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Re: passive energy dissipation

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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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