Cannot show pore pressure response

[yuweihwang]

Hello STKO,

Sorry to bother you,
I recently found that the STKO post-processor cannot show the pore pressure response when I use 20-8 BrickUP element.
Is there any way to fix this issue?

Many thanks

[STKO Team]

Can you send us a file (the SCD preprocessor file) so that we can test it?

[yuweihwang]

Hello STKO,

Here is the file. Many thanks for your help.

[STKO Team]

I can see the water pore pressure.
If you refer to the face that you see it only on the corner (8) nodes of the quadratic 20 node element, it's because only the corner nodes have pressure DOFs.

This comes from the theory of mixed finite element. Pressure should be interpolated with 1 order less than the displacement to satisfy the inf-sup (Ladyzhenskaya–Babuška–Brezzi). This can be done in quadratic elements using pressure DOFs only at the corner nodes.
In low order elements, you don't see this because they cannot use a lower interpolation for pressure. In those cases they use a different approach.

[BuuNguyen]

Hello Yuweihwang, and STKO team
Firstly, I am sorry Yuweihwang for downloading your file without permission.
I have looked at your model, you simulated the same procedure as an example in Opensees wiki (for boundary conditions)
Have you tried to use ASDE absorbingboundary condition Auto which is available in newest version of STKO?
I am thinking that the procedure to simulate dynamic analysis is:
I/ Static analysis
1. Add constraint of boundary condition for static analysis (fix all DOFs: translation and rotation)+ add constraint for PWP
2. Update stage 0
3. Run elastic gravity analysis
4. Update stage 1
5. Run plastic gravity analysis
II/ Update parameter for dynamic analysis
1. Update permeability
2. Remove BC constraints
3. Activate ASDE absorbing BC
III/ Dynamic analysis
1. Run dynamic analysis

In your opinion, is it correct?
If I have super-structure. Can you suggest the procedure, STKO team?

Thank and best regard.
Happy new year, and best wishes for you in the next year.

[STKO Team]

the procedure seems correct.
If the super-structure is a frame structure you can have a look at our latest webinar

[BuuNguyen]

Yes, I have a look at the 25th webinar. It use quad element, not consider pore water pressure.
The problem is that if I use UP element, the pore pressure at the Absorbing boundary is set to be 0. it not reasonable, right?
How can I set the alalysis procedure to consider pore water pressure and absorbing BC?
Thank

[STKO Team]

The absorbing elements do not deal with pore water pressure. Why would you need it?
Just place the correct pressure constraints you want at the soil boundaries (nodes between soil and absorbing)

[BuuNguyen]

As in attached figure 1,
the depth of my model is 30m, water level at 2m from the ground surface.
The pore pressures of all element close to the boundary are 0.
The problem is The pore presure should be increase with depth, but it reaches 0 at the bottom.

Now I checked my model, then it is reasonable if neglecting the additional layer which is added for Absorbing boundary condition.
If I hide the BC layers, its fine (as in Figure 2)
Thank for your support,
Buu

[STKO Team]

YEs this is exactly what I meant.
It's not a problem if you see a pressure = 0 at the bottom layer of absorbing boundaries (those nodes are not real soil nodes).
The important thing is that what happens inside the SOIL domain is what you expect!
good job