Flood Load on Horizontal Pipeline through River

[steel_possum]

If a horizontal pipeline passes through a flow of water, I know there will be a lateral flood load on the pipeline. This can be calculated using ASCE7-16 Section 5.4.3. Will there also be uplift load due to the hydrodynamic effects (I am not talking about buoyancy; I have already accounted for that) that is perpendicular to the flow direction? The reason I ask this is because aerial pipe crossings can experience vortex shedding (see Kellogg: Design of Piping Systems (Rev. 2nd Ed.) p. 259) resulting in forces perpendicular to the direction of flow that can be equal in magnitude to the lateral load. Would a pipe passing through water experience any force like this, or anything causing it to have uplift/downdrag perpendicular to the flow direction?

 

[LittleInch]

Yes is the short answer.

VIV is a serious fatigue issue.

Also impact with anything in the water.

Any more details?

It sounds like a bad idea to me.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[human909]

I think you have answered your own question. YES. And unless the project is massive it is often better over designing rather than trying work out very complex interactions.

Water is a fluid so it air and they both behave similarly as analysed by fluid mechanics. Obviously water usually moves much slow and is much denser so the effects can be more or less pronounced.

Regarding vortex shedding. I am no expert, so don't take my word as gospel. However I would be more worried about the stiffness of your pipe between supports rather than the strength. Also vortex shedding is more likely in laminar flow. It once bit on a structure I designed. Some braces on a A-frame were overly slender (still easily fine for calculated ULS winds). I visited site on a mild-moderately windy day and they were vibrating excessively. The scary part was the mild twist they were inducing in the I-beams of the A-frame. And this was at wind speeds about ~20% of design speeds. (At higher and lower wind speeds they were fine).

 

[Heaviside1925]

Echoing LittleInch Big Yes.
This should be checked. Should be a fairly simple check, i.e. pass/fail but it would need to be performed by a firm that can provide this service. Besides the overall geometry of the pipeline, consider the support on either side. If this direct buried, consider how flexible the pipe would be perpendicular to the fluid steam. In water both the viscous and inertial forces are very much higher than air, so the forces driving resonance can be much higher. Even if the design does not pass, there are some fairly excepted mitigation measures that can be taken to help break up the vortices.

 

[1503-44]

Uplift is experienced when water flows faster over the top of pipe than the bottom, like an aircraft wing. This is especially true when the pipe is within 2 diameters of a flat edge (stream's bottom or flat barrier). Von Karman vortex shedding effects are dependent on Reynolds number of the current, pipe effective length and moment of inertia.

https://en.m.wikipedia.org/wiki/Kármán_vortex_street

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[steel_possum]

Thanks guys. @LittleInch I'm not worried about deleterious VIV since the forcing frequency due to the river velocity is much lower than the natural frequency of my pipe. I guess what I am wondering is if the pipe will still experience some vortex shedding loads even though the forcing frequency is significantly lower than the natural frequency of the pipe between the supports. I am thinking that the forces perpendicular to the river velocity force (Fa) will be somewhere between 0.7*Fa to 1.0*Fa for the following reasons:

1. Kellogg (on page 259) says the magnitude of the forces (in a wind load situation) acting at right angles to the direction of flow will be "essentially equal to the dynamic pressure acting on the projected area of the cylinder". This is of course for a non-resonant condition.

2. API RP 2A-WSD R2020 says in Section B.5.3.1.2.13 that "In the absence of dynamic excitation, the maximum local lift force amplitude FL,max per unit length of the member is related to Umax, the maximum value of U [velocity] during the wave cycle, by the equation: FL,max = Clmax*(w/2g)*D*(Umax)^2" where it goes on to explain that the coefficient Clmax is approximately 0.70*Cd for both smooth and rough cylinders.

I am already considering impact loads, and there is no way to avoid this design situation. Like it or not the pipe must be designed for this situation. (Little more info, it is the portion of a force main pipe at a 45 degree angle turning down into the ground from a bridge abutment. It has a 20ft unsupported length in this angled section, and part of it passes through a flood zone.)

 

[1503-44]

You will get the force, but not the resonance vibrations, if you are outside 0.7 to 1.4 x Fn.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

Got a drawing or a sketch or photo?

45 degrees down or horizontal?

Some physical protection would seem like a better idea to me, but I guess if a sewer line broke in a flood it might not be so bad.

but you will still get alternating forces on the pipe which could induce movement. A bit more detail would help to see what we're looking at.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[steel_possum]

45 degrees down

 

[Heaviside1925]

You could get transverse vibration at half Fn but considering this is a transient state, i.e. flood conditions, I would not feel fatigue issues would occur. Since you are designing for impact loads, I would suspect that will provide enough support that vibration shouldn't be an issue.

 

[LittleInch]

Especially as it's at 45 degrees you won't see any significant load or forces IMHO.

Damage from debris is the most likely issue.

Concrete coat it?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.