Tuned Mass Vibration Damper for Poles

[Dinosaur]

I work at a DOT and we have thousands of poles supporting lights and signs. When I was in Grad School, we talked about using tuned mass vibration dampers to reduce vibration forces on structures. Our aluminum poles come with a device that sounds as if it may be one of these devices. However, our steel poles do not. I do not know why this is so.

I would like to do my homework first, and then propose that we install tuned mass vibration dampers on some of our larger steel structures to reduce the amount of loading on these structures due to wind. So far my research indicates we can resonably expect to reduct the base moments by 33-40% with this technology. It is extremely simple, until you have to do the math. Eigenvectors send a chill down my spine, but with the help of a well documented example, I could likely handle a two or three DOF model.

Is there anyone among you that can steer me in the right direction so I may master the mathmatics of this problem? Thanks in advance for any help you may be able to provide.

 

[MikeHalloran]

I don't think you necessarily need field- adjustability, because of other design standards. I'll expand on that a little:

Different structures designed to the same standard (i.e. beams given standard loading, max deflection as a fraction of span, that sort of thing) will generally resonate, or at least respond very strongly, at the same frequency. That's why most floors will sing nicely at ~3..4Hz, and why airplanes commonly have 'flutter' modes at ~6..7hz.

So, if all the poles in your population were designed as efficiently as possible to the same standard, then there's a fair chance that a damper effective for one will be effective for all.


That said, I worry that a TMD may not be effective enough. I've noticed that a lot of highway signage structure has recently gone to cantilevers of breathtaking proportions, all with single tubular columns welded to a sturdy bolted flange. I assumed, it appears wrongly now, that some serious aero style engineering had been done, because especially with signs attached, those columns _are_ aero structures.

I've also noticed news reports of large signs falling down, which used to happen ... never.

This bodes ill.



Mike Halloran
Pembroke Pines, FL, USA

 

[Dinosaur]

Mike,

Yes to pretty much everything you said, and this is why I am frustrated with the civil engineering community on this subject. We are not supposed to be constructing any more of these in my area right now, but even if we didn't, there would still be 1000s to study and possibly fix.

The resonant frequency of most of these structures is around 0.8 - 2.0 Hz. It mostly depends on whether there is a veavy variable message sign or a comparatively lightweight aluminum panel sign on the structure. I think an adjustable device would be best for our use.

 

[GregLocock]

Another option would be an untuned absorber.

This consists of a heavy puck sliding around in an oil bath. As the top of the pole moves around, the puck sternly resists the motion, and damping is developed in the oil film. For obvious reasons these are less efficient than tuned dampers.

A portable accelerometer and frequency analyser will tell you what frequency a given pole is shaking at, although if it is only .5-2 Hz so does a stopwatch.



Cheers

Greg Locock

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