displacement control and load control

[Lei Lei]

Thanks!

Here, I tried to use displacement control instead of load control for the same scenario to find the difference between them.

However, the displacement control model didn't converge. So I want to know why and how to deal with it.

Appreciate for your reply!

[STKO Team]

What's your error?
In my case it is running, but :
1) I would control the DOF 2 (vertical Uy) not DOF 1(horizontal Ux).
2) I would make the path time series negative, otherwise you are enforcing un uplift

[Lei Lei]

Thanks for your reply!

I modified the displacement control of the sliding support, and modified the limitation of the Ux to the limitation of Uy. It can compute completely.

I want to apply a downward displacement to the loading points, so I define the Uy to be negative in load pattern. Do I need to do any other settings (like the path time series). But The results look a little off.

I set the output as follows:
------------------------------------------------------------------------
recorder Element -file $filename/Mid_span-Moment.txt -time -ele 20 localForce;
recorder Node -file $filename/Mid_span-Disp.txt -time -node 24 -dof 2 disp;

recorder Node -file $filename/Load_point-Disp.txt -time -node 4 -dof 2 disp;
recorder Node -file $filename/Support-Force.txt -time -node 5 -dof 2 reaction;
------------------------------------------------------------------------

Take node 4 as an example：

The first column of data - time step - is all negative instead of positive, and the second column of data - node displacement - is all positive instead of negative. The result was the opposite of what I expected. At the same time, the first column of time steps is not monotonically increasing. So I want to know what exactly is exported under displacement control.

By the way, what's the difference between the 'numIncr' and the 'Target Displacment History'? Is 'numIncr' the number of analyzing steps per cycle or the number of analyzing steps for the whole process?

Looking forward for your reply!

[STKO Team]

I want to apply a downward displacement to the loading points, so I define the Uy to be negative in load pattern. Do I need to do any other settings (like the path time series). But The results look a little off.

Since you are using a Displacement History (in the displacement control analysis) via a Path time series, the values of the path time series should be negative. As it is not, your arch is uplifting! Change the values in the time series to negative

the first column of data - time step - is all negative instead of positive

In a displacement control analysis the time is the lambda (load multiplier). Since you defined a downward load, but an upward displacement, the load multiplier must be positive to make the load positive to match the positive Uy displacement. Please, change the sign of your imposed displacements in the path time series

By the way, what's the difference between the 'numIncr' and the 'Target Displacment History'? Is 'numIncr' the number of analyzing steps per cycle or the number of analyzing steps for the whole process?

The numIncr is the number of increments to reach the final displacement.
When you use a Cyclic Displacement History (via a Path time series), STKO first computes the total Length of your displacement path. Then it used numIncr increments to walk through that displacement path.
However, since you have a very dense timeSeries (> 250k points), STKO will make at least 1 increment for each point.
I would suggest you to downsample that time series

[Lei Lei]

Thanks for your reply!

In a displacement control analysis the time is the lambda (load multiplier). Since you defined a downward load, but an upward displacement, the load multiplier must be positive to make the load positive to match the positive Uy displacement. Please, change the sign of your imposed displacements in the path time series

Do you mean the load multiplier is negative?
As I understand from your response, the real displacement loaded at the defined point is computed by the time serious factor multiply load factor multiply values in the time series and the target history is computed by the time serious factor multiply time values in the time series, is it?

And how about the load control?

Looking forward to your reply!

[STKO Team]

Do you mean the load multiplier is negative?
As I understand from your response, the real displacement loaded at the defined point is computed by the time serious factor multiply load factor multiply values in the time series and the target history is computed by the time serious factor multiply time values in the time series, is it?

What I mean is:
In a Displacement control, you define 1) a Reference Load (call it LRef) and a Target Displacement (call it U).
Now, the analyis will compute the lambda (load multiplier) such that lambda*LRef leads to the Target displacement U.

So if for U to be positive you need a positive value lambda*Lref, if LRef is positive lambda will be positive as well, otherwise it will be negative

[Lei Lei]

Thanks for your reply!

For the same case, two control methods, force control, and displacement control are used to simulate. In the same situation, the calculation result of displacement control obviously fluctuates with the calculation step change.

Are there any reference conditions for selecting the number of steps of displacement control analysis?

I am looking forward to your reply!
Best wishes!

[STKO Team]

I think you are using a too large tolerance.
Also, the min max material produces a sudden drop and can introduce convergence issues.
Try with an hysteretic material, you can define a softening branch to make it more stable