Rayleigh Command

Paolo · Post by **Paolo** » Sun Feb 26, 2006 2:31 pm

Hi all users!
I have a question about the command:
rayleigh $alphaM $betaK $betaKinit $betaKcomm
What is betaKcomm?
I think it is the stiffness matrix at last-committed state of the dynamic analysis, but, if so, how can be known by the program before doing the analysis?
In some examples rayleigh is used with
alphaM = 0
betaK = 0
betaKinit = 0
Is this a good choice?
Thank you for the attention.
Best regards.

Nero · Post by **Nero** » Mon Feb 27, 2006 5:06 am

A material has two state stiffnesses: the trial stiffness, and the committed stiffness. The trial stiffness is cacluated at each iteration (as the non-lineararities are stepping to converge), whereas the committed stiffness is the final trial stiffness that occurs at convergence. So each analysis time step may have multiple trial stiffnesses, but has only one committed stiffness (which is committed at the very end). Kcomm is this stiffness that is converged upon at the end of the last time step.

This value is not known before analysis, but in most (all?) materials it is initialized to a starting value. Your damping matrix will change over the course of the analysis if you have non-linear materials and you are using either betaK or betaKcomm- the difference is when- K will change at each convergence iteration, whereas Kcomm will change once a time-step. Kinit uses the initialized K cacluated when the material is generated, and thus does not change at all during the analysis.

The choice you make is dependant on the system you are looking to analyize.

silvia · Post by **silvia** » Mon Feb 27, 2006 9:31 am

i typically do an eigenvalue analysis to determine the inital stiffness.