Question about Rayleigh command

[babibibak]

Dear Silvia,

How can I specify correct rayleigh coefficients for a ten story concrete building with moment resistant frame?
by the way, the building does not have any dampers.

[fmk]

look at the theory for rayleigh damping .. you need to pick your damping ratio and the 2 mods you are going to use, do some eigenvalue calculation and then there is a little math involved.

then, you have to think for a bit .. because rayleigh damping was dreamed up for linear elastic problems. most commercial fe codes use the initial stiffness, i.e. C matrix does not change. In OpenSees, we give you the option of using one or a combo of the initial, current or last committed stiffnesses.

[khosravifar]

Hey Frank,

Could you please explain more on what you mean by current/last/committed stiffnesses (or any reference)?
I'm using San Diego's model (i.e. highly nonlinear). Within one time step, the solver iterates to find the yield surface, so there is an initial stiffness and a current stiffness within one time step, but the intial stiffness changes from one time step to another. So, now if I use initial stiffness Rayleigh coefficient, does it use the stiffness when the Raileigh command was called, or it uses the initial stiffness within each time step?

Thanks,
Arash

[fmk]

initial stiffness i would have thought obvious .. it is that at time 0 when structure is in undeformed config.

the stiffness at the end of the last time step is the last committed stiffness

the stifness at the surrent trial step is the current stiffness.