Direct integration vs Modal history in SAP2000

[Hunter64]

Hi all.

I am analyzing my bridge model using non-linear direct integration and modal FNA analysis but the results from both tests are different. Modal FNA analysis give smaller result vs. direct integration.

I did try using displacement and acceleration time history for direct integration and those results are the same, but the modal FNA analysis is about 0.5 of the DI result. Is this supposed to be the case? I know that the numerical solutions used are different.

Also, I am considering multi-support excitation, but I think it's not computed properly using modal FNA analysis. I'm asking because bridge model with pile and soil springs take a very long time, say 24 hrs, to compute.

Thanks all.

 

[stressed]

You didn't mention the nature of the nonlinearity in your bridge model. In order to run NL time history analysis with FNA, the nonlinearity will need to be limited to NL links. FNA will not consider geometric or material NL behavior, although material NL behavior could be simulated with NL multi-linear plastic links. Also, you need to run FNA using Ritz vectors, not Eigen, and the Ritz vectors need to have loads applied for every load pattern and acceleration load. If you have NL links, you'll also need to have loads applied for Links "All". You'll need to run enough Ritz vector modes to capture adequate mass participation.

Google "CSI FNA time history" and you'll find a number of technical write-ups

 

[Hunter64]

Thank you stressed, that is insightful

For the first case, i did not specify any NL links. The 2nd case (bridge-pile) now considers NL links. I will try your suggestions.