Discussion of “Thrust Restraint Design of Concrete Pressure Pipe” by Mehdi S. Zarghamee, Daniel W. Eggers, Rasko P. Ojdrovic, and Daniel P. Valentine

Abstract

The authors are to be complimented on their development of a design procedure for restraint of pressure-induced thrust at bends in pipelines, using concrete pressure pipe, which includes the effects of soil resistance to lateral movement of tied-pipe. The authors’ procedure is a needed extension of a beam-on-elasticfoundation design procedure for thrust restraint of fiberglassreinforced plastic pipe and fittings developed in the late 1970s Ameron 1980 , but it is found to be lacking in some respects. Throughout this discussion, is substituted for the authors’ for compatibility with AWWA 1996 . In the authors’ Eq. 3 the third term, 2klb cos 2 /2 referred to herein as T3, represents passive soil-pressure forces that act on the bend to resist thrust T. The fourth term of Eq. 3 2fulb sin /2 referred to herein as T4, represents friction forces that act on the bend to resist thrust. Both terms are incorrect because they are not based on movement of the bend into the soil in the direction of the thrust force and because all bends are treated as having only a single mitered joint. The correct third term is T3 =k lk where lk is the effective length of the projection of the outer surface of the entire bend on a plane perpendicular to T. That projection is a rectangle of length 2lb cos /2 and width Do, with half ellipses appended to its ends. The major axis of the ellipses is Do and the minor axis is Do sin /2 . The effective projected length, which is equal to the projected area divided by Do, is lk=2lb cos /2 + /4 Do sin /2 . The correct fourth term is T4 = f l where l is the sum of the centerline lengths of each of the mitered sections of the bend. Using the guidelines for the design of mitered bends in AWWA 1996 , l =2 lb −R tan /2 + ns−1 R tan /2 , where lb is equal to L1 from Table 1 of AWWA 1996 , R is the radius of the bend, ns is the number of mitered sections and = / ns−1 . For Do=1,829 mm 72 in. values of T3 /T3 range from 0.9 to 1.4 for values of from 7.5° to 90° and R from 1.0Do to 2.5Do. Values of T4 /T4 range from 15.3 to 2.0 over the same ranges of and R. The differences represented by the ratios, especially T4 /T4, are significant. The authors’ depend upon axial friction forces on tied pipe from vertical loads We, Wp, and Wf to resist thrust force, but they neglect friction forces produced by lateral soil pressure against the pipe. The total lateral force producing friction along the springlines of pipe is equal to 2Vo, and, because lateral friction pressure is greatest near the bend, it could be significant in reducing the amount of longitudinal reinforcement of the first few tied pipe as well as the number of tied pipe. It seems overly conservative to neglect the effect of friction from lateral soil pressure.