## eigen value

02-Getting started with the modelling
karabi
Posts: 3
Joined: 21 Jul 2020, 12:46

### eigen value

I am modeling multibay 3D (G+3)building frames. In my study, I am working
on 3D buildings with different bays such as 3x5, 4x5, 4x7, etc. These are
all symmetric plans and each bay length is 4m. I have considered rigid
diaphragm because my buildings are of RC. So I have considered the most
probable center node(though exact center node not defined) as the master
node and the remaining nodes as slave nodes. I have run the eigenvalue
analysis to check if my modeling is correct. For 3x5 and 4x7 I do get nodal
displacement in the y-direction and zero nodal displacement in x for mode 1
and vice versa in mode 2. For 4x5 frame I get nodal displacement in the
y-direction and zero nodal displacement in x for mode 1 however, for mode 2
I get nodal displacement in x-direction as well as y. As far as I know,
I should not get such an eigenvalue solution for 4x5 frame. It's a
symmetric plan. So the first two modes should not have a rotation in Rz.
huffte
Posts: 892
Joined: 22 Jul 2011, 10:19
Location: Cookeville, Tennessee, USA
Contact:

### Re: eigen value

If the exact center of mass has, indeed, not been defined, then you likely have eccentric mass distributions at one or more levels, which would indeed produce multi-directional displacements, even if the stiffness is symmetric. Both mass and stiffness must be perfectly symmetric in order to expect no orthogonal effects.
Last edited by huffte on 31 Aug 2020, 16:01, edited 1 time in total.
Tim Huff
karabi
Posts: 3
Joined: 21 Jul 2020, 12:46

### Re: eigen value

Hello Huffte,

Thank you so much for your reply. Can you please instruct me to define the exact center in Seismostruct if there is no beam-column joint(node) that lies exactly at the center of the plan? how to define such a node? because taking any other node near the center as the master node results in these types of multidirectional nodal displacements.

with best regards,
Karabi
huffte
Posts: 892
Joined: 22 Jul 2011, 10:19
Location: Cookeville, Tennessee, USA
Contact:

### Re: eigen value

You can place a node in the exact center and connect it to the corners with rigid links. Depending on your intended model, you may have to use rigid links to more than the corners.
Tim Huff
karabi
Posts: 3
Joined: 21 Jul 2020, 12:46

### Re: eigen value

Hello Tim Huff,

I am so thankful to you. I tried a few combinations with a node in the exact center and connected it to the corners with rigid links. Finally I got the desired results of nodal displacements. Thanks once again.

with best regards
Karabi