Two simultaneous prependicular ground motions

pejman_opensees · Post by **pejman_opensees** » Sat Apr 24, 2010 9:38 am

Hi everybody,

I have a 2D moment resisting structure with fully fixed ground supports. I would like to apply a horizontal as well as a vertical [b]velocity time history[/b] ground motions on the supports. Can I do this via multiple support pattern in combination with imposed and ground motion commands?
I assume these commands are solely used for free DOFs. Can they be used on fixed nodes to simulate ground shakings?
If no, what is the alternative command in OpenSees?

Post by **fmk** » Mon Apr 26, 2010 9:39 am

you can input both .. AND you have to free the fixed dof's that these motions are being applied to .. if you need to do gravity before the input motions, fix the nodes and then use 'remove sp nodeTag? dof?' to free the dof again.

pejman_opensees · Post by **pejman_opensees** » Mon Apr 26, 2010 12:15 pm

Wont I encounter rigid-body motions and subsequently non-convergence if I free the supports?
Based on what you stated, If two simultaneous (vertical and horizontal) velocity ground-motions should be applied on supports using ground motion command, two out of three DOFs should be freed in fully fixed supports and only one direction remains fixed.
Wont it cause rigid-body motions?

Post by **fmk** » Mon Apr 26, 2010 12:47 pm

no because when you use imposed motions i create single point constraints for those dof's .. the only difference between them and when sp are created using the fix command is that they are treated as being non-homogeneous time varying constraints.

pejman_opensees · Post by **pejman_opensees** » Thu Apr 29, 2010 10:30 am

Thanks. It indeed worked with OpenSees 1.7.3 and 2.2.0 very well. However, a message appears which is unknown to me
WARNING: GroundMotion::getDisp(double time)-integration is required to get the ground displacements from the ground velocities.

Let me mention that I did add trap ezoidal integration for groundMotion command.

By the way, the same model was run using 2.1.0 and I received the weirdest results which were totally wrong.

Post by **fmk** » Thu Apr 29, 2010 11:13 am

using trapezoidal to integrate ground motions from the acelerations is tyoically rather poor .. it is better if you can provide the filtered ground motion displacements as input.