Analysis using ASDAbsorbingBoundary3D

[kolozvari]

Hello, this is a follow-up question regarding the SSI analysis of a tall RC core wall building. I have reached the point where I am able to run the structural model with nonlinear walls and coupling beams and a complete soil domain. I am comparing results from the SSI analysis with fixed-base model results and they don't make sense. I spent a lot of time trying to figure out what is going on, but the SSI model results seem pretty insensitive to various parameters.

The first figure compares the maximum drift profile during a single MCE-level ground motion between the SSI (solid blue line) and fixed-base (solid red line) models. As you can see, building in the SSI model experiences about 1/2 of the intersotry drift compared to the fixed base model, which is too much difference. In an attempt to understand what was going on, I made the soil super stiff (properties of concrete), adjusted the absorbing boundary parameters correspondingly, and removed the contact modeling by creating super stiff zero-length elements between the basement walls and the soil (results are shown with dotted line). I did this because I expected to get results of the SSI model with super stiff soil similar to the fixed-base model. However, as you can see (dotted blue line), drifts are very similar (actually a bit smaller) than in the original SSI model, which is the part that doesn't make sense to me. I expected that the structure in this model should have similar drifts compared to the fixed-base model.

To further investigate this, the second picture shows the accelerations recorded at various points along the height of the soil domain. As expected, in the original SSI model there is some amplification of the acceleration through the soil ( blue vs magenta line). Also as expected, in the model with super stiff soil all points along the height of the soil domain have very similar acceleration because all points are essentially moving together like a rigid body. I also plotted the maximum absolute accelerations profile (third figure), which also looks strange for the two SSI models.

Questions:
1) Do you have an idea why are the SSI model drifts about 50% of the fixed-base model?
2) How come the structural model is totally insensitive to the stiffness of the soil? As said earlier, it is expected that the structure in the SSI model with super stiff soil has a similar response to the fixed base model since the base is super stiff and the input acceleration is essentially the same as in the fixed-base model. However, the SSI model with super stiff soil and the SSI model with produce very similar drift profiles
3) What other tests can I do to get to the bottom of this issue? Currently, both SSI results do not make sense to me for the reasons mentioned above.

If it would be useful, I can send you any of the models mentioned above.

Looking forward to your response.

Thank you.

[kolozvari]

Hello, here is a follow up information. I have noticed that when I remove the soil on the sides of the basement walls, but keep the soil beneath the building, the drifts in the SSI model increase drastically (see below). I find this interesting, but I am having a hard time understanding why is this. The difference is just too much. I am aware that this is not a reasonable modeling approach, but I was just trying different things. I am curious to hear what you think.

Thank you.

[STKO Team]

If the behavior of the soil influences or not the behavior of the structure may depend on many factors.
Probably in this scenario, there is no significant interaction between soil and structure.
What puzzles me is why the SSI model is giving lower drifts.
When you did the fixed base model what simplification did you do? Did you model the basement walls? if yes, how were they constrained?

[kolozvari]

Thank you for your reply. They are not constrained. The model is only fixed at the base. For that reason I posted another question on a separate thread regarding applying springs at the basement walls of the fixed base model to see if I can get similar results to the SSI model.

Something feels off since the difference is too large between the fixed base and ssi models. I tripple checked everything and structural models are identical and the soil modeling has been also verified. I would really appreciate any further advice.

Thank you.

[STKO Team]

Hello, here is a follow up information. I have noticed that when I remove the soil on the sides of the basement walls, but keep the soil beneath the building, the drifts in the SSI model increase drastically (see below). I find this interesting, but I am having a hard time understanding why is this. The difference is just too much. I am aware that this is not a reasonable modeling approach, but I was just trying different things. I am curious to hear what you think.

What is fine here is that, if you remove the soil around the walls, the results start matching. This makes sense because in the fixed base model you did not put any constraint on the walls.

If the difference between considering/neglecting the constraining effect of the soil on the wall is too much, depends on how flexible was the lower part of the building.

Another idea is that you may be overestimating this constraining behavior if you are considering the soil perfectly attached to the walls. Did you attach them ? or did you use a contact element?

[kolozvari]

I use contact elements, but they do not make a lot of difference. I tried running analysis with and without contact elements. I will try to run fixed-base model with soil springs around basement walls and let's see what happens. Thank you.

[STKO Team]

You're welcome

[kolozvari]

Just to follow up on this issue. I realized that in the case I obtained larger drifts in the structure, the results are actually processed wrong. It was plotting velocities instead of displacement. I am not sure how is this possible since I am using your Python API code that does not even have the option to process velocities (see below). Can you please clarify what is happening here?

1.JPG (75 KiB) Viewed 243 times

Also, I am trying to run the model without absorbing boundaries (AB) on the side - only AB at the base. When I unassign the AB properties from the side faces I receive the following error (see below). I am not sure why is this happening. I re-did this a couple of times. Please clarify.

2.JPG (85.18 KiB) Viewed 243 times

[STKO Team]

Also, I am trying to run the model without absorbing boundaries (AB) on the side - only AB at the base. When I unassign the AB properties from the side faces I receive the following error (see below). I am not sure why is this happening. I re-did this a couple of times. Please clarify.

If you use the Auto version of the ASDAbsorbingBoundary, you must provide all 4 sides + bottom face.

If you want a custom assignment of absorbing boundaries, you should manually extrude the volume in the location of interest (bottom in your case) and assign the non-auto version. Please refer to the webinar about the use of asborbing boundaries to see how to use the manual version.

Just to follow up on this issue. I realized that in the case I obtained larger drifts in the structure, the results are actually processed wrong. It was plotting velocities instead of displacement. I am not sure how is this possible since I am using your Python API code that does not even have the option to process velocities (see below). Can you please clarify what is happening here?

It's strange. Can you share your python script?
What is result_id? If it is the ID of the result in the worktree, then it depends on what is the order of the results that you requested in your MPCO Recorder

[kolozvari]

Thank you for the clarification.

I think I have finally gotten to the bottom of the issue where SSI model reported much smaller drifts in the superstructure. I believe it is related to damping. The way I applied damping is to apply 1% at f1 and 5f1 of the soil to the entire model (see Rayleigh in screenshot 1) and then overwrite the damping that is applied only to the structure (see Rayleigh Superstructure in screenshot 1) by using the region command as shown below (see screenshot 2). The selection set in screenshot 2 corresponds to the superstructure.

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Capture.PNG (173.07 KiB) Viewed 233 times

However, I don't think this is working correctly. It seems that the damping applied to the entire model (meant only for the soil) is not being overwritten by the Rayleigh Superstructure damping. I have proven by removing the Rayleigh Superstructure damping and applying the same level of damping to the entire model. The results make much more sense. In other words, I don't think that applying (overwriting) damping with the region command works correctly the way I assigned it. Can you please advise on how to do this? What does the WriteOnList do?

Please let me know. Thanks