×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Tower Vibration Problem

Tower Vibration Problem

Tower Vibration Problem

(OP)
Hi Friends, I have a strange problem to discuss...

I work for National Grid. Few days back, I got a video in which a person was applying force on the lowermost diaphragm/horizontal bracing of the latticed steel tower. He was applying force manually (which probably wouldn't be more than 50 kgf) at the centre of this horizontal bracing. Result was a great amount of vibration observed in the horizontal and diagonal bracings in the lower portion of latticed tower, which perhaps are of the magnitude of over 200 mm. This caused a great panic in the management. The tower was a new one without any conductors erected yet. I was asked to recheck the analysis. I replied that towers are not designed for such load application, but this isn't the answer my managers want to hear. They just want to stop/reduce the vibration. Now the same towers have been used in here for over 15 years. And I'm unable to satisfy my bosses with the reason for such huge vibration. Can anyone explain the probable reason of vibration.
Thanks

RE: Tower Vibration Problem

Have you tested another tower? If it is common theme then the natural frequency is probably close to the applied frequency.

http://www.nceng.com.au/
"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

RE: Tower Vibration Problem

Almost every structure can be destroyed by the proper set of loads and actions. And, at some point we just can’t design practical, economical structures if we don’t draw the line someplace. It is management’s place to weigh the risks and costs, with our help, and to help us set those limits. These types of towers are inherently very light structures, designed to a pretty fine limit. Can you raise those horiz. braces a few feet to make them less accessible to these jack ass’? Could you put some diag. braces each way and to the center of that horiz. brace to make it less vib. sensitive? Could you put a bounty on those kinds of jack ass’ and hire sharp shooters to control the situation. What happens to the same tower when you do have insulators and conductors in place, don’t they do a lot to dampen the vib. situation?

RE: Tower Vibration Problem

Most proably it happend because of slenderness of the braces.can u send sketch of this tower?,what is ur slenderness ratio?

RE: Tower Vibration Problem

The members probably have just enough compressive stress in them to oscillate under relatively small directly applied concentrated loads normal to the long axis. They are not designed for that and the only service load that would come close to that would aolian vibration (from wind).

Your condition is not overly concerning.

RE: Tower Vibration Problem

Tell them to repeatedly push their cars sideways, it will oscillate, it is not designed to not oscillate because it is not normally subject to repeated sideways loading; same thing with the tower.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

RE: Tower Vibration Problem

Ask the managers, other than the unnatural load applied to the tower by the party in question, please identify a natural load we should be concerned about that the tower will actually see that will induce the same ordered response.

I also agree that this is a non-issue.

Mike McCann
MMC Engineering

RE: Tower Vibration Problem

Some days it seems that vandals are everywhere.

You could add cables from the center of that accessible diaphragm going up and down to other nodes, or a strut going up, or partially sister it, or half a dozen other things to reduce the tower's sensitivity to vandals or to make it a less attractive target for them.

If management says it's a problem, then it's a problem. Simply declaring it to be a non-issue is a non-solution. ... and a career decision.

Pick something to do that will solve the specific 'problem' as cited by management, make plans, figure out what it will cost, and submit it as a response. Let management decide whether to add it to every tower, or just the ones most accessible, and whether and to what extent to retrofit something.

Does National Grid keep records of vandalism? There might be historical data to help you support a recommendation of partial execution. ... or, not.



Mike Halloran
Pembroke Pines, FL, USA

RE: Tower Vibration Problem

Good advice, MikeHalloran....keeps a young engineer from being in the political firing line.

RE: Tower Vibration Problem

This is an interesting post.
Do you know who this person is that was applying the load ?
Could it be an engineering student or young experimenting with resonace ?
Surely there must be a reason why he was doing this other than vandalism.

I remember as a young engineer I used to check structures by testing them manually and got reponses even in fairly large structures.
I got a feel for structures that way.
Any one else do this ?

RE: Tower Vibration Problem

Sounds more like a "reporter" trying to "make a name for himself" by scaring the utility with a "this tower wiggles when it is vibrated" TV story.

RE: Tower Vibration Problem

Sorry, but management stating that there is an engineering problem does not make it such. That statement should be reserved for competent engineering professionals only.

Mike McCann
MMC Engineering

RE: Tower Vibration Problem

I dont think a reporter would do anything like that!
Only someone with some sort of knowledge of structures would attempt anything like this.

RE: Tower Vibration Problem

I didn't say it was an >engineering< problem.
... but it could be stated in a way that makes it an engineering problem, or that allows an engineered response to what is basically a nontechnical problem.

I.e., you could state that the goal is to prevent or suppress a disproportionate response to an anomalous input.


Nontechnical solutions are possible, also. I have no problem with declaring vandals who attack the grid to be terrorists, putting a bounty on their heads, and deploying snipers to deal with them, er, pre-emptively.

