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RE: Vibration problems

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>This would require reasonably accurate frequency analysis in the design 
>stage for the design of an appropriate mass damper and vibration 
>measurements, for mass adjustments, after completion of the structure. 
A tuned mass damper really seems like overkill (the equivalent of 
shooting sparrows with a howitzer). Skyscrapers use such things to avoid 
large displacements from dynamic loads. It's hard to imagine such a thing 
would be cost effective for a walkway.

You might want to look at the frequency assessment a little closer. 
Girders which span 100 feet must be pretty heavy. I should think if 
they're at the root of the problem that the girder system itself must be 
very soft. Is that the case? Have you looked at the mass of the platform 
compared to the mass and frequency of the girder system alone? If it's 
really the girder system that drives the frequency, you're pretty much 
stuck with it, since you can't change the girders. The conclusion is 
pretty straightforward--you can't hang your platform off the girders. I 
can't help thinking that if you really do have a problem it goes deeper 
than just the platform. If this were my problem, the first place I'd 
start looking is the mass calculation. I assume this was done with FEA 
software--have you verified the results carefully?

Christopher Wright P.E.    |"They couldn't hit an elephant from
chrisw(--nospam--at)        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)

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