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Aerial Sewer Crossing

Aerial Sewer Crossing

Aerial Sewer Crossing

(OP)
I'm new to this forum, and I apologize in advance if this question has already been answered. I'm designing a 36-inch ductile iron gravity sewer line with several aerial crossings in the range of 20 to 60 ft each. These crossings pass over small waterways and we have a nationwide 404 permit to install.

For the longer crossings, we plan to use steel pipe to avoid the need for intermediate supports. Most crossing details I've seen show a steel casing pipe and internal carrier pipe configuration. Is there any reason we can't just make a single steel pipe crossing without a casing? In other words, the pipeline would transition from ductile to steel at each end of the crossing.

RE: Aerial Sewer Crossing

In theory no, I don't believe there is any reason why this can't be done, howerver there are a few practical issues.

1) Connection between steel and ductile iron is not simple. I can only assume for a ductile pipe you are using push fit spigotted pipe. Steel pipe doesn't normally come like that so you will need some special connectors

2) Think about the potential for corrosion of the steel. It tends to corrode differnetly (faster) than ductile iron and is usually a lot thninner or very expensive if you go for steel pipe as thick as ductile iron.

3) The pipe is presumably spanning the crossing as a simple beam. To resist this bending as a part of the system, you will need to hve at least the length of the crossing and probably more buried either side before you transiiton back to ductile iron, otherwise this bending load goes into the ductile iron, whereas if you're just using it as a bridge, you don't have that issue, hence less expensive steel.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

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