Negative acceleration in FRF Analysis in Robot 2025

[pslambard]

I am running a harmonic analysis for a steel stair structure in Robot 2025, and I'm surprised to see negative acceleration values in the results, as well as shifts between positive/negative accelerations, which makes the FRF graph look odd. I've never had this issue before, and when running the same analysis on the same model in Robot 2024, the acceleration is always positive, so it seems to be specifically a Robot 2025 issue.

Maybe this is not a big issue and the solution is to take the absolute value, but I'm not sure. 

[okapawal]

Hi @pslambard 

FRF results if version 2025 might be incoherent.  You may take absolute values for acceleration but it is better to rely on results from version 2024 or 2026.

We wanted to deliver internal forces for FRF analysis in version 2025, but it is not that simple.  Harmonic analysis with damping is calculated in 360 time phases.  So it is a question what results is to be saved to each frequency step in FRF analysis. If we want to return maximum forces from all the time phases than actually the displacement, velocity and acceleration is returned from a phase which is not relevant to the maximum displacement in the selected node.  As a result you will get envelope of forces, but the displacement is not compatible with the real movement of the node.

We reverted the solution back in 2026 release.  Internal forces are not available for member in cases of FRF analysis.  But the nodal displacement, velocity and acceleration is reliable.