RE; Force displacement behaviour and extraction

[ryancwl95]

Hi,

I am modelling a monopile using a combination of beam and continuum elements, embedded in sand, which I have modelled using SanisandMS. The top of the pile is subjected to a monotonic push, and I would like to extract the force required to move the pile as well as its displacement at ground level.

I have attempted to plot the force-displacement curve, but the results did not match my expectations. The results from OpenSees, compared with another finite element software for verification, showed some plastic behaviour. However, after a ratio of \( u_x/D = 0.3 \), the results indicated linear elastic behaviour, which seems odd. After running the analysis in a different software, I found that the results from OpenSees were quite different. Below is the graph showing the results:

Screenshot 2025-11-23 000055.png (21.29 KiB) Viewed 141 times

To extract the reaction force (in Opensees), I retrieved results from the "Reaction Force" in the OpenSees results tab at the node where I applied the specified displacement. I then plotted this against the displacement of the pile node located at ground level.

Below is the model schematics:

Screenshot 2025-11-23 000734.png (138.75 KiB) Viewed 141 times

Could you please advise me on the issue I'm facing? Could it be an error related to the mixture of solid and beam elements I'm using? or maybe my extraction method is wrong? Let me know if you need more details!

On a separate note, I also tested this with a single brick element in a triaxial compression scenario. The force-displacement results are consistent with those from another finite element software. The method for extracting force (reaction force) and displacement is the same as in the monopile example. For this singular element, OpenSees displays results indicating plasticity, and the force-displacement curve reaches a plateau after a certain displacement.

Thanks for your help, and look forward to your reply.

regards
Ryan