convergence problem with UCSD soil models

[hadisayadpour]

I use opensees and UCSD soil models (PressureDependMultiYield) to model a soil medium. My model is 3D and I use 5000 to 20000 bbarBrick elements in my different models. My problem is at gravity analysis. As you know the gravity analysis is composed of two stages that the first one is elastic and the second one is plastic. The elastic one is runnig well with no errors but in the plastic behavior stage the model fails to converge. I have used any kind of test, algorithm and integrator but it did not work. Some times the increments converges but with large number of iterations and the analysis is really time consuming and I don't know that it is true or not .I'm really exhausted and would be thankful if you guide me to make my models converge.
If you have experiences in this field, What is your estimation about the run time of these models?
By the way I should apply some earthquake records on the models. what is your estimation about this type of analysis?

[deepak]

I had the same problem when using PDMY material. One solution to prevent the problem to happen is to update the materialstage to 0 ( during linear elastic). Also update the material shear modulus and bulk modulus to the value you originally defined in you material. So, you are providing the same data twice.

I don't know why it works but it works for me. I hope this works for you as well. Somebody has to fix the problem in the source code in future.


Best,
Deepak

[hadisayadpour]

thanks Deepak.
I fixed the gravity step but now I have difficulties with the transient step of analysis. It does not converge and I'm really tired working on it. Do you have any advice for me?

[deepak]

What type of property you are using?
Can you post your model and material property?

[hadisayadpour]

Hi Deepak,
I use PDMY material and SSPbrick element. My model is really huge and I can not post it.
I have another problem with gravity step. I do gravity analysis in two steps. there is no problem with the elastic stage. When I update the material to consider the plastic behavior of soil, it converges but the settlement of the soil surface will increase with the time and if I increase the time of analysis the settlement will increase as well. The settlement at the end of the elastic stage is rational but after plastic stage the result is completely unacceptable. Do you have any idea about it?
Thanks

[dks]

Hi,i'm facing the same problem now. Did you get any solution to this problem? My problem is also not converging when I'm updating the material parameters after the gravity stage and when i switch the material to plastic i'm obtaining large deformations with each load step. Do you have any suggestion?? It will be really helpful.

Thanks