topic Re: Correct modal analysis in Robot Structural Analysis Forum

Correct modal analysis

vise92 — Tue, 15 Dec 2020 12:51:27 GMT

Hello everyone! How do I know if the results I have found for the modal analysis (frequency and period) are correct? The structure is a polygonal pole 12 meters high with a 3.5 meter high flagpole on top (see attached image). For the modal analysis I made these choices (see attached images) and on top of the polygonal pole I added an additional mass of 4000 N (condition number 4). But now I don't know if the frequency and period of the structure are right. How can I do? Thanks a lot to everyone!

Re: Correct modal analysis

vise92 — Tue, 15 Dec 2020 12:53:44 GMT

Re: Correct modal analysis

DennisVDijk — Tue, 15 Dec 2020 13:16:20 GMT

Hello,

[1] You should add more nodes along the length of the mast and flag pole. use "division" into part, take 15 for the mast and 8 for the pole.

[2] modal settings:

Re: Correct modal analysis

vise92 — Tue, 15 Dec 2020 14:04:58 GMT

Thanks for the reply. Just insert nodes or do I have to divide the segment into several segments? Thanks a lot!

Re: Correct modal analysis

DennisVDijk — Tue, 15 Dec 2020 14:44:15 GMT

Select a bar and use "division"

Re: Correct modal analysis

vise92 — Tue, 15 Dec 2020 14:51:22 GMT

Ok! Thanks a lot! I had done so! I came up with these results and comparing them with another program are very different: in Robot I have the frequency of 2.97 Hz, in ProSap I have 1.96 Hz. Same loads and same settings ... Arrived at this point, how do I understand who is giving me the right value? To me honestly 2.97 Hz seems a bit high, doesn't it?

Re: Correct modal analysis

DennisVDijk — Tue, 15 Dec 2020 15:35:40 GMT

your 1st mode shape must be looking like this:

the frequency can be rigth, it depents on the stiffness of the mast and pole.

If you are not sure you can check it by hand calc.

I have attached my model.