No dominant mode in dynamics 1

[goodbusiness]

Hello, I'm hoping to get some help with a particular problem for some models I've encountered when using dynamic (seismic) analysis. The problem is when have I have no single mode with a particularly large mass participation in the x or z directions. I know that generally, you want to acheive a mode with 70%+ mass participation in-order to add dominant mode signage to your RSA results. I'm currently testing to see if restraining low participation local modes will help, but in general:
* when you have no single mode with a high mass participation, is it still reasonable to add dominant mode signage?
* what are some general ideas or adjustments I can make to designs to achieve less evenly distributed mass participation results?

Thanks,

 

[JoshPlumSE]

* when you have no single mode