Damping Ratio for Secondary System 1

[rickfischer]

What is an appropriate damping ratio for use in a seismic analysis of a secondary system? The device is a sheetmetal cabinet assembled sometimes by welding but mostly with threaded fasteners.

 

[tomirvine]

The generic assumption in mechanical engineering is 5%. Some "large structures" are as low as 1%.

So you run the analysis with both values.

Ultimately, damping cannot be predicted. It can only be measured.

Tom Irvine

http://www.vibrationdata.com

 

[rickfischer]

Thanks for the reply.

Since my original post, I found NUREG/CR-6919, "Recommendations for Revision of Seismic Damping Values in Regulatory Guide 1.61", . This paper gives recommendations for damping ratios for structures, piping, HVAC ducts, mechanical and electrical equipment, etc. This can be found at:

http://www.nrc.gov/reading-rm/doc-collections/nuregs/contract/cr6919/cr6919.pdf

 

[GraemeL]

That looks like a good paper to back up the Regulatory Guide. It is interesting that there appears to be no mention of how damping is affected by the stiffness of the secondary item - i.e. its fundamental period or by its level of ductility. Haven't read it cover to cover, so may have missed it.

It is my understanding that for rigid items, damping has no effect on its seismic response and it is only as the items fundamental period starts to get above 0.06 secs does damping start to kick in.

Also, as an items response to an earthquake increases and things start to yield, the level of damping has a decreasing effect on its dynamic response. Hence for structures with a strutural ductility factor of 2 or greater, it is normal to disregard the effects of damping and assume a response as per the 5% damping curves.

So if your secondary item is meant to remain largely elastic and is not rigid, damping is a consideration in its response. Whereas if it is rigid or is going to yield and deform substantially, damping will not be important.

 

[Zhuge]

because you want this secondary system to behave elastically during big earthquake, I suggest you to take a 1% damping.