Tuned Mass Vibration Damper for Poles
Tuned Mass Vibration Damper for Poles
(OP)
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.
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.





RE: Tuned Mass Vibration Damper for Poles
The standard idealisation for TMDs is as a two-degree-of-freedom problem. The mathematics is quite tricky, but no more so than for any other 2-dof problem. Once I had designed my system on the basis of the above book, I checked the result by done a full FE analysis on the structure (using an FE program that offered damper elements). It checked out fine. Even more important, the actual bridge when built also checked out fine.
RE: Tuned Mass Vibration Damper for Poles
So, I'd talk to your test people to find what data they have on the fatigue life of the poles, before getting too enthusiastic, and also see if they have any accelerometer data for the vibration at the top of the poles.
I can't see why it would be worse than a 2DOF problem, and with a bit of malarkey we can just pretend that the pole is a cantilever in bending.
So I would work out, or measure the modal mass at the top of the pole, and then set up a 2 dof spring mass damper model to estimate the reduction in amplitude. You can do that in Excel. So far as I am aware there is no analytical solution to a discrete SMD on the tip of a distributed cantilever beam.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Tuned Mass Vibration Damper for Poles
RE: Tuned Mass Vibration Damper for Poles
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Tuned Mass Vibration Damper for Poles
RE: Tuned Mass Vibration Damper for Poles
From some of the figures given, it looks like you can get pretty dramatic reduction in peak amplitude at any frequency, even if the mass ratio (auxiliary mass over main mass) is small.
For a given mass ratio, in the optimum tuning, he adjust the natural frequency ratio f of damping system to main system to get equal heights of the two sidebands. Then he adjusts the damping to determine the value of damping which minimizes the magnitude of those two sidebands. At least that's what it looks like to me.
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RE: Tuned Mass Vibration Damper for Poles
My assumption (right or wrong?) is that the excitation should be viewed as broadband or random from flow turbulence. Perhaps one could attempt to predict vortex shedding frequencies, but those would change with wind velocity, direction, and maybe profile. So it seems the design objective would not be to shift the resonant frequency, but to reduce the magnitude of the highest resonant peaks within the credible region of excitation. I think the damped dynamic vibration absorber is capable of that.
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RE: Tuned Mass Vibration Damper for Poles
Here is a photo:
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I tried to take a picture but the quality is understandably bad (I was driving at the time).
I heard second-hand that these are some kind of vibration device and I’m curious if that’s what they are.
Is this similar to the ones you have seen on aluminum poles Dinosaur?
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RE: Tuned Mass Vibration Damper for Poles
When a TMD is excited at the design frequency, the damper buzzes, but the protected structure hardly moves.
The 'rod' between the masses is often made of wire rope, which provides a little internal damping of its own, and I think spreads out the response a bit.
Mike Halloran
Pembroke Pines, FL, USA
RE: Tuned Mass Vibration Damper for Poles
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RE: Tuned Mass Vibration Damper for Poles
Yes, the mass dampers you have there in your photo are similar to those used on aluminum poles.
Several folks have correctly indicated that tuned mass dampers (TMDs) are used to reduce the response as the structure approaches the first natural frequency. This is important to our highway structures because they will sway in the wind at their first natural frequency even though the excitation is a random forcing function. Also, truck gusts will cause many to vibrate as well.
A properly tuned TMD will reduce the maximum response over 30% from the peak of the unmodified structures first mode. However, there are other things besides finding the original strusture's first mode that need to be considered. Also, I will need to be able to document the new response spectrum to a magnitude of about 1.5-2 x first freq in order to show some of our folks this is a positive thing.
Thanks for the help. Keep it coming. Dinosaur
RE: Tuned Mass Vibration Damper for Poles
Absent failure or foreboding incident not mentioned so far here, you have to prove that the problem you are proposing to spend resources to solve, actually is a problem.
It's entirely possible for a steel structure, even a large one, to flex noticeably over its entire service life, before it fails from fatigue. The odds are not so good for aluminum, hence (speculation on my part) the TMDs on/in the aluminum poles.
You may also have to prove that it's _your_ (or your outfit's) problem. I.e., if other parties engineered the structures in question, you need to review the contracts in before reviewing the design documents.
But you knew all that.
Since even just reviewing the contracts involves a lot of effort, you probably need to set up a crude 2DOF model as suggested, and evaluate it for deflection, _and_ for fatigue, with and without TMDs.
Mike Halloran
Pembroke Pines, FL, USA
RE: Tuned Mass Vibration Damper for Poles
You pretty much hit the highlights of proposing a change. As for the problem, well there definately is a problem. One of our structures fell due to a complete non-ductile mode failure at the pole base. This particular structure came down between the Cab and the Trailer of a truck traveling on the interstate. Think how amazing that is! We have had a few other non-ductile failures and these are occuring just above the base plate circumferential weld.
Now I say non-ductile failure mode, but what do I mean? Well these are steel structures of a grade between 36-50 ksi material. A material in this range should demonstrate a significant yielding where the maximim stress occurs before fracture. There is no such evidence of yielding, just a fairly straight grainy crack. This leads me to believe fatigue is a major contributer; however, I suspect there is a large residual stress built in due to the fabrication tolerances.
If I can reduce the free vibration deflection by 30%, there will be a substantial increase, possibly close to double, of the fatigue life, and more if we happen to drop below the CAFL. Regarding the premise that my organization needs to "Own" the problem ... well we are the DOT. Generally our customers want us to own problems like this. It is a legal matter whether we truly own it though; although, the structures are ours and the roadway below is also.
Does anyone else bear a greater responsibility such as the engineer? In this case I don't think so. If it can be shown the structures were designed to meet the code in effect when they were delivered, then I don't think the engineer will be considered negligent. The design code went through a substantial revision in the 90s I believe and I am unsure they have captured the essence of the vibration problem to this point. They included an entire new section with fatigue levels that need to be addressed but there still appears to be problems within the structural community about interpreting this.
For the cost of less than 5 new structures, I believe we could retrofit every cantilever sign support in our state, reducing the fatigue stress levels for free vibration by 30% and maybe more. It will take me years of paperwork to back that up because this is not a subject covered by my position responsibilities and I therefore have no resources to dedicate to pursuing. But I will continue to look for information to help me make the case. If I'm really lucky, the true problem will become known and fixed long before I'm ready to make my case.
RE: Tuned Mass Vibration Damper for Poles
Johnny Pellin
RE: Tuned Mass Vibration Damper for Poles
Back to TMDs, in this case you'd tune the damper to the problematical mode of the pole, and you probably need damping in the coupler, to pull energy out of the system. The problem is that the optimal damping is not very heavy tyically, certainly anything over 30% of critical seems to be too much for metal structures. If you use too much damping the TMD never really gets going, so it doesn't pull enough energy into the damper. I've rarely had much luck calculating these things, but in retrospect a phase angle of less than 10 degrees is too little, and 25-30 is typical, and 45 can be OK but is often too much.
Another alternative is to use an undamped tuned absorber, and just split the mode. I don't this will work in this case - in fact it would give you twice as many modes to worry about... but if vortex shedding is the problem then it could detune the problem away from the vortex frequency.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Tuned Mass Vibration Damper for Poles
After Hurricane Wilma, there was a shortage of traffic signals. Wilma took them down and shredded them, and it took most of a year to catch up.
The 'old way' to suspend traffic signals above an intersection was with a cat's cradle of wires between four concrete poles. Wilma knocked the signal boxes off, but left the poles and the wires mostly intact.
The 'new way', now appearing, is two tapered tubular columns each supporting two very long tapered tubular beams at right angles to each other, with signals for four to six lanes attached to each beam. They have an interesting vibration mode that's excited at some wind speeds, with the column bending around an axis that's diagonal to the intersection, and the signals at the tip of each arm going up and down over a range of a couple of feet at about half a Hertz.
They sure look zoomy, and I guess they'll survive a hurricane, but I wonder how long they'll survive normal service. The poles flex quite a lot, and I swear you can see the column bend too.
Mike Halloran
Pembroke Pines, FL, USA
RE: Tuned Mass Vibration Damper for Poles
This is exactly what I am talking about. The design code has not captured this phenomena properly, in my opinion, and I am worried we will continue to get fatigue failures until we recognize this problem. The fact that these structures have no redundancy makes this a severe safety problem. However, in the civil engineering community, most folks act as if this sort of talk about vibrations is making this into a space shuttle design problem. Dynamic load, such as wind, in civil engineering has been handled by determining a safe magnified static load that will produce a structure stronger than the dynamic effect. It works in many cases, but I don't know if we could find a safe static load for the wind effects in this case because the problem is so dependent on the geometry of the problem.
Now picture that same cantilever signal support with a vibration absorber on the tip. Properly proportioned, the deflection of the pole should be reduced by approximately 30% and the pole will vibrate at a frequency near but not quite matching the original frequency. There would be a corresponding reduction in the maximum bending stress at the pole base, and that could be enough to save the pole.
This is my hypothesis and I am looking for some assistence in pursueing the mathmatics to show the potential benefit.
RE: Tuned Mass Vibration Damper for Poles
It seems like estimating the damping in your original structure and in your damper will be the most challenging part of the calculation.
Where did you come up with the 30% number?
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RE: Tuned Mass Vibration Damper for Poles
Unfortunately, my DOT is not on-board with the problem statement and has some ways to go to catch up with the work going on at these other DOTs.
As for estimating the damping, I agree that would be a major challenge; however, I plan to circumvent the whole thing by simply measuring the response and back calculating the damping. There are any number of signal supports withing a half hours drive from the office I could study in the field but right now nobody sees any need to do that because they don't agree vibration can cause a failure.
RE: Tuned Mass Vibration Damper for Poles
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
RE: Tuned Mass Vibration Damper for Poles
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.
RE: Tuned Mass Vibration Damper for Poles
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
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.