BRIDGE CROSSINGS WITH DUCTILE IRON PIPE
The Ductile Iron Pipe Research Association (DIPRA) periodically receives requests from engineers and contractors concerning recommendations on the design and/or installation of pipelines spanning waterways, highways, and railroads. Because the variables involved in such installations present numerous alternatives and challenges for designers and contractors, DIPRA does not provide recommendations and does not assume responsibility for design or installation practices on such projects. DIPRA does, however, recognize the engineering complexities inherent to bridge crossing pipelines and offers information to assist those involved with this type of installation and points out typical design criteria which should be considered for bridge crossings. Adaptation of the entire pipeline as a unit applied to a bridge structure involves close detail to many parameters in both structures. The following sections cover these parameters in detail.
Ductile Iron pipe is centrifugally cast in 18- and 20-foot nominal laying lengths. Nominal diameters range from 3 to 64 inches, with a variety of pressure and special thickness classes. Although Ductile Iron pipe is usually furnished with a cement-mortar lining, optional internal linings also are available for a wide range of special applications. Also, Ductile Iron pipe is normally furnished with an external asphaltic shop coat for a “finished” appearance, although shop-applied primers for special painting systems also are available for above ground use.
Ductile Iron pipe is furnished with several different types of joints: push-on, PUSH-ON JOINTS mechanical, restrained, ball and socket, flanged, and grooved and shouldered joints. Typically, bridge crossings involve push-on joints, restrained joints, or combinations thereof. Push-on joints (see Figure 1) are excellent for bridges with properly designed and constructed supports. Ample deflections in these joints are possible when properly Fastite Joint braced support structures are provided to carry the weight of the pipe and its contents and resist any forces acting against the pipe supports. Normally, expansion and contraction of the pipe due to temperature changes can be adequately provided for with such joints; if more adjustment is needed, expansion couplings (see section on Tyton Joint Expansion/Contraction Couplings) should be considered. Mechanical joints (see Figure 2) are often used for fittings but are not generally Figure 1 used for straight runs of pipe. To accommodate possible pipe movement caused by thermal expansion and contraction, the push-on joint may be a better choice than the mechanical joint due to its deeper socket depth. Since standard push-on and mechanical joints are not “restrained,” due consideration should be given to proper design and construction of supports or anchorages to resist thrust forces, dead loads, impact and shock loads, and thermal changes. The restrained joint complements the push-on and mechanical types by maintaining flexibility and also by providing both ease of assembly and a “locking feature” to resist pull-out. Numerous types are available employing modifications of the push-on and mechanical joint designs (consult with the pipe manufacturer regarding the use of standard push-on joints with gripping type gasket products on bridges). In a Figure 2 pressurized system, some flexible restrained joints are subject to significant joint extension. Therefore, when utilizing restrained joints, proper design and construction techniques normally should include provisions for extending each joint so as to engage its restraints. This may be accomplished by extending the joints fully during assembly and/or by hydrostatically testing the horizontal portion of the crossing separately (using restrained closures) before making connections to offset bends or riser pipes. Cumulative joint...