Rayleigh Damping Coefficients in OpenSees and SAP2000

[hadisayadpour]

Hi every body;
I modeled a SDOF system with a massless column and a lumped mass at top of that. I used Rayleigh damping coefficients in order to model a 5% damping ratio.
I created the same model in SAP2000 and used the same damping Coefficients but after transient analysis, the results are different from those from OpenSees.
So, the question is:
What is the difference between them and how can I fix this problem?

thanks

[mhscott]

More details on your model are required. Is the material response linear or nonlinear? Are you using mass or stiffness proportinal damping?

[fmk]

if you use rayleigh damping with the coefficients in sap being on the mass and stiffness matrices, the results will be exactly the same with OpenSees if the models and analysis are exactly the same. the problem arises for people who are using different models (different element or material formulations, and often different node placement, element connectivities, boundary condituions, etc.) and analysis options. The problem is typically that one and sometimes both! of the input files are incorrect.

if you are using OpenSees and ZeroLength or Truss elements, do note the -doRayleigh option on the command line in the manual (it makes a difference if you want to use rayleigh damping!)

[hadisayadpour]

@mhscott:
the material is linear and I used both mass and stiffness proportional damping in OpenSees, but it gives different results with the same coefficients in SAP.

@fmk:
the connectivity, element, material properties and every thing are exactly the same but as I said, it gives different results with the same coefficients in SAP.

thanks

[fmk]

to find the source of your error i suggest:
1) if you have a single dof model you are only provide mass or stiffness proportional damping (you don' provide both!) ..
2) check the -doRayleigh option if you choose the stiffness proportional option and you are using truss or zerolength elements ..
3) run it though a motion for which the results are known (e.g. elcentro picking a period and damping ratio out of Prof. Chopra's book) to find the source of your error.

look at the sdofTransient script in the verification examples. It tests a number of harmonic responses (damped and undamped) and also a uniform excitation example. It compares results from OpenSees versus theoretical exact response and results from Prof Chopra's book:
http://opensees.berkeley.edu/WebSVN/lis ... ication%2F