Seismosoft | Earthquake Engineering Solutions

Is it possible to specify zero local damping? It appears the choosing 'none' under local damping does not mean the same thing as choosing 'none' on the global level.

Regards,
Matt.

Hi mattfox.

I'll give you my interpretation based on the Help System in the Inelastic Frame Element subtopic. The help file reads:

"Users are reminded also that damping defined at element level takes precedence over global damping, that is, the "globally-computed" damping matrix coefficients that are associated to the degrees-of-freedom of a given element will be replaced by coefficients that will have been calculated through the multiplication of the mass matrix of the element by a mass-proportional parameter, or through the multiplication of the element stiffness matrix by a stiffness-proportional parameter, or through the calculation of an element damping Rayleigh matrix."

I admittedly have not done an exhaustive study of these effects, but I interpret this to mean that if local damping is unspecified, then no modification to the structure damping is employed; i.e., the global damping matrix is left unmodified.

If the local damping is specified on the other hand, then the system damping for the corresponding degrees of freedom are adjusted in accordance with the Help System.

So it may be that one could assign local damping not quite equal to zero for a given element, but virtually zero by specifying very small, yet non-zero, coefficients in local damping (I'm not sure what would happen if one specified local damping but entered zero for the coefficients - it may be worth a try). Unless of course both global and local damping are zero, in which case the system is truly undamped other than that induced by hysteretic behavior of fiber elements and links.

I hope this helps and if someone wishes to correct my interpretation, I'll welcome the knowledge.

Best of luck mattfox.

Thanks huffte,

My question was more directed towards resolving what the ambiguous option of 'None' under local damping means. Does it mean (a) There will be no damping for the element considered (i.e the word 'None' has an equivalent meaning to when it is used to specify no damping on a global scale) or (b) 'None' means no local damping is specified and global damping is applied as usual. I think this is a question for Seismosoft and possibly a (small) area that needs clarification in future versions of the software.

On the same topic I am now interested to find out more on how local damping is applied. Does anyone have any good references? What interested me the most is that it appeared I could apply Rayleigh or mass proportional damping to an element with no mass. How can this be?

Hopefully someone can help!

Regards,
Matt.

Matt,

This response comes certainly too late now, but we just wanted to confirm that, as stated in the Help System, the correct answer to your question is indeed option (b).

With regards to the other point, if an element has no mass, then mass-proportional damping defined at element level will inevitably end up by being zero, since the damping parameter will be multiplied by a zero element mass matrix.

Renewed apologies for the immense delay,

Seismosoft Support