Sign post with risk for galloping (wind)

[ThomasH]

Hi all,
A colleague asked me something that I thought you might be able to help with.
If you look at the very simple figure below there is a support column (black) with diameter 1 m and height 30 m. Then you have two signs (blue) that are ~15 m high and the total width for the two signs and the column is 11 m.
The natural frequency is 0.9 Hz and the mass for the top part with column and sigh is ~1000 kg/m.

In the Eurocode EN 1991-1-4 (Wind) there is an appendix for "Vortex shedding and aerodynamic instability". There is a factor for galloping instability a_G that becomes 0.4. That factor is a function of the depth and the width of the sign. The sign (a horizontal section) is a rectangular section with depth 0.7 m. That means that for the total width d/b = 0.7 / 11 = 0.06.
The table in Eurocode gives values for d/b = 2/3 to 1/3 and they are 1.0 to 0.4. But extrapolation is not permitted. If my colleague uses 0.4 (probably conservative) the result is that there is a risk for galloping. I have not found any source where I can get a better value for a_G.

Does anybody have an idea for a solution without using extreme methods?

He is designing a sign post that is relatively large.

 

[JAE]

Bump.
Anyone?

 

[GeorgeTheCivilEngineer]

It's not something I've looked at in detail but the obvious answer is there could well be a risk of galloping. Can the size/mass of the sign be changed?

Otherwise it may need some sort of damping.

 

[JStephen]

I'm not familiar with the code in question. With the range of depths, it sounds like that section is for buildings, not signs. Is there a separate specification for signs?
I have seen small roadside signs fluttering in the wind, I think they vibrated torsionally, which is not the normal assumption for vortex shedding. That may be addressed in some of the highway sign requirements, rather than the building codes.

 

[rb1957]

that's some sign ... 0.7m * 11m !! ... I'd add some stiffeners.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[ThomasH]

that's some sign ... 0.7m * 11m !! ... I'd add some stiffeners.

Perhaps I was a bit unclear, the two blue rectangles with the column between them is 11m wide in total. And the height of the blue rectangles is 15 m. The horizontal section is 0.7 m * 11 m, 0.7 m is the out-of-plane depth.

I don't think this necessarily is code related. I have some literature on aerodynamics and galloping can be an issue for rectangular sections. But I am not sure if this structure should be considered as rectangular in the context.

JStephen, you mention torsional vibrations. That could be due to galloping. I read that one option for a structure with possible galloping problems is to modify the structure to get torsional movement instead of lateral. Unfortunately the source did not specify how the modification should be made.

According to my colleague the size and masss is not yet finalized. Perhaps things will improve. But I have a feeling that we are stuck with this problem.

Thanks for your input.

 

[TLHS]

The AASHTO standards for luminaires and signs has guidance on vortex shedding design and the associated fatigue considerations, I'm pretty sure.

 

[rb1957]

yeah, I know nothing (Sgt Shultz ?) about your codes. I'd've thought the key structural stiffness was the cross-section and the length (height) above ground.


"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[3DDave]

Have you considered -

https://onlinepubs.trb.org/onlinepubs/idea/finalreports/highway/nchrp141_final_report.pdf

- tuned mass dampers using eddy current to remove energy from the motion.

 

[rb1957]

simple masses would be enough, no ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[RPGs]

I know nothing (Sgt Shultz ?)

Thank you for the laugh!

 

[rb1957]

you're welcome ... I'm here all week ...

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[BridgeSmith]

Unfortunately, the potential for several types of aerolastic instability will be difficult to rule out for a structure of this scale. I did the basic design for our new high mast light towers, but we handed it off to a consultant to do the detailed computer modeling and wind tunnel testing. Some of the results were a surprise to us.

That said, the AASHTO standard TLHS mentioned is a good starting point.

 

[ThomasH]

Thanks for your input. I think we'll try with AASHTO and see what that gives us.

 

[azcats]

I have the same experience as Schultz w/ this code, but here are my uninformed thoughts:
1. If you're outside the d/b ratios in the table, could they be telling you that these effects needn't be considered? This kinda makes sense as most examples you see of this phenomenon are more blunt/short like squares or cylinders. A long object would seemingly help promote more laminar flow around itself.
2. The 15x11 face would be a heck of a damper against winds in the parallel direction.
3. Resisting the winds in the large projected area direction may make the structure so rigid that any shedding effect could be negligible.
4. This doesn't appear to meet the type of structures that are usually in the vortex shedding conversation: tall slender structures like chimneys, light masts, comms. towers, etc. Seems like a big difference having a huge load at the top.

I do realize that none of this is a technical response that helps you.

 

[JStephen]

Here's an example from youtube of the vibration I've seen. Signs I've seen do this were smaller, shorter. They MAY have had channel-type posts (low torsional stiffness), I don't remember. They weren't this severe, and wind didn't seem to be this high at the time. This example may be largely due to the adjacent building. Examples I've seen were out in the open, though.

https://youtube.com/shorts/lhxGmLDMHo4?si=drHR2DzX5M_FYraf

 

[azcats]

I'd bet all my Eurocodes that stop sign is on one of those u-channel sign posts.

 

[dik]

Torsional vibration may be a bigger issue.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik