Well, of course, you should use stiffness and damping in series. In the case of a viscous damper, use Maxwell formulation or simply combine two axial materials in series.

Damping and stiffness in parallel is only applicable for viscoelastic dampers.

This webpage conveys some useful stuff.

http://opensees.berkeley.edu/wiki/index.php/Maxwell_Material

Dear Amin, actually at first I used ViscousDamper material, the same material that you linked but I got some warnings that showed there are some convergence problems in static analysis in this material and I asked about it in OpenSees Community, here is the link:

http://opensees.berkeley.edu/community/viewtopic.php?f=2&t=60689

There, one of OpenSees developers told me to ask about the problem from the publisher of ViscousDamper. So I decided to use Viscous material and a stiffness material separately and then make a connection between them.

Now if I want to use parallel, is it possible and common to use viscoelastic material as a brace in steel moment frames?

And Besides what is the best material for stiffness?

Elastic or Elastic Perfectly Plastic or steel01?

I think you need to check a couple of things.

Check stability condition of your model. In Opensees all structures should be stable. If your model is unstable without those dampers, it would be also unstable even when you add those series-based viscous dampers. Stability of a pinned frame can be assured by introducing some minor flexural stiffness for beam/column connections or using a fixed connection for column bases.

I'd like to clarify that you can also use stiffness and damper in parallel. But in this way you should assign a very small value for the stiffness such that it has very minor contribution on the damper force.

About your second point, for a well designed damper, it doesn't matter if you use either elastic or inelastic materials for the stiffness component. As stiffness component should remain mainly elastic and the main energy dissipation capacity would be provided by viscous damping itself. So, in favor of simplicity, use Elastic command. Steel01 is well-suited for yielding dampers and you can use it if you wish to consider yielding dampers as well.

Above comments are all based on the assumption that your diagonal dampers don't buckle. For a buckle prone diagonal damper, more rigorous model is needed.

BTW, if accidentally you are going to carry out a kind of parametric study with large number of analysis on these dampers, I propose Matlab/SimuLink as it is much faster than Opensees or any other FE software.

Good luck in your job.

Dear Amin,

First of all thank you for generous and willing attitude. The steel moment frame alone, is stable and I've got thousands of runs from and before this all verify it with other models. After I used damping and stiffness in series rather than viscousdamper material all convergence warnings are gone.

Thank you again

Ryleigh