Rotational Component of Ground Motion

[kesavapraba]

Dear STKO Team

I have following questions:

1. If I have ground motion acceleration time histories in all six DOFs (3 translations + 3 rotations), how do we apply them to a structural model in STKO? Using "UniformExcitation" command or MultiSupport excitation?

2. Regarding MultiSupport excitation analysis: While going through Plain groundMotion command, I find the statement in the Wikipage "For earthquake excitations it is important that the user provide the displacement time history, as the one generated using the trapezoidal method will not provide good results.", what does it mean?

3. Can I give only velocity time history instead of acceleration as an input in thegroundMotion command?

Thanks in advance.

[marafini.f]

Dear Prabakaran,

1. If I have ground motion acceleration time histories in all six DOFs (3 translations + 3 rotations), how do we apply them to a structural model in STKO? Using "UniformExcitation" command or MultiSupport excitation?

The difference between uniformExcitation and multiSupportExcitation is not related to the DOFs of application. The first applies a uniform excitation to all fixed nodes in the model, (not a condition, just an analysis step), results are relative in respect to the results at the node fixed in the model. The second applies a ground motion excitation to the selected nodes in the model, results are absolute in respect to the results at the node selected in the model.

If you want to apply a uniformExcitation in all DOFs, you will simple need to have 6 analysisSteps, one per each DOF. If you want to use a MultiSupportExcitation than you need to define a groundMotion applied to all DOFs.

2. Regarding MultiSupport excitation analysis: While going through Plain groundMotion command, I find the statement in the Wikipage "For earthquake excitations it is important that the user provide the displacement time history, as the one generated using the trapezoidal method will not provide good results.", what does it mean?

I didn't read this sentence, but I think it means not to use approximated, or linearly interpolated time history. If I understand it better I will tell you more.

3. Can I give only velocity time history instead of acceleration as an input in thegroundMotion command?

Of course, when you open the groundMotion condition you see there are different options, depending on if you defined acceleration, velocity or displacement time history, and the DOFs you want to apply it to. Of course, since it's a condition, you will need to select the nodes you want it to apply it to.

Hope all of this is clear, let me know if you have any more questions.

Francesca

[kesavapraba]

Dear Ms. Francesca

Thanks a lot for your reply. Your answers are helpful. I understand it. Just one question.

If you want to apply a uniformExcitation in all DOFs, you will simple need to have 6 analysisSteps, one per each DOF. If you want to use a MultiSupportExcitation than you need to define a groundMotion applied to all DOFs.

.

Dr Massimo said that the origin of the global axes will play role in structural responses when rotational components are applied using UniformExcitation. I did not understand it. Can you please briefly explain it here how, so that the answer will be useful for others as well?

Thank you

[marafini.f]

So, I asked Massimo about this more.
For earthquake excitations it is important that the user provide the displacement time history, as the one generated using the trapezoidal method will not provide good results

It's referred to the GroundMotion in the MultiSupportExcitation saying that if such command takes only acceleration it will calculate velocity and displacement using only numerical integration (trapezoidal method) and that's not very accurate. So if you have the displacement time history and velocity time history it's better to input it.

As for this question.

Dr Massimo said that the origin of the global axes will play role in structural responses when rotational components are applied using UniformExcitation.

The concept is that, if you apply a rotational excitation, the rotation always refers to a center of rotation (acceleration will be translational = rotational_acceleration * distance_Center). We did not test this, but from a quick peek at the source code, it seems that this is done referencing the global 0, not the center of mass of the structure. You should make some attempts and verify this assumption.

Let me know if you discover anything.

Francesca

[kesavapraba]

Dear Ms. Francesca

Sure, Thank you for your reply.

[marafini.f]

You are welcome!