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Hi Admin,

I am attempting to model a monopile embedded in sandy material, subject to displacement-controlled monotonic horizontal loading under drained conditions. At the end of the analysis, I would like to extract the force-displacement curve resulting from the displacement and compare it with results from other FEM software for validation.

Currently, I am modeling the soil using the Drucker-Prager model (nDmaterial DruckerPrager) with SSPbrick elements. For the monopile, I am also using SSPbrick elements to represent its shape and thickness, assuming a modulus of elasticity (E) of 2E12 kPa to treat the pile as fully rigid.

The steps of my analysis include: 1) a gravity stage (purely soil), 2) pile installation, and 3) loading.

However, I am uncertain about how to apply the load at the top of the pile. Below is some screen shot of my model and the pile top for illustration. I have encountered several methods for implementing the displacement, including: 1) applying equal displacement to each node at the top of the pile; 2) selecting one node as the load application point (master node) and constraining the other nodes at the top of the pile to this master node, allowing for equal degrees of freedom in the x-direction; and 3) creating a central node and connecting the inner pile nodes to this new central node using an elastic beam-column with the same material properties as the pile, to which the displacement will be applied.

I have been informed that when using slaves tied to a master node (where the imposed constraint is applied), the reaction forces are recorded at the slave nodes. This could prevent me from accurately reading the reaction force at the node where I applied the displacement. Is that true?

How do I apply displacement (ux) correctly for accurate reaction forces (Fx) readings?

In this case you can just apply a Load->sp, to apply an imposed displacement in the Ux direction for all nodes of the top face.
Use that top face (create a selection set) to record reactions

STKO Team wrote: ↑

Mon Mar 17, 2025 8:50 am

In this case you can just apply a Load->sp, to apply an imposed displacement in the Ux direction for all nodes of the top face.
Use that top face (create a selection set) to record reactions

Hi there,

Thank you for your reply. I am currently using OpenSees for post-processing. I wanted to clarify your comments: to calculate the total horizontal force required to achieve the "x" displacement (sp - applied at all the nodes on the top face), do I need to sum the total reaction forces (Fx) for each node at the top? Additionally, does this mean I can ignore any reaction forces at the nodes in other parts of the model that result from this pile movement/analysis?

Also, to double check, this is my command: recorder Node -file C8_DP_E2e12_t1/S_reactionforces.out -precision 12 -time -node 1 2 3..... -dof 1 2 3 reaction, this would mean I'm extracting nodal reaction forces in the x y z direction.

Or is there a better way I can record the total horizontal force required to achieve the displacement?

Thank you for your help! I look forward to your reply!

Sincerely,
Ryan Chia

Hiya,

I was wondering if there is any follow-up regarding this query.

Thanks in advance!

To calculate the total horizontal force required to achieve the "x" displacement (sp - applied at all the nodes on the top face), do I need to sum the total reaction forces (Fx) for each node at the top? Additionally, does this mean I can ignore any reaction forces at the nodes in other parts of the model that result from this pile movement/analysis?

Sum the reactions for all nodes involved in the SP constraints. they represent the force required to apply that displacement

Also, to double check, this is my command: recorder Node -file C8_DP_E2e12_t1/S_reactionforces.out -precision 12 -time -node 1 2 3..... -dof 1 2 3 reaction, this would mean I'm extracting nodal reaction forces in the x y z direction.

yes