DAMPING

[mohammadjjj]

Dear Users
I'm searching about MODAL DAMPING Command in Opensees (or applying Modal Damping) in Opensees
Would you please help me ? Pleaseeee

[fmk]

there are 2 commands:
modalDamping $ratio
modalDampngQ $ratio

the latter ignores adding any part of C to the tangent matrix. the commmand uses the existing eigenvectors, so the eigenvector command must have been issued first. a wipe currently does not remove the factor, so if in a script you want to play with modal and then rayleigh you need to call the modal command with a 0 ratio before the analyze. you can issue a rayleigh command with he modal to damp out high frequency modes that are not covered as Prof. Powell recomends in the PERFORM manual. The issue never popped up (in the responses i have looked at) so don't know how much of an issue it is as at least for the disp response the response is governed by the fundamental modes.

[mohammadjjj]

If I want to assign different damping to different region which is possible for Rayleigh command, how it could be used for modalDamping? In addition, in modalDamping only one damping ratio can be assigned for all modes, how it could be accurate? (even if different frequency and mode shapes for each mode are multiplied)

[fmk]

you cannot .. it would be possible to apply different damping ratios to different modes .. the modes however know nothing of regions nor should they .. this is modal damping after all .. and what do you mean how could it be accurate? .. if you are saying that it will not give the same as rayleigh you are correct, but who ever said rayleigh was accurate? .. it is like comparing apples and oranges .. they numerically do different things

[mohammadjjj]

I don't mean that Rayleigh gives an accurate result, I just have two questions about modalDamping as follows:

1- Is it possible to assign different damping ratio to different regions in different modes using modalDamping command?
2-How could different damping ratio be assigned to different modes in modalDamping command?

Thanks