Mike Halloran
Pembroke Pines, FL, USA

RE: Tower Vibration Problem

By the way, was that an impact load? Since this would be a guyed tower, fence the base and all ground anchors so that it's off limits to all except authorized personnel. Such towers have a low factor of safety unlike self standing towers and should not be the subject of trivial activities around the base.

RE: Tower Vibration Problem

chicopee,
Where did you get the idea that this is a guyed tower?

RE: Tower Vibration Problem

Yea. Maybe it's a gal'd tower.

Mike:

OK. I see your point. I guess I'm just tired on non-technical managers calling the shots structurally. Where I currently work, my structural opinion is taken at face value and not challenged. That is refreshing for a change.

Mike McCann
MMC Engineering

RE: Tower Vibration Problem

(OP)
Thanks Folks

I just printed this page and showed the responses of you guys to my boss and everybody here was laughing. :)
That video was made by a guy who always points problems in everyone's work.

Anyhow, I modeled the towers on SAP2000 and applied 100 kg force at the same point where that guy was pushing.
Result is vibration off course, but the magnitude is very small, about 10mm but in the video it's much more. I guess there is some construction eccentricity almost always present during the tower fabrication, but can this create so much difference in the calculated and actual results?

RE: Tower Vibration Problem

Are you sure that the video was legitimate? Before jumping to any conclusions, I would want to repeat what the troublemaker and/or whistleblower did. No, eccentricity would not make that much difference.

RE: Tower Vibration Problem

(OP)
Well, I can't question the legitimacy of the video, that doesn't come in the protocols of a junior engineer like me... :/

RE: Tower Vibration Problem

Of course you have the right to question the video, junior or not, or your boss should. They can be doctored and photo shopped can’t they? You were asked to review the situation, you did and came up with only about 5% (10mm vs. 200mm) of the vibration the video seems to show. If you/we don’t or can’t trust our design, analysis and building methods, having been developed over many years, we are in big trouble as engineers. What frame of reference do you have in the video to judge the 200mm movement. Sometimes the angle of view, or the focal length of the lense can make things look worse than they really are. Telephoto lenses do some very funny things. Was there some resonance pushing this movement? It might be worth inspecting that particular tower to see that there were no loose connections, or other anomalies, which added to the apparent problem. What affect does the fact that the conductors are not hung yet have on the situation? Something sounds a little fishy here. If you find nothing, charge the trouble maker for the extra analysis and inspection time.

RE: Tower Vibration Problem

You make it sound like it’s more of a localized situation than the tower shaking overall……if it’s just an individual member(s) moving…….I wouldn’t get too excited: I’ve gone into industrial facilities and moved around braces (out of plane) that were meant to take nearly 50 kips in a lateral event (albeit some as tension only braces). The test is: is the whole structure moving…….if that is the case than check the FEA model for (overall) stiffness.

RE: Tower Vibration Problem

Hokie66, I can not imagine the degree of vibration explained in the OP to be on a self standing tower, and now checking the additional replies from Nibbi that may be the whole thing is a hoax and which in my mind was a waste of time for replies and despicable.

RE: Tower Vibration Problem

chicopee,
There are a lot of dynamic phenomena which are difficult to imagine. The Galloping Gertie bridge was one...probably not a good example, as that was indeed suspended/guyed. I didn't find it beyond reason that resonance could cause the affect observed in the video described by nibbi, but neither would I accept it without further investigation.

RE: Tower Vibration Problem

(OP)
chicopee, I dont know why you thought this 'whole thing is a hoax'.

I saw the video and responded immediately that we cant stop this vibration induced by applying push force on the centre/joint of horizontal and diagonal bracings, simply because the towers are not designed for this force.

But I was asked to check again, why so much vibration is being induced in the lower portion of tower (off course it's a localized vibration). I knew this whole effort was a waste of time, but I had to model the towers and check the response, which is what I've written earlier. (10mm deflection as compared to much larger one seen in the video). And That's why I published the problem here so as to get your views. Anyhow, after so much deliberation and not being able to satisfy my (Electrical Engineer) Boss, I have now distanced myself from this problem, with apologies. Some problems are better left unresolved !

RE: Tower Vibration Problem

If your boss is doing his job, he will find someone qualified in structural dynamics to investigate.

RE: Tower Vibration Problem

The SOB probably removed a bunch of bolts from the tower, then shook it.

Mike McCann
MMC Engineering

RE: Tower Vibration Problem

Am I reading this correctly that the tower was un-loaded when the lateral force was applied?
I don't know the magnitude of what this will carry, but adding gravity load should dampen this.

Load it up and have him try again

RE: Tower Vibration Problem

Were the tower legs anchored properly yet in the video? Agree with the unloaded condition changing how the truss would react also - did you model the tower without the conductors load, and did you model the added mass of the bozo jumping on the strut?

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources