Constant Damping

[danusaht]

Based on the documentation online, it seems that only the Rayleigh damping model is available to define a damping matrix for nonlinear dynamic analysis with OpenSees. Is there a built-in command in OpenSees to define a damping matrix based on specified modal damping ratios?

[fmk]

there is only rayleigh damping or explicit damping through viscous materials.

the rayleigh coefficients are typically chosen
for 2 specific modes and desired damping ratio. typically user selects damping ratio at start of analysis. however, as mode shapes change in nonlinear system there is nothing stopping user from changing damping coeeficients during analysis.

[danusaht]

First of all thank you!!!

Second, I wasn't able to find the source code for the rayleigh command, is it available?

Thank you again

[fmk]

it can be found in the file OpenSees/SRC/tcl/commands.cpp

all it does is loop over elements and nodes setting the factors provided.

[Fahimeh]

fmk wrote:
> there is only rayleigh damping or explicit damping through viscous
> materials.
>
> the rayleigh coefficients are typically chosen
> for 2 specific modes and desired damping ratio. typically user selects
> damping ratio at start of analysis. however, as mode shapes change in
> nonlinear system there is nothing stopping user from changing damping
> coeeficients during analysis.


Could you please explain what you mean by "as mode shapes change in
> nonlinear system there is nothing stopping user from changing damping
> coeeficients during analysis."
Thanks so much in advance.

[fmk]

you could if you waned to inside the analysis loop, redo the eigen analysis and recalculate the rayleigh damping factors, and then reissue the rayleigh ccommand.