Reactions are different between SAP2000 and OpenSees

nvquan · Post by **nvquan** » Tue Jul 01, 2008 7:51 pm

Hi Silvia and all,

I do a time history analysis for my strucuture in OpenSees and SAP2000 and compare the results. Displacements and accelerations are in very good agreement. But reactions do not match at all. Difference is like 30%.

Do you have any idea about this? What can be wrong? Are the reactions in OpenSees recorded in a different way ?

Thank you.

Post by **fmk** » Wed Jul 02, 2008 5:00 am

is there damping in your model?

nvquan · Post by **nvquan** » Wed Jul 02, 2008 7:17 am

fmk wrote:is there damping in your model?

No, I am trying no damping first.

silvia · Post by **silvia** » Wed Jul 02, 2008 7:26 am

have you compared a static analysis first?

nvquan · Post by **nvquan** » Wed Jul 02, 2008 7:30 am

silvia wrote:have you compared a static analysis first?

Hi Silvia,

Yes, I applied 100kips horizontaly on both model and I got the same reactions. But when I run time history analysis, the reaction plots are identical, but the value is different.

nvquan · Post by **nvquan** » Wed Jul 02, 2008 7:40 am

nvquan wrote:
silvia wrote:have you compared a static analysis first?
Hi Silvia,

Yes, I applied 100kips horizontaly on both model and I got the same reactions. But when I run time history analysis, the reaction plots are identical, but the value is different.

I read in this thread that reactions in OpenSees including inertia forces or some thing like that. Is this the problem?

http://opensees.berkeley.edu/community/ ... 29c98afbec

silvia · Post by **silvia** » Wed Jul 02, 2008 9:23 am

yes, that's why i wanted you to check static analysis.
damping will also add damping forces.
There are different options Frank has put in, but an easy way to make you get what you want, is to get element forces instead, which is the quantity you really want.

majidghaderi · Post by **majidghaderi** » Fri Nov 20, 2015 7:25 am

Hi, I have developed 9 story SAC building benchmark in SAP2000 and OpenSees. First three modes are in good agreement in both SAP2000 and OpenSees. But when I analyze the structure by Time history with same record, method, and damping, the outputs such as drifts are different. What do you think about problem?
Thanks

Post by **fmk** » Mon Nov 23, 2015 5:10 pm

the models are the same nitially so you have the geometries, element cconnectvity, masses and initial material properties correct. If the results are different it can be due to material properties and element formulations. If you are using SAP2000 I take it you are using the DispBeamColumn element and not the ForceBeamColumn one.

getting nonlinear models from 2 different codes to give similar results for transient nonlinear analysis is not always simple. remember that once the results start to diverge, it is basically like starting different transient analysis with same loading but different initial conditions.

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