welded steel pipeline above ground thrust issues?

[minorchord2000]

I am a structural engineer and I have a question: I have a 24" diameter steel pipeline under about 40 feet of hydraulic head with occasional changes in direction of 45 degrees each. The pipes are welded together using groove welds. Do I need to account for thrust forces at the bends even though the pipe has welded joints? The pipe run is close to the ground and is supported by pre-cast concrete planks.

 

[1503-44]

Welded pipe allows you to not have to build thrust restraints to keep the elbows from popping off, as the welds transfer those tension forces into the pipe walls. That is not to say that you are home entirely free. Those forces need to go somewhere. Friction between pipe and your concrete supports will take some and whatever is left over will stay in the pipe as axial stress until the pipe reaches the next elbow, which is doing the same thing. That being said, pipe will stretch some between opposing elbows. Friction between pipe and supports will doubtfully be enough to resist all of the stretching, so some movement will occur, some due to pressure and some due to temperature change. Stress due to temperature change will be compressive to the pipe, if it is fully or partially resisted by the friction of the supports, That compression load will reduce the tension in the pipe. If movement is permitted, those compressive loads will reduce, but that tends to put the full tension load back into the pipe wall again.

It boils down to, no large thrust restraints are required (to keep the elbows attached to the pipe), but you must design the pipe for hoop stress and axial tension, or maybe compression, if movement is not possible due to high Friction. If you allow movement, compressive pipe axial wall stresses reduce and stresses shift towards and may cross into tension caused by the elbow loads again. So as long as the pipe is designed for both hoop stress and whatever axial tension or compression stress remains, or the range of those stresses, no problem, as long as the movement is not excessive.

 

[LittleInch]

A lot depends on the length of pipe, it's temperature variations and where anchors or line spots exist.

In general no you don't need elbow restraints on welded pipes. However you do need to allow the pipe to move, however small an amount, so often need to guide the pipe to stop it moving all over the place.

A lay out drawing would be good to see.



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