Force-Displacement

[mline41]

Dear STKO Team,

I have modelled a soil-pile interaction, and I am trying to get Force-Displacement graphs for my model. Do you have any advice? I have tried to use the monitor function under analysis and use the control node selection set, but I cant seem to extract the data. Please could you have a look, and advise how I can extract a force displacement graph.

Model is attached.

Many thanks in advance.

[STKO Team]

Just move the monitor before the analysis.
Also, in the monitor, it's better to use "Average" (instead of "Add") for the Displacements(or Acceleration or Velocity), because if a node is on more that 1 processor, the monitor will sum all of them, and since they have the same value on each processor you will see a wrong result.
Instead, for results such as Reaction Force it's ok to use Add, because each processor adds its contribution to that node.

[mline41]

STKO Team, thank you for your reply.

I am confused by the graph output, (attached) of reaction force against displacement for the top of the pile.

1. I was expecting a straight line, (increasing reaction force with increasing displacement, as my model currently is purely elastic), why is the graph fluctuating from negative and positive values of reaction.

2. the reaction force values are very small, why is this? My applied face load to the top of the pile is 4 MN.

3. I am trying to produce a force displacement graph for a purely elastic model that shows how the displacement at the top of the pile increases linearly with the force applied, please could you help?

My model is also attached.

Thank you very much for your help in advance.

[STKO Team]

It's because you are monitoring the reactions on the free surface (reactions are where constraints are applied).
Try this:

Force-Displacement Graph Issue 2.zip

(307.63 KiB) Downloaded 127 times

[mline41]

Dear STKO Team,

Thank you very much for your response and the modifications you provided to my model. I appreciate the guidance, but I would like to better understand the reasoning behind each change so I can properly apply these concepts in future analyses.

I have a few specific questions regarding the adjustments made:


Addition of the "reac" Selection Set

- I noticed that a new selection set, "reac", was created, representing the fully fixed bottom face of the soil. However, I do not see it explicitly applied in the monitor settings.
- Could you clarify the purpose of this selection set? Was it intended for reaction force monitoring, or does it serve another function?


Reaction Force Monitoring in the X-Direction

- My applied force is in the Y-direction, so I originally monitored both displacement and reaction force in Y at the top of the pile.
- However, I see that you changed the reaction force measurement to the X-direction.
- Could you explain why monitoring reaction forces in X is more appropriate in this case? Does it relate to pile constraints, unintended lateral forces, or some other numerical effect?



Scale Factor of -1 Applied to Reaction Force

- I noticed that the reaction force in the monitor settings now has a scale factor of -1.
- Was this adjustment made to correct the sign convention of the reaction force?
- Does OpenSees/STKO store reaction forces as negative values by default when resisting motion?


Clarification on Proper Force-Displacement Monitoring

- If I want to generate a purely elastic force-displacement curve showing how the top pile displacement increases linearly with applied force, what would be the best setup? Does the current set up measure the force applied at the top of the pile with the displacement applied at the top of the pile?


Thank you in advance

[STKO Team]

Sorry I attached the wrong file (I didn't save it before sending it)

The reaction for is needed at the bottom because reactions are available at fixed nodes.
The sign -1 is because the reaction force will have the opposite sign to the force applied on top of the pile

[mline41]

Dear STKO Team.

Please see attached powerpoint clearly outlining some questions I have. The Models are also attached.

Thank you in advance

[STKO Team]

In your models you have different results because you are comparing different things

• In one model the soil is elastic, in the other id DP (though this is not the main source of difference).
• In the model where you apply displacements in UY, you are also fixing Uz to 0! this imposes a different displacement pattern to the pile (and a stiffer response... shear beam vs cantilever)

  

  check_disp.png (125.32 KiB) Viewed 2654 times

• Please, always check displacements to understand the behavior of your model! don't look just at Force-Displacement graphs
• In the applied force, when you monitor the reactions, make sure to monitor all the faces where you have a fix in UY.
• Also in the 2 models you are monitoring different points for displacements... of course if you monitor the average of the soil hole, you will see a smaller displacement because, due to contact, the back nodes of the soil will move less (separation from pile, again look at the displacement to understand this point)

Summary

:
The correct model is the one with applied displacement in Uy (but you have to remove the applied displacement Uz=0, otherwise your beam cannot rotate as a cantilever).