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RE: Vibration problems

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Christopher Wright is right.  I replied before my coffee took effect.

Harold O. Sprague

> -----Original Message-----
> From:	Christopher Wright [SMTP:chrisw(--nospam--at)]
> Sent:	Tuesday, October 02, 2001 11:31 AM
> To:	SEAOC Newsletter
> Subject:	RE: Vibration problems
> >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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