Tower Vibration Problem 1

[nibbi]

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

 

[rowingengineer]

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."

 

[dhengr]

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?

 

[ahmedhegazi]

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

 

[Ron]

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.

 

[paddingtongreen]

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

 

[msquared48]

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

 

[MikeHalloran]

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

 

[Ron]

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

 

[civeng80]

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 ?

 

[racookpe1978]

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.

 

[msquared48]

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

 

[civeng80]

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

 

[MikeHalloran]

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

 

[chicopee]

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.

 

[hokie66]

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

 

[msquared48]

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

 

[nibbi]

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?

 

[hokie66]

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.

 

[nibbi]

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