Modal analysis

[skilak]

I am trying to obtain dynamic properties (natural frequencies) of a portal frame 5x5m (pinned bases) using 305x305x137 UC. The modal anaylsis gives me the lowest fn=6.28Hz. Now, I would like to use Elasto-Plastic properties of the sections thus 305x305x137EP UC (elastic-perfectly plastic). The modal analysis gives me the lowes fn=1.87Hz. How is that?

Please could you explain and advise if the above is right?

Regrds,

[Artur.Kosakowski]

The modal analysis takes the stiffness matrix from the static case defined prior to it.

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[skilak]

and where I can compare those matrixes for the two cases? 305UC and 305UC EP? for either prior case I have, both look the same in terms of results e.g. buckling, bending, etc. So for the magniutde of loads in prior cases I cannot fugure out the difference.

Can I use elast-plastic properties for modal or not?

cheers

[Artur.Kosakowski · Accepted solution]

I have created two examples based on your description. The results are similar but you have bear in mind that for EP analysis the shape of I section is simplified.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.