Question: How to apply "Prescribed Motion" by nodal acceleration

[youngkyu]

Dear STKO members,

While modeling a slope subjected to seismic loading condition, I would like to apply acceleration time series to a node of the model bottom. In other words, I would like to simulate the "Prescribed Motion" boundary condition that is described in the FLAC dynamic manual on the page of 1-27.

In STKO, I can see the option to use NodeForce that is not what I want.

Also, I tried to use "Ground Motion" command of OpenSees. But, it did not output the acceleration time series at the model bottom same as the input motion that I entered.

The attached describe what I am trying to simulate in OpenSees. They contain the geometry of my model, STKO input file, and FLAC dynamic manual for your reference.

Thanks in advance!
Youngkyu

[STKO Team]

Before solving the problem with the ground-motion, may I ask you what you would like to achieve with the equalDOF in X along the vertical sides? It is not clear to me.
If you are trying to set the free-field columns this is not the correct way.
What you are imposing now is that all nodes of the vertical sides should move of the same amount in X.

[youngkyu]

Regarding the equal DOF at both lateral boundaries,
I would like to make the slope model existing in a rigid box which does not need the free-field columns.

In other words, the two lateral sides are assumed to move along with the prescribed motion specified at the model bottom.

Thanks.
Youngkyu

[STKO Team]

Ok so in this case you don't need EqualDOF at all.
You can simply fix in X all the nodes you want to apply the ground motion to.
It didn't work before because the nodes should be fixed (it's a prescribed motion)

Here's a working version of your model:

NoBox_PIMY.zip

(186.25 KiB) Downloaded 305 times

Also I think you have a mistake (which I did not correct), please check it:
You input the gravity acceleration instead of the body force. This is fine for U-P elements, because they internally multiply it by the material mass density. But the standard quad element you are using requires the REAL body force: G*RHO

[youngkyu]

Thanks for your edit and checking my mistake.
Your edit is what I wanted to achieve.
You are right that I should put the body force for that element to give the gravitational loading.

Now, I can see how the prescribed motion works in OpenSees.

Best,
Youngkyu

[STKO Team]

You're welcome