## How to get Eigen Values with fully loaded frame???

Forum for OpenSees users to post questions, comments, etc. on the use of the OpenSees interpreter, OpenSees.exe

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ankurjain
Posts: 55
Joined: Sun Aug 16, 2020 10:08 pm

### How to get Eigen Values with fully loaded frame???

Sir, I have prepared a model and applied various uniform loads over it. But When I analyse it for Modal Analysis, I am not getting correct values of Eigen. When I researched a bit, I found that the loadings are not getting considered in Modal analysis.

Do i need to convert my loadings to Nodal masses first ?

If yes than for a big structure how it is possible to convert all loads to nodal masses to get Eigen values.

Kindly help me out sir.

selimgunay
Posts: 879
Joined: Mon Sep 09, 2013 8:50 pm
Location: University of California, Berkeley

### Re: How to get Eigen Values with fully loaded frame???

In OpenSees, weights and masses are considered separately. Having weights does not mean that you have the corresponding masses, you need to input the masses yourself.

ankurjain
Posts: 55
Joined: Sun Aug 16, 2020 10:08 pm

### Re: How to get Eigen Values with fully loaded frame???

selimgunay wrote:
Mon Oct 19, 2020 7:34 am
In OpenSees, weights and masses are considered separately. Having weights does not mean that you have the corresponding masses, you need to input the masses yourself.
But sir for a very big 3D model, how it is possible that i convert the UDL loads to Nodal masses and then apply them on the nodes ?
Will it be practical to do so ?

Also, So many researcher have analysed big big models in Opensees as i have seen in various papers...How did they find the Eigen Values in Opensees ??

Is there any way that I can just assign loads to various elements and calculate their Eigen values ?

selimgunay
Posts: 879
Joined: Mon Sep 09, 2013 8:50 pm
Location: University of California, Berkeley

### Re: How to get Eigen Values with fully loaded frame???

You need to define masses with the mass command: https://opensees.berkeley.edu/wiki/inde ... ss_Command or distributed mass on the elements, example: https://opensees.berkeley.edu/wiki/inde ... mn_Element. If you don't have these mass commands, your structure would not have any mass and the mass matrix is zero.

ankurjain
Posts: 55
Joined: Sun Aug 16, 2020 10:08 pm

### Re: How to get Eigen Values with fully loaded frame???

selimgunay wrote:
Mon Oct 19, 2020 12:32 pm
You need to define masses with the mass command: https://opensees.berkeley.edu/wiki/inde ... ss_Command or distributed mass on the elements, example: https://opensees.berkeley.edu/wiki/inde ... mn_Element. If you don't have these mass commands, your structure would not have any mass and the mass matrix is zero.

For Time History Analysis also OpenSees won't consider any loadings given on the elements. Right ???
Because I have been trying to do Time History Analysis , but i found no change in the Acceleration response of a particular node whether i remove load or add any amount of load. Though if i change nodal mass, then the acceleration response do change .

ankurjain
Posts: 55
Joined: Sun Aug 16, 2020 10:08 pm

### Re: How to get Eigen Values with fully loaded frame???

selimgunay wrote:
Mon Oct 19, 2020 12:32 pm
You need to define masses with the mass command: https://opensees.berkeley.edu/wiki/inde ... ss_Command or distributed mass on the elements, example: https://opensees.berkeley.edu/wiki/inde ... mn_Element. If you don't have these mass commands, your structure would not have any mass and the mass matrix is zero.
For Time History Analysis also OpenSees won't consider any loadings given on the elements. Right ???
Because I have been trying to do Time History Analysis , but i found no change in the Acceleration response of a particular node whether i remove load or add any amount of load. Though if i change nodal mass, then the acceleration response do change .

selimgunay
Posts: 879
Joined: Mon Sep 09, 2013 8:50 pm
Location: University of California, Berkeley

### Re: How to get Eigen Values with fully loaded frame???

That is correct. Mass and gravity loading are two separate things in OpenSees