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RE: Vibration problems

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>A cheaper solution would be
>to first try to add mass to the platform.  You are correct in that for a
>given forcing function the 2 methods of control are stiffness and mass.
>Vibration dampers control the stiffness.
I'm gonna stick my neck way out and quibble with Harold Sprague, but 
adding mass to the platform lowers the natural frequency of the system. 
And vibration dampers control amplitude at resonance by absorbing 
energy--no effect on frequency. You can add mass to move a resonance 
point below some fixed excitation frequency, but that's about it. I've 
also seen shock mounts which are soft enough to decrease a frequency but 
the trade-off is that the displacements tend to increase as the 
accelerations drop off, which may not be desirable.

Increasing a structural frequency by a factor of two is a tricky job. 
Since the frequency goes with the square root of stiffness/mass, you'll 
need to decrease the mass by a factor of four, since your stiffness is 
fixed. The fact that some of your mass is in the roof girders makes it 
even tougher. 

I've tried increasing structural frequencies a few times with machinery 
and invariably ran into the fact that it's very difficult to add 
stiffness without adding mass. If you double the stiffness and increase 
the mass by, say, 30%, your frequency increase is only about 25%. You 
might try looking at your frequency assesment and make sure you haven't 
over-simplified it. It's very tempting to assume pinned supports and 
ignore inconveniently complicated support conditions, but I suspect 
you'll be very hard-pressed to increase the frequency as much as you want 
if you fail to take advantage of everything. You might want to try 
attaching your platform to a stiffer point on the girder, closer to the 
girder supports. Could be your platform mass could be decreased, too.

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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