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

 

[dhengr]

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.

 

[WARose]

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.

 

[chicopee]

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.

 

[hokie66]

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.

 

[nibbi]

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 !

 

[hokie66]

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

 

[msquared48]

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

Mike McCann
MMC Engineering

 

[hawkaz]

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

 

[btrueblood]

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?