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Design Data 21 Design Data 21 Curved Alignment Changes in direction of sewer lines are usually Figure 1 Deflected Straight Pipe accomplished at manhole structures. Grade and align- Figure 1 Deflected Straight Pipe ment changes in concrete pipe sewers, however, can be incorporated into the line through the use of deflected L straight pipe, radius pipe or specials. t L 2 DEFLECTED STRAIGHT PIPE Pull With concrete pipe installed in straight alignment Bc D and the joints in a home (or normal) position, the joint ∆ space, or distance between the ends of adjacent pipe 1/2 N sections, is essentially uniform around the periphery of the pipe. Starting from this home position any joint may t be opened up to the maximum permissible on one side while the other side remains in the home position. The difference between the home and opened joint space is generally designated as the pull. The maximum per- missible pull must be limited to that opening which will provide satisfactory joint performance. This varies for R different joint configurations and is best obtained from the pipe manufacturer. ∆ 1/2 N The radius of curvature which may be obtained by this method is a function of the deflection angle per joint (joint opening), diameter of the pipe and the length of the pipe sections. The radius of curvature is computed by the equa- tion: L R = (1) 1 ∆ 2 TAN ( 2 N ) Figure 2 Curved Alignment Using Deflected Figure 2 Curved Alignment Using Deflected where: Straight Pipe R = radius of curvature, feet Straight Pipe L = length of pipe sections measured along the centerline, feet ∆ ∆ = total deflection angle of curve, degrees P.I. ∆ N = number of pipe with pulled joints /N ∆ ∆/ = total deflection of each pipe, degrees N ∆ N L /N 1 D ∆ From Figure 1, the deflection angle is further L /N defined as: 2 N 2 P. T. ∆ 1 ∆ /N = SIN-1 PULL or SIN-1 PULL (2) P. C. 2 N 2(D + 2t) 2B ∆ c RADIUS /N ∆ where: Normal /N ∆ PULL = joint opening, inches DirectionLaying of D = inside pipe diameter, inches t = wall thickness, inches American Concrete Pipe Association • www.concrete-pipe.org • [email protected] 1 © 2012 American Concrete Pipe Association, all rights reserved. DD 21 (11/12) Bc = outside pipe diameter, inches L D The joint opening and pipe length required to provide a R = - + t (5) ∆ ( 2 ) curved pipeline alignment may be calculated using the TAN unit values found in Table 1 on page 3. The table tabulates N the radius of a pipeline constructed of standard eight-foot laying length pipe with a 1-inch joint opening (PULL). where: Other pipeline radii may be calculated by changing, first, = total deflection angle of curve, degrees the joint opening, and if necessary, the pipe laying length. N = number of radius pipe An eight-foot laying length is standard for most concrete pipe manufacturers. Other lengths may require special L = standard pipe length being used, feet manufacturing procedures. Changes in the design radius = total deflection angle of each pipe N are directly proportional to the pipe laying length and inversely proportional to the joint opening. The specific From Figure 3, the radius of curvature can be expressed pull per pipe joint is found by the equation: in terms of the drop and is given by the equation: Lx Ru = total deflection angle of curve, degrees PULL = PULL8 (3) L(D + 2t) D x ()L ()R () R =N = number- of +radiust pipe (6) 8 x DROP ( 2 ) L = standard pipe length being used, feet Rx = (Lx/L8)(PULL8/PULLx) Ru (4) L 1 R = B = total deflection- angle of each pipe (7) where: N c (DROP 2) PULL = the joint opening From Figure 3, the radius of curvature can be expressed LBc Ru = the Unit Radius (Taken from Table 1) (8) in terms DROPof the drop= R+ andB is given by the equation: Lx = Length of deflected pipe c/2 where: Specific radii may be calculated by the following pro- Bc = outside diameter of the pipe, feet cedure: • Select the unit radius of curvature for the speci- Figure 5 presents R/Bc ratios for drops from one fied diameter pipe from the chart. inch through 15 inches and commonly manufactured • Increase or decrease the joint opening (PULL) pipe lengths. Since the maximum permissible drop in Equation 1 to obtain the design radius. If the for any given pipe is dependent on manufacturing required joint opening exceeds the pipe manu- feasibility, it is essential to coordinate the design of facturers recommendations, select a pipe with a radius pipe with the pipe manufacturer. Many manu- shorter laying length. Four and six foot are com- facturers have standardized joint configurations and mon non-standard pipe lengths. Check with the deflections for specific radii and economics may be pipe manufacturer for availability of non-standard realized by utilizing standard radius pipe. lengths. As illustrated in Figure 4, when concrete pipe is • Recalculate the pull for the shorter pipe. installed on curved alignment using radius pipe, the As illustrated in Figure 2, when concrete pipe is installed pipe sections are oriented such that the plane of the on curved alignment using deflected straight pipe, the dropped joint is tangent to the theoretical circular curve. point of curve (P.C.) is at the midpoint of the last unde- Projection of the joints do not converge at a common flected pipe section and the point of tangent (P.T.) is at point, but are tangents to a common circle of diameter the midpoint of the last pulled pipe. equal to the length of pipe sections. The point of curve (P.C.) is at the midpoint of the last straight pipe and the RADIUS PIPE point of tangent (P.T.) is one half of the standard pipe Radius pipe, also referred to as bevelled or mitered pipe, length back from the straight end of the last radius pipe. incorporates the deflection angle into the pipe joint. The The required number of pieces of radius pipe is equal pipe is manufactured by shortening one side of the pipe. to the length of the circular curve in feet divided by the The amount of shortening or drop for any given pipe is centerline length of the radius pipe (L-1/2 DROP). Where dependent on manufacturing feasibility. Because of the possible, minor modifications in the radius are normally possibility of greater deflection angles per joint, sharper made so this quotient will be a whole number. curvature with correspondingly shorter radii can be ob- Minimum radius of curvature obtained from equa- tained with radius pipe than with deflected straight pipe. tions (1) and (5) are approximate, but are within a range As in the case of deflected straight pipe, the radius of of accuracy that will enable the pipe to be readily installed curvature which may be obtained by radius pipe is a to fit the required alignment. A reasonable amount of field function of the deflection angle per joint, diameter of the adjustment is possible for radius pipe by pulling the joints pipe, length of pipe sections and wall thickness. in the same manner as with deflected straight pipe. The radius of curvature is computed by the equation: BENDS AND SPECIAL SECTIONS Special precast sections can be used for extremely American Concrete Pipe Association • www.concrete-pipe.org • [email protected] 2 © 2012 American Concrete Pipe Association, all rights reserved. DD 21 (11/12) short radius curves which cannot be negotiated with Find: The required pull per joint for deflected straight either deflected straight pipe or with conventional radius pipe or the required drop for radius pipe. pipe. Sharper curves can be handled by using special short lengths of radius pipe rather than standard lengths. Solution: From Table 1, for a 42-inch diameter pipe, the These may be computed in accordance with the methods radius of curvature for a 1-inch pull is 408 feet. discussed for radius pipe. Certain types of manufactur- The required pull for 170 feet is: ing processes permit the use of a dropped joint on both 7. 5 408 PULLX = 1 = 1.125" ends of the pipe, which effectively doubles the deflection. ( 8 )(170)( ) Special bends, or elbows can be manufactured to meet any required deflection angle and some manufacturers To evaluate the required drop for radius pipe produce standard bends which provide given angular to negotiate the roadway curvature, it is first deflection per section. necessary to calculate the R/Bc ratio: One or more of these methods may be employed to meet the most severe alignment requirements. Since R 170 = = 40 manufacturing processes and local standards vary, Bc 4.25 local concrete pipe manufacturers should be con- sulted to determine the availability and geometric Enter Figure 5 on the vertical scale at R/Bc 40. configuration of special sections. Proceed horizontally until the line represent- The following example illustrates proper use of the ing L = 7.5 feet is intersected. At this point the Tables and Figures. horizontal scale shows the required drop to be Given: A 42-inch diameter concrete pipe storm 2.2 inches. Or sewer is to be installed on curved alignment (7.5) (4.25) Drop = = 0.185 ft. = 2.2 in. corresponding to the roadway curvature. 170 + 4.25/2 The pipe will be manufactured in 7-1/2 foot lengths with a 4-1/2-inch wall thickness. The Answer: Radius pipe with a 2-1/4-inch drop would be curve data for the roadway curb is: required. It is important to consult local point of intersection station P.I.
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