Displacements are very tiny compared to acceleration response

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bozturk
Posts: 8
Joined: Mon Jan 09, 2023 1:01 am

Displacements are very tiny compared to acceleration response

Post by bozturk » Wed Aug 30, 2023 4:50 am

Dear STKO Team,

I am currently working with a model-scale pile-soil-bridge model and trying to validate it using shake table experimental data for the same configuration. My dataset comprises accelerometer and displacement sensor readings.

Upon analyzing the acceleration response from my numerical model, I've observed a favorable match with the measured data obtained during the experiment. However, I've encountered an issue with the calculated displacements. These calculated values are consistently several orders of magnitude smaller than the displacements measured during the experimental test, as recorded by laser meters.

In my attempt to address this discrepancy, I've double integrated my acceleration data (while acknowledging the associated error). This process yields significantly larger values than the displacement readings noted in the file.

In my analyses, I have fixed the nodes at the base of the model and applied uniform excitation. My understanding is that in uniform excitation, the prescribed acceleration time history is applied to all nodes, generating "relative" results. As such, I've summed the calculated acceleration values with the seismic motion input.

However, I feel confused when it comes to handling displacement. Given that I'm inputting acceleration and have fixed the base of the model, I would anticipate that these displacement values should be "absolute" due to the nodes at the base are fixed.

When I compare:
- The summation of recorded accelerations with the input motion closely aligns with the input motion itself.
- However, upon inspecting the displacement data, I find that the values are on the order of e-20, essentially negligible. While my initial thought was that this might be due to fixing the base nodes in the loading direction, even when I release them, the resulting displacements at these nodes remain tiny.

I'm reaching out for assistance, as I've been stuck with this issue for a while and am unable to make progress. If you have any insights or suggestions, I would greatly appreciate your input.

For your reference, I have included the Eigenvalue modal analysis report below (from modal analysis procedure that STKO developed). Additionally, I've attached a clean copy of my model (I've made it specific to this topic, though I might have overlooked some parameters to delete—please disregard those). A sketch of my model has also been attached to provide a clearer understanding, in case you find the opportunity to review it.

I'd really appreciate any thought or helps that I could get to solve this issue.

Best Regards,
Burak O.

Sketch:
Screenshot 2023-08-15 at 10.02.17 PM.png
Screenshot 2023-08-15 at 10.02.17 PM.png (222.11 KiB) Viewed 3178 times
Modal Analysis Report:
Screenshot 2023-08-30 at 12.34.22 AM.png
Screenshot 2023-08-30 at 12.34.22 AM.png (146.33 KiB) Viewed 3178 times
File:
Last edited by bozturk on Sun Sep 03, 2023 7:35 am, edited 2 times in total.

STKO Team
Posts: 2306
Joined: Tue Oct 29, 2019 8:45 am

Re: Displacements are very tiny compared to acceleration response

Post by STKO Team » Thu Aug 31, 2023 2:19 pm

There could be many reasons.
But your model has not been done with STKO and we cannot go through 70 thousand lines of code to find the issue. I'm sorry but I think you should create a smaller version of your model so that it's easier for you to debug it

bozturk
Posts: 8
Joined: Mon Jan 09, 2023 1:01 am

Re: Displacements are very tiny compared to acceleration response

Post by bozturk » Sun Sep 03, 2023 7:40 am

STKO Team wrote:
Thu Aug 31, 2023 2:19 pm
There could be many reasons.
But your model has not been done with STKO and we cannot go through 70 thousand lines of code to find the issue. I'm sorry but I think you should create a smaller version of your model so that it's easier for you to debug it
No worries, you are right! I've started from scratch and created a new model file to investigate the issue thoroughly. Once I pinpoint the problem, I'll be sure to provide updates on this topic so that if anyone encounters a similar issue in the future they can reach to this topic through Google search.

STKO Team
Posts: 2306
Joined: Tue Oct 29, 2019 8:45 am

Re: Displacements are very tiny compared to acceleration response

Post by STKO Team » Fri Sep 08, 2023 10:29 am

ok

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