Natural Frequency 1

[yarzharlt054]

Hello,

I would like to know how to calculate the natural frequency of a building using SAP2000.
I know how it works on 2 -D frames but not sure about a 3D structure. Would the natural frequency be different in each direction?

Thanks !

 

[structSU10]

If you have a rigid diaphragm, you have mass at each floor lumped at the master node for the floor. Then run an eigenvalue analysis to determine its frequencies. They will be, more than likely, different in each direction.

 

[yarzharlt054]

Can I just mode a 3d structure and run the eigen analysis? Or I have to analyze each direction separately? The tutorial of the Sap 2000 not working well on 3d structure.

 

[structSU10]

I have only minor experience with SAP2000, so I am not sure on the exact way to model this, but the mass needs to be properly defined in each direction, as well as the mass MOI about the rigid diaphragm about the vertical axis. I am unsure if you can have the program calculate that for you. One model, with proper mass distribution, will certainly give you the answer in each direction.

 

[dcarr82775]

You have it almost by default. Build your model, and put in your dead loads. 'MODAL' is a default load case SAP runs unless you tell it otherwise. It automagically does 3D. After running the model you can pull up the natural frequencies in the tables, but I like to look at the mode shapes (using the deflected shape viewed set to the MODAL load case) to see which ones are the ones I care about.

If you have floors/levels you will need to tie all the beams into a diaphragm or you will get 1001 mode shapes of individual beams which is probably not what you are looking for.