Help interpreting “expansion joint” meaning--Minnesota plumbing code

[nelson_rg]

Not a plumber. Homeowner/DIYer doing research. I'm looking at digging a trench for a sewer line for a garage and connecting it to main sewer line in my house. There's a part of the Minnesota plumbing code that I'm a bit confused about. I'm trying to figure out what they mean by "expansion joint".

For a sewer line outside of the building thermal envelope using 4 inch Schedule 40 PVC, the MN code has a table that says that the "Expansion Joint Length" for a 30 foot run of sewer line is 68 inches. They then have drawings of the "expansion joint" that shows a 3 part jut-out in the sewer line for the lack of a better description. You can see the drawings here: https://www.dli.mn.gov/sites/default/files/pdf/pe_list_plastic_pipe_expansion.pdf

Does this mean that 1 time in the 30 foot run of sewer line that I need to install this 68 inch long section of "expansion joint". Expansion joint seems like an odd term for this intentional bend in the sewer line.

Does anyone know the purpose of this? Is this just because straight sewer line runs are prone to problems? And that an occasional jut-out bend in the line prevents problems?

I just have a feeling I'm really misinterpreting something here.

 

[Breplum]

It relieves stress from expansion and contraction due to temperature changes. All pipe, interior and exterior experiences same.

 

[Sponsor]

 

[wwhitney]

Expansion joint seems like an odd term for this intentional bend in the sewer line.

If you prefer, it's an expansion-accommodating feature. A long straight run between two fixed points, when it expends, will do a weird S-shaped buckling, or if it contracts, will pull apart. While the two-dimensional patterns in your attachment can all grow a bit in length by just bulging outwards, or shrink by doing the opposite.

Cheers, Wayne

 

[nelson_rg]

Thanks for the responses @Breplum and @wwhitney -- Got it, that all makes sense. Will plan for this

 

[MNplmbr51]

The table noted is just for piping that is located above grade. If the piping is located underground provisions for expansion are not required.

 

[nelson_rg]

Hi @MNplmbr51 -- Ahh yes, that makes sense now. I was looking another version of this chart in the 2020 MN Plumbing Code (pages 27–28) and I think I see where I misinterpreted some info. Thanks for clarifying!