Cannot show pore pressure response

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yuweihwang
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Joined: Wed Apr 29, 2020 4:54 am

Cannot show pore pressure response

Post by yuweihwang » Wed Jan 13, 2021 9:18 pm

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
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Joined: Tue Oct 29, 2019 8:45 am

Re: Cannot show pore pressure response

Post by STKO Team » Thu Jan 14, 2021 9:59 am

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

yuweihwang
Posts: 37
Joined: Wed Apr 29, 2020 4:54 am

Re: Cannot show pore pressure response

Post by yuweihwang » Thu Jan 14, 2021 8:09 pm

Hello STKO,

Here is the file. Many thanks for your help.
Attachments
TestEPWP.rar
(7.01 MiB) Downloaded 67 times

STKO Team
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Re: Cannot show pore pressure response

Post by STKO Team » Fri Jan 15, 2021 10:33 am

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
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Re: Cannot show pore pressure response

Post by BuuNguyen » Mon Dec 27, 2021 7:05 am

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
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Joined: Tue Oct 29, 2019 8:45 am

Re: Cannot show pore pressure response

Post by STKO Team » Tue Dec 28, 2021 2:26 pm

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

BuuNguyen
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Location: Paraguay
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Re: Cannot show pore pressure response

Post by BuuNguyen » Mon Jan 03, 2022 8:46 am

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
Posts: 1346
Joined: Tue Oct 29, 2019 8:45 am

Re: Cannot show pore pressure response

Post by STKO Team » Mon Jan 03, 2022 12:04 pm

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
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Joined: Fri Oct 29, 2021 3:28 am
Location: Paraguay
Contact:

Re: Cannot show pore pressure response

Post by BuuNguyen » Thu Jan 06, 2022 9:38 am

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
Attachments
Figure 1.PNG
Figure 1.PNG (442.83 KiB) Viewed 843 times
Figure 2.PNG
Figure 2.PNG (343.54 KiB) Viewed 844 times

STKO Team
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Joined: Tue Oct 29, 2019 8:45 am

Re: Cannot show pore pressure response

Post by STKO Team » Thu Jan 13, 2022 6:12 pm

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

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