Hello,
Pls. find attached model and screenshot.I'm just wondering why Mode 2 in Modal analysis is showing to be "twisting",normally the first 2 modes are always deflecting in orthogonal directions. Am I correct on this?
Any ideas will be greatly appreciated.
It seems normal. In that case, modes 2 and 3 are neighbors. To convince yourself of this, you can change the section of bars 3 4 24 25 to BR 300x600 and release ART-ART beams 30A32 35A37. After that you have the first two modes in the main axes.
I've tried this on a different software and my assumption is correct ,the first two modes shapes are acting on the two principal axes,not twisting.
Hi @saclovitzky
How do you interpret that for mode 2 the participating mass is zero, then is there really twisting?
M. Agayr
Did you find this post helpful? Feel free to Like this post.
Did your question get successfully answered? Then click on the ACCEPT SOLUTION button.
As i told you in msg 4, there is not really twisting as the mobilzed mass on mode 2 is zero.
You said : I've tried this on a different software and my assumption is correct
Could you post here frequencies and mobilized mass for mode 1 2 3 you obtain with the other software
M. Agayr
Did you find this post helpful? Feel free to Like this post.
Did your question get successfully answered? Then click on the ACCEPT SOLUTION button.
Yes i tried it
Look at screenshot in msg 4, what i say is that in spite of theorical tsiwting in mode 2, no mass it activated by this mode
M. Agayr
Did you find this post helpful? Feel free to Like this post.
Did your question get successfully answered? Then click on the ACCEPT SOLUTION button.
Mass was already converted in your first model (density was taken into acount automatically).
In the secon model you converted the mass but you must tick neglect density
Yes i saw that mode shap shows a twisting, but with no mass mobilized, no efforts will be created.
May be the support team could give its opinion
M. Agayr
Did you find this post helpful? Feel free to Like this post.
Did your question get successfully answered? Then click on the ACCEPT SOLUTION button.
If you complete your model with sismic cases you'll see below that there is no efforts neither reactions in twisting mode 2.
M. Agayr
Did you find this post helpful? Feel free to Like this post.
Did your question get successfully answered? Then click on the ACCEPT SOLUTION button.
@saclovitzky Please elaborate - why do you think that the first two modes always have to be orthogonal?
The modes of modal analysis depend on stiffness of the structure in different directions. I could easily create a structure that has first 9 modes in the weaker direction (see the picture).
In your case the stiffness difference in different directions is also large (in one number of columns is a lot smaller and the span of the beam is a lot larger than in the orthogonal direction)
Can't find what you're looking for? Ask the community or share your knowledge.