topic Re: mass eccentricity not taken into account in Robot Structural Analysis Forum

mass eccentricity not taken into account

adrian_mihali — Sat, 19 Apr 2025 11:07:52 GMT

Hello!

I have a problem in RSA 2024: the mass eccentricity is not taken into account in the analysis.

I tried defining the modal analysis and the seismic analysis manually and also using the Modal with automatic definition of seismic cases option. RSA correctly generated the seismic cases with the eccentricity in the directions I selected but when comparing the deformation diagrams for members, there is no difference between X- and X+ or between Y+ and Y-.

When starting a new project, on the first run, the mass eccentricity is taken into account but after multiple runs and changes in the model, it doesn't do that anymore and I can't figure out what is the problem.

I attached the model, maybe someone can help me figure it out.

Thank you!

Re: mass eccentricity not taken into account

Simau — Sat, 19 Apr 2025 15:54:47 GMT

Hi @adrian_mihali

1) You don't need to create a modal analysis for each seismic excentricity as the loads converted to mass are the same.

2) Did you tick this box

You may then compare displacements in X+ and X- with signed quadratique combinations

Example attached

Re: mass eccentricity not taken into account

adrian_mihali — Sat, 19 Apr 2025 18:46:11 GMT

Thank you for your answer. I will use your solution after I familiarize myself with the concept behind it. I didn't know what the Signed tick box means.

I managed to obtain what I was looking for (the influence of the mass eccentricity) by copying everything in a new model and runing the analysis. It looks like it's a bug, maybe..

Re: mass eccentricity not taken into account

Simau — Sun, 20 Apr 2025 20:13:18 GMT

Hi @adrian_mihali

In a symetrical structure there no effect in deplacement in case of + - X execentricity if you look at Seismic X, look Insteade at Seismic Y Direction.

The same for deplacement in case of + - Y excentricity if you look at seismic Y