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Re: Damping Ratios

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>Can anyone refer me to a good reference for determining damping
>ratios? 
USNRC damping ratio values for seismic design are as follows
                                           OBE   SSE
Piping (large diameter)        2%    3%
Piping (small diameter)       1%    2%
Welded structures               2%    4%
Bolted structures                4%    7%
Prestressed Concrete          2%    5%
Reinforced   Concrete          4%    7%

OBE means an operating basis earthquake--an event where the plant is 
expected to remain operational and structural damage is expected to be 
slight--not much permanent deformation. The SSE (safe shutdown) event is 
severe enough to produce yielding but not collapse in structures. The 
different service levels will give you some guidance in design.

You want to be a little careful with the foregoing. First, the damping 
ratio is really just a convenient fiction. It refers to viscous damping 
which doesn't occur in real structures. The actual energy loss in real 
structures varies with stress levels and frequency, and the given values 
are for use in nuke plant design with different stress levels and local 
stiffnesses than residential or non-industrial structures. Third, vortex 
shedding is non-linear, in effect self-excited vibration and different 
from seismic work, so it's a good idea to look for rules of thumb in the 
business you're addressing. Use anecdotal information very carefully.

There are ways of calculating damping energy loss for different types of 
systems, but it get very complicated very fast. See the Shock and 
Vibration Handbook for details. 

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


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