Ductile Iron Pipe Support Locations

[rsaunders]

Does anyone have any pointers on how to identify the maximum support distance from a joint/fitting for a span of ductile iron pipe? The pipe in question is 8" diameter and carries water. Looking through the Ductile Iron Pipe Research Association literature and webpage, the closest information I found is included in the brocure for design of pipes on supports. Their recommendation is to provide a support at each joint location. In the condition I have, the pipe has been excavated over a section of the length and now spans 8'-0" between supporting sections of soil. The stresses on the span are not a problem isn't a problem according to the stress calculations, however the 8'-0" span has a joint located at midspan.

Supporting at the joint is not feasible. The excavation is actually for a foundation that will span over the pipe, so providing support would link the pipe to the foundation and we need to keep the two isolated.

Any information is greatly appreciated.

 

[dicksewerrat]

Move the pipe.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[rsaunders]

Brillant. I designed a complex foundation to span over this and it never occurred to me to move the pipe.

This is a critical utility and shutdown of the pipe would cost more time and money than a redesign of the foundation. This is a less than optimal solution for everyone involved, but the only remaining hurdle is verifying the pipe span at the fitting.

 

[rsaunders]

To break it down analytically, I have looked at the pipe properties as follows:

S = 15.8 in3
E = 24,000,000 psi
self wt = 30lb.ft
weight of water = 22lb.ft
Allowable stress = 48000psi per DIPRA

I submit that we can consider that the 8'-0" span with a joint in the middle conservatively as 2 cantilevered sections. For a 4'-0" cantilever, the applied stress is 316 psi. Further, the deflection (assuming a fully rigid support) is .002in (L/24000). This means that concerns of deflection and/or rotation at this joint are virtually nil.

Based on these results I conclude that even if the joint has no capacity, the pipe is capable of supporting itself across these span conditions without worry about the joint capacity.

Any input/insight is welcome.

 

[dicksewerrat]

What happens when the pipe fails in 5 years? Can it be replaced without damaging the foundation? Is there a plan in place to do the repair. Scheduled shutdowns take less time than emergency shutdowns. Murphy's law applies to anything buried. Ask the local Water Dept. what damage an 8 inch line can do when it fails.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[paddingtongreen]

What is the maximum water pressure in the pipe?

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[JedClampett]

I would add a support at each end of the excavation. Even though your pipe works as a 4'-0" cantilever, the edge of the excavation is by no means a rigid support. While backfilling, some soil might get disturbed under your pipe, increasing the cantilever length. And the backfilling is not done by watchmakers. You're likely to have soil dropped on the pipe from pretty high and other abuse to the pipe. So make it a close as you can to your analyzed condition, a continuous beam.
If I understand your problem, you have a foundation you're pouring over the pipe. So pour a little pad and support the pipe from the bottom of the excavation using a cradle or even a fabricated pipe support. Pretend that the exacavation is never going to be filled. That soil at the bottom is likely to be native and have a good capacity.
Since whenever I mess with utilities under a structure, I like to have a belt and suspenders, I would also consider encasing the pipe in addition to adding supports at the end of foundation. That way, you've avoided any questions about the pipe leaking after your done and gone.