STRUCTURE MODES

[George Ntounis]

Hello.
Am doing a structural analysis of a 4 story building with basement.
The modal analysis results are kind of weird. I get the first two modes for UX with almost similar factors and the third is for UY.
Which of two is the fundamental period for UX. Obviously thw third is for UY.
Isn't these results strange?
Any comments or suggestions?
Thanks in advance.
Am uploading the modal analysis results and a 3D Model image.
[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1708367555/tips/MODEL_tyl5aq.pdf[/url]

 

[chris3eb]

The first 3 modes are typically some combination of UX, UY and RZ (torsion). It looks like your first two modes both have a fairly significant UX and RZ components which is not uncommon when UX and RZ are coupled (ie when x-direction movement causes torsion).

I'm not really understanding the 3d model - is it a frame building with a core wall in just the lowest level?

 

[George Ntounis]

Thank you so much.
It's the basement floor which is part of the upper construction area. I use area elements to model the walls.
There is an elevator core which it's modeled with equal frames connected with rigid frames in each floor.