Clarification on DEMRCFORC (demrcf) Output vs RCFORC in DEM–Structure Coupling

Hello

I am currently working with DEM–structure interaction models using *DEFINE_DE_TO_SURFACE_COUPLING, and I’ve encountered a persistent and fundamental discrepancy between two output forces:RCFORC and DEMRCFORC. 

In my model:

• DEM particles fall under gravity and settle onto a flat structure surface

• At final steady state, only one single layer of DEM particles remains in contact with the structure

• There are no inter-particle contacts active, and DEM motion has ceased

Under such conditions, I expect the vertical force in demrcf (Z-direction) to match the total weight of DEM particles (since this is the only external vertical load), and to be consistent with rcforc.

However, I consistently observe the following:

• RCFORC (Z-component) ≈ total weight of DEM particles → reasonable

• DEMRCFORC (Z-component) is 2–3 orders of magnitude smaller than RCFORC → unexpected

My key questions are:

1. What exactly does demrcf (DEMRCFORC) represent in terms of contact force physics?

2. Does it include the total contribution from all DEM particles in contact with the structure?

3. Could DEM–DEM interactions indirectly affect the value in demrcf (e.g., via force chains)?

4. Is this output known to be partial (e.g., segment-level only), and not a full sum over all particle contributions?

5. In such scenarios, is rcforc the more reliable quantity to assess the net force on the structure? 

6. Furthermore, in the documentation titled “DEM_method_LS_DYNA_draft_mrj_09_10_12.pdf”, there is an example showing that the RCFORC and DEMRCFORC outputs match closely in a simple case, where DEM particles settle on a surface. However, in my case, despite using similar conditions (gravity loading, full contact, final static state), the two force outputs are very different, particularly in the Z-direction.

Thank you in advance for your time and support!

​The left image is from the documentation, and the right image is from my model.