Learning Forum

[d.g.]

Dear all, I’m running a spectrum analysis with Ansys, and I found a stranger behaviour in final results. I modeled a simple column, 30x30x300 cm, subject to its own load. I run a modal analysis, and then a response spectrum, using 4 shape modes and a SRSS combination mode. I analysed Ansys results: as regard the deformation and the normal stress, Ansys results correspond with manually ones; as regard principal stress, on the contrary, I don’t obtain the same results. Moreover, I noticed that Ansys gives not only positive but also negative values, that is impossible according to the definition of the SRSS combination method itself. I tried also to repeat the analysis using another combining method (CQC) but the problem still exists. What can be the cause? Thankyou.

[peteroznewman]

It could help if you would attach the archive of your project to your post and say which version of ANSYS you are using.

[d.g.]

I'm using ANSYS 18.0. How can I attach the file?

[peteroznewman]

In Workbench, save the project, then File, Archive... and create a .wbpz file.  If that file is < 120 MB, you can attach it using the Attach button on any of your posts.

If the file is > 120 MB, click on Model, Clear Generated Data, then save and archive to get a smaller file. Don't include results.

[d.g.]

Ok, it was really a size problem 

[peteroznewman]

I have downloaded your model and solved it, now I need some time to study RS analysis as your question is deep.

[d.g.]

Ok, I will wait... thank you

[d.g.]

Hi peteroznewman, you got some news?

[peteroznewman]

Sorry, I have not had time to study this yet. I plan to do some this weekend.

Others are reading these posts and may also answer this question.

[d.g.]

Hi everybody, you got some news about my trouble? Do I deduce that there is a mistake in Ansys algorithm?

[Dave Looman]

In Mechanical derived stresses like s3 are not combined in the SRSS comination, they are computed after the combination from the SRSS'd component stresses. As a result, the derived stresses from a response spectrum analysis are not mathematically valid. They tend to be in the right ball park, but anomalies like this one are possible. In APDL the sumtype,prin command is used to request the combination be done directly on the derived stresses which is more conservative and avoids the possibility of a negative value. n