Force from reaction generates unwanted high accelerations in elastoplastic models

[sebi_magi]

Dear STKO Team,

I'm trying to implement a model very similar to what you did in the SSI Absorbing boundaries webinar:

https://www.youtube.com/watch?v=OGII3a0 ... o7CFo_yJmP

The differences are that the geometry is in 2D, and I am not using viscous damping on the sides because that does not takes into account the shear modulus degradation (We discussed that in a past topic). So my boundary conditions are Free-Field Periodic.

When I use an Elastic Isotropic Model everything goes well, but problems arise when (i) I use elastoplastic models (i.e, PM4SAND, PDMY02), (ii)remove the fixities and add the force from reaction, and (iii) update the up-element permeabilities to 1e-6 in order to simulate undrained response; when the dynamic analysis starts, a very high acceleration propagates propagates from all the boundaries, taking a lot of time to dissipate and interfering with the earthquake loading. Note that all the tests I did, material stage is elastic, so this is not an issue with the plastic stage. I think the main issue is the force from reaction, although updating the permeabilities and the constitutive models may also contribute, but I'm not sure how.

In order to dissipate the force from reaction accelerations, I tried to run another gravity analysis after removing the fixities and imposing the LK2D condition, but the model fails to converge. I'm not sure why this happens since LK2D adds fixities to the model.

Do you have any ideas how I could solve this problem? Or this boundary conditions are not compatible with non elastic models?


I attach my model. Thank you very much for your time

[STKO Team]

In this model, I don't have convergence problems. Could you share the model that gives you issues?

[sebi_magi]

I did not make myself clear enough. There are no convergence issues with the model I sent you. Please look at the .mpco file recorder_full. The first step of the dynamic analysis (stage 3, step 110) exhibits a very large unwanted acceleration, that contaminates the real dynamic load I imposed, which appears some time steps after.

I want to get rid of those large unwanted accelerations. I don't understand quite well why would they appear (they dont appear using elasticisotropic material).

Kind regards,

Sebastián.

[sebi_magi]

Dear STKO Team

I solved this issue already using much simpler boundary conditions.

I have another question

I am using the RandomizedMaterialWrapper to map soil variabialities on the PDMY02 model and using setParameter command with a randonmaterial table to update permeabilities and match void ratios of the elements with the void ratios of the PDMY02 model defined above.

I was wondering if the syntax of the setParameter random table is the same than the RandomizedMaterial Wrapper table.

I was using something like this. Will it work? Will it work on parallel? The elements are SSPQUADup

Code: Select all

5
void,alpha
5915
1,0.55,1e-08
2,0.65,1e-08
3,0.7,1e-08
4,0.77,1e-08
5,0.85,1e-08
-90.0,0.0,0.0,1
-88.0,0.0,0.0,1
-86.0,0.0,0.0,1
-84.0,0.0,0.0,1
-82.0,0.0,0.0,1

Lastly, do you think it is possible that the Monitor command could also plot the pore water pressure? It would be helpful to watch the changes in velocity/acceleration and the pore-water pressure build up in the frame of soil liquefaction.


Kind regards,

Sebastián

[STKO Team]

I was wondering if the syntax of the setParameter random table is the same than the RandomizedMaterial Wrapper table.

I was using something like this. Will it work? Will it work on parallel? The elements are SSPQUADup

Yes, it uses the same format. The issue is that in OpenSees, the PDMY02 does not allow you to "update" those parameters:

PDMY02_Only_Params.png (73.26 KiB) Viewed 1676 times

That's why we introduced the randomizer wrapper, that changes the parameters directly in STKO.

Lastly, do you think it is possible that the Monitor command could also plot the pore water pressure? It would be helpful to watch the changes in velocity/acceleration and the pore-water pressure build up in the frame of soil liquefaction.

We are currently adding this and other features to the monitor to enrich it