocation 14.1 Applied Load In-service pipeline relocation is practiced routinely in the industry in order to