Pipe stress analysis and buckling

[tomecki]

I'm looking at installing some pipes in a water treatment clarifier. The pipes are 16" diameter and 60' long. They will be horizontal and simply supported at either end. The pipes never carry any additional static loading besides their own weight.

The results of simple beam stress calculations for several wall thicknesses result in stresses of about 20% of yield. (Interestingly, for the range of wall thicknesses I tried, the max stress was independent of the wall thickness).

To get to my question: I'm wondering if I should be concerned about any other failure modes. Namely, I'm worried that the top of the pipe which is loaded in compression might buckle. Is this something to be concerned about?

Thanks

PS. I hope I chose the right subforum for this question.

 

[tomecki]

I forgot to mention the pipe sees no pressure or vacuum.

 

[rb1957]

1) the running load due to weight is directly proportional to thickness, so too is I of a ring (= pi*R^3*t) ... so bending stress is independent of thickness.

2) is there no load from water ? 'cause 1) will lead you to conclude that a pipe of zero thickness can support it's weight (ok, very small thickness).

3) for symmetrical sections in bending, I always look at the compression side first (since buckling/crippling is less than fcy, so the compression side is more critical than the tension).

4) other failure modes ... vibration due to flowing water, wind loads ??

Quando Omni Flunkus Moritati

 

[JStephen]

Yes, it is an issue, may or may not be the controlling factor here. The AISC code, various tank standards have allowable bending stresses for thin-wall cylinders that could be applied.

A long pipe like that can also be subject to vibration from wind.

 

[tomecki]

Thanks!
I will look into the buckling on the compression side.

As for the other loads mentioned:
- there is no water load because the arrangement is such that the water level on the inside and outside of the pipe is always equal
- vibration due to water movement - the clarifier is a settling tank with extremely low water velocities
- wind - the pipes are indoors
So, I should be ok once I figure out the buckling stress.

I appreciate the help!

 

[gfdoug]

Other methods of failure to consider:

-Thermal Expansion: Will the pipe be anchored at each end? If it's a straight run of pipe, you may be subject to thermal loads in the Maximum and Minimum temperature cases
-Static Earthquake: You may want to check what sort of earthquake loading factors that are in the area.