Lateral building modes of vibration in SAP2000

[KZSteel]

Hi all,

I've modeled a fairly complicated building in SAP2000. The diaphragms are not continuous throughout the lateral system, and individual plates are instead connected through flexible bridges that transmit lateral forces to the floor plates that are connected to cores. I'd like to get the first three modes of total building lateral vibration, so I turned to my SAP2000 model to do this. I set the mass source to the element masses, and created a modal load case. Running the model, instead of getting total building modes of lateral vibration, I get endless modes of vibration of individual floor spans vibrating vertically. How do I convince SAP2000 to give me the lateral modes I'm looking for?

Thanks for any guidance!

 

[sbouvia]

I think you can go in and change the vertical stiffness of what I am assuming you are using is shell elements to be very stiff in the vertical direction. What it appears that you are more concerned with is the horizontal diaphragm stiffness so changing the vertical stiffness should help out. If the vertical stiffness of the shells is correct and you still want to stop the vertical modes of vibration you might have to add the intermediate supports if you have note already done so.

Hope this helps.

 

[DCBII]

I ran into this same problem doing a large building in RISA. I had so many degrees of freedom in the model that it was picking up isolated modes that didn't have any real bearing on the overall behavior of the structure.

I determined the best way to get it to work was to make simplifying assumptions that reduced the number of degrees of freedom in the model. Look at the isolated/localized mode shapes and decide how best to get rid of them or stiffen them. RISA has rigid diaphragm options built into the program that eliminate the need for plates when modeling rigid diaphrams. This cuts a lot of degrees of freedom out of the model. Does SAP2000 have anything like that?

Plate models are tough get meaningful mode shapes out of, because there are so many degrees of freedom (plates bring a lot of nodes with them).

 

[JoshPlumSE]

You have two options:
1) Adjust the stiffness of your structure so that these modes do not occur.
2) Adjust the mass so that these modes do not occur.

Option #1 would be to use (like sbouvia suggested)a plate element with a higher flexural stiffness which will tend to suppress (or at least stiffen up) these vertical modes.

Option #2 would be to re-model you mass in a way that avoids these vertical modes. I call this "discretizing" the mass. By that I mean you should lump the mass at "discrete" locations. Do so in a way that reduces vertical modes, but still captures your lateral modes. I usually tell people to lump the mass at the main beam - column intersections. That will pull the mass away from the mid-span of the plates and will eliminate those vertical modes.

I suppose you also have a 3rd option. You could solve for Ritz Vectors rather than the true modes. Ritz vectors can be biased to avoid modes with low mass participation.