How to obtain relative displacements in direct transient analysis?

Antonio Tommasuolo

Hi,
I'm performing some linear transient analysis, I was able to get good results with the modal transient analysis. But if I try to set the same problem as a direct transient problem I get a rigid motion of the plate. I was having the same issue with the modal transient that was solved using the PARAM ENFMOTN set to REL.

Is there something like that for the direct transient analysis or I can only perform a post process operation and subtract the SPCD displacements "by hand"?


thank you for every help, find attached the fem files of both analysis.
Antonio Tommasuolo

modal_transient.fem

direct_transient.fem

Adriano_Koga

In fact, modal trasnient is better in this case, as it could directly get rid of this.

not sure if this helps, but in physical test, a pre and post correction signal is applied to compensate these high displacements and get rid of the undesired motion (which is natural due to the enforced acceleration profile (half-sine)).

https://www.sentekdynamics.com/page-8-how-to-select-a-vibration-testing-system

I also saw once that we could apply a gravity load on the opposite direction to stabilize the motion. I never tried this, but might be worthy trying.