SDOF

[mja165]

Hi

[vesna]

to define the damping you need to define Rayleigh command: http://opensees.berkeley.edu/wiki/index ... ng_Command

e.g.,
rayleigh $alphaM 0 0 0

in this case c=$alphaM*Mass;

If you know mass and damping (c) you can calculate alphaM.

[mja165]

Thank you very much.

[vesna]

It does not matter for the linear cases. You can use one or the other.

[mnazari]

I have a SDOF system, but a rotational spring is defined at base to model the multilinear elastic behavior of the system. Do you think I should just use one of these coefficients (Mass or Stiffness?) since it makes lots of change in the results.

[fmk]

for the typical model, most people use a term on the mass and another on the stiffness to get the required damping at 2 periods. most people have a nonlinear system, but typically choose the periods based on initial periods. as periods change the amount of damping the the structure changes as the periods shift.

Based on what most people do you would only need to specify 1 term: mass or initial stiffness value and you should get the same answer. Now even for a SODF system you could use 2 mass and current stiffness (as opposed to initial stiffness). Think of what Rayleigh damping does, run the analysis and look at what period range you will see even for the 1dof case, and then pick appropriate terms based on the damping you want over the period range your model will see.