Frequency Response analysis with external excitation force not working

[structuralmechanic]

Hello there,

I have 2 major issues while running frequency response analysis on a simple structure (A steel portal frame). Firstly, when I try to run an analysis with just a external exiting force, the set up automatically adds "Base" (All fixed nodes in the model) as the excitation nodes. Secondly it does not recognize the effect of external force applied on to the structure. Say I apply a external force with 100 lb magnitude and make it act at the natural frequencies of the structure, it does not yield any result. I am assuming that this is a bug in the analysis. I am attaching all the necessary images with this post. Using 2018 Simulation mechanical.

[John_Holtz]

Hi @structuralmechanic 

I am not able to reproduce this. What happens if you delete row 1 of the table to delete the base acceleration?

If you are still having problems, please create an archive of the model ("app button > Archive > Create") and attach the .ach file to the forum. (I think the forum will accept an .ach file. If not, just rename the file and change the extension from .ach to .zip.)

[structuralmechanic]

Hello John,

I think I found out the problem. The analysis by default adds a Node set 1 and calls it Base, this will have all the fixed nodes in it. If I now add more node sets say Node Set 2,3,4, and come back and delete Node Set 1 as I dont want to apply base excitation in this particular problem, then the analysis automatically adds the Node set 1 back into the analysis.

But, If I first delete the Node Set 1 that is there by default and then add my Node Set 2,3,4 but call them Node Set 1,2,3 instead than the deleted "Base" node set will not re appear.

Let me know if you can reproduce the same.

Thank you for your quick response.

Karthik