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RE: passive energy dissipation

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Agreed, probably should have been more specific... time history analysis is
what I meant by dynamic analysis.

-gm

-----Original Message-----
From: Christopher Wright [mailto:chrisw(--nospam--at)skypoint.com] 
Sent: Tuesday, January 11, 2005 11:35 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: passive energy dissipation


On Jan 11, 2005, at 12:06 PM, Gerard Madden, SE wrote:

> Model the structure, using the damper properties (use etabs, sap, or 
> the
> like). It's a dual system with CBF's... use 100% of load. Then run the
> analysis with the braces/dampers inactive with 25% of the lateral 
> force.
>
> Probably should do a dynamic analysis if you are going to the trouble 
> of
> using dampers to really get the behavior down.
There are a few caveats here. For the most part structural analysis 
programs don't extract damped eigenmodes or frequencies because 
structural damping is small and the overall results won't be 
significantly affected. For seismic analysis damping is normally 
included in the response spectra. Another problem with discrete dampers 
is figuring out the damping coefficient, which is different than the 
damping ratio of the structure, since the damping ratio includes the 
equivalent mass associated with the applicable eigenmode.

If you're actually going to use discrete dampers, your best bet is to 
run a time history analysis. It's going to be a huge pain in the ass if 
you use a synthetic earthquake record, but you might have some luck if 
you confine your attention to a few governing modes and see if you can 
associate a damping ratio with the damping coefficients by running a 
step load input and noting the amplitude decay. You can back-calculate 
the damping ratio from the ratio of the amplitude of successive peaks. 
The actual relationship is in most vibration handbooks.

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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