Pipe hangers suspending pipe in soil

[ajk1]

How do you determine the force that goes to pipe hangers suspending 6" diameter cast iron drain pipe in weak soil, from structural suspended basement slab above?

 

[PEDARRIN2]

A plumber is not going to support buried piping from structure above. He will either excavate and backfill with appropriate materials to provide support or he will support the piping from the basement slab and the soil will be below the suspended pipe.

 

[ajk1]

PEDARRIN2 (Mechanical) - Thank you for the response. Very much appreciated and very interesting indeed. However are you sure all mechanical engineers specify it that way? I know for a fact that there are cases where the pipe hangers are installed and then the excavation filled and the slab poured. If they do not fill the excavation, then they will have to construct formwork across it when they are ready to pour the slab, and that would be quite an expense.

 

[PEDARRIN2]

I cannot speak to what other engineers specify.

If forced to do it, I would specify the distance between hangers be cut in half (every 5 feet), maybe even use a thicker diameter all thread rod (0.875"), maybe even use a hanger that fits over the OD of the pipe, like an Anvil 212 (and use two all thread rods). Depending on the amount of pipe, I might require all three. It is not that much extra compared to what extra excavation and backfill would require.

The different hanger would be because I would be concerned about pipe shift from the areas where loose soil moves around it (at fittings or joints) causing the pipe to lose its uniform downward pitch, maybe even forming a valley.

 

[dbill74]

My direct answer to your question is I don't know, but it can be significant. Factors include weight of filled pipe, weight of soil, and suction forces as the soil moves.

A plumber is not going to support buried piping from structure above. He will either excavate and backfill with appropriate materials to provide support or he will support the piping from the basement slab and the soil will be below the suspended pipe.

99.5% of the time this is correct. However, there are some locations where soil conditions do not permit this. New Orleans, LA for example is ripe with water logged soil that has a tendency to erode out from under buildings. In these locations it is standard practice to use structural slabs and support pipe from the slab.

During a recent project I saw pictures of how one particular company gets around the issue. Their solution is to hang the pipe with rods that are bent over the rebar in the concrete slab, then attach styrofoam "shields" on either side of the supports fully covering the pipe. The shields would take the weight of the soil and the forces associated with moving soil off of the pipe.