Seismosoft | Earthquake Engineering Solutions

Dear Seismostruct support,

I have a very basic question about the Rayleigh Damping since I am a beginner in this part.

I would like to apply Rayleigh damping to a four storey three bay RC frame for dynamic time-history analysis. In Seismostruct user manual, I understand I should choose two modes. In my RC frame, I can get four reasonable modes from eigenvalue analysis. Should I choose the first and second modes for Rayleigh damping? Or first and fourth modes for Rayleigh damping? The damping ratio is 8% for all modes. Thank you very much.

Best regards,
Zhiyu

It would seem that this is a question best answwered by you, zhwang. You have to decide what is most important. It is possible to determine the fraction of damping in the other two modes, given the assigned damping in whichever two modes you select to have 0.08. This could be helpful to you - you could pick two modes, say the first and the fourth, and calculate how much effective damping you will end up with in the second and the third. Then you could pick the first and third to have 0.08 and calculate how much would be in the second and fourth. Then you have to use your engineering judgement to decide which is preferrable. You could do all possible combinations if needed. The calculation is straightforward. It may be found in the following reference, which should provide additional valuable insight. Rayleigh damping has isues which are discussed in the paper. Note that SeismoStruct does have the option of stiffness proportional damping, either initial or tangent based, in addition to Rayleigh damping. Of course, there are reasons you may wish to stick with Rayleigh damping, depending on the goal of your work and study. Best of luck zhwang. The reference should be rather easy to locate via a web search on the title.

Charney, Finley A., Unintended Consequences of Modeling Damping in Structures, ASCE Journal of Structural Engineering, April 2008, 134(4), pp. 581-592.