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Local Stresses at Pipe Supports

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Title: Local Stresses at Pipe Supports

I have been asked to do a stress check on a proposed ductile iron pipeline which will be suspended on roller type hangers on a highway bridge.  The ductile iron pipe people recommend hanging the pipe with a single hanger behind each bell, where the section is thicker, in order to avoid overstressing the pipe. My own preference, and the highway departments standard mounting detail, supports each length of pipe with two hangers, one at each quarter point. Its easy to compute the primary stresses in the pipe due to flexure and water pressure, however I am struggling with local stresses in the vicinity of the hangers. There are readily available formulas for computing stress in a pipe at cradle supports, but a roller is more like a knife support, particularly in this case, where the designer wants to use oversized rollers to facilitate installation, so there is no cradling effect at the roller.

In an attempt to evaluate the stresses, I estimated the contact area between the pipe and roller using Roarks formulas for two crossing cylinders, then used this area and the reaction load to estimate stress with formulas in WRC-107 (Bijlaards curves). The membrane stresses looked reasonable, but the pipe wall bending stresses were way over yield. I have seen plenty of ductile iron and steel pipe supported on fairly narrow channels with no apparent problems and am wondering if I am missing something here or should just ignore this issue. Does anyone out there have any suggestions regarding ways to evaluate local stress in this situation. I have spent more time than I want to trying to Google a reference with no real luck.

I need to wrap this up. Contact me off list if you prefer.

Jim Lutz, P.E., S.E.

206 505 3400 Ext 126

206 505 3406 (Fax)

jim.lutz(--nospam--at)bhcconsultants.com

BHC Consultants, LLC

720 3rd Avenue, Suite 1200

Seattle, WA 98104-1820