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## Unable to get a correct displacement time graph of one point on my model

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Hello Dear researchers,

I am running a time history analysis of a 3D steel moment frame built on a soft soil. I have input a ground motion at the bed rock. The analysis ran successfully and when it comes to plotting the displacement time graph of a random node on the structure, I use the 1D plot group then the point graph. But my big problem is that, the displacement has shifted away from it equilibrium axis(x= 0) which is not normal according to me. I am wondering if anyone else have experienced this problem before or is there any researcher who can assist me with some suggestions.

2 Replies Last Post Mar 24, 2023, 6:48 a.m. EDT

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Posted: 1 year ago

This type of problem is not uncommon. The root cause is often that the input signal does not have zero mean (in several numerical senses). What type of load are you using? (Displacement, velocity, acceleration...)?

The typical case here is that a measured velocity is used. If you integrate that signal over time, physics tells us that that the integral should be zero (unless you have an event like the recent earthquake in Türkiye, having remanent displacements).

To get a zero mean, it may be necessary to (somewhat ad hoc) add a bias to the input signal.

However, there can also be some drift induced by the time stepping itself. Try tightening the tolerance, and see if that makes any difference.

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Henrik Sönnerlind
COMSOL

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Posted: 1 year ago
Updated: 1 year ago

This type of problem is not uncommon. The root cause is often that the input signal does not have zero mean (in several numerical senses). What type of load are you using? (Displacement, velocity, acceleration...)?

The typical case here is that a measured velocity is used. If you integrate that signal over time, physics tells us that that the integral should be zero (unless you have an event like the recent earthquake in Türkiye, having remanent displacements).

To get a zero mean, it may be necessary to (somewhat ad hoc) add a bias to the input signal.

However, there can also be some drift induced by the time stepping itself. Try tightening the tolerance, and see if that makes any difference.

Henrik Sönnerlind, thank you very much for your feedback and sorry about the late reply. I am using the acceleration at the bedrock level. Yes i have tried to reduce the tolerance but the problem is still there. Please excuse me for adding a question to an unsolved problem. in fact since i am running a nonlinear problem, i have considered the contact feature between raft foundation and soil interface , also material plasticity in solide mechanics module. Due to these considerations, the system has automatically included the geometric nonlinearity which is ok! but my problem is that when i run the analysis, there is no convergence , i mean the analysis stucks up.I do not know whether i am missing any bounary condition definition somewhere else. Please i need your advice. LEKEUFACK BECO

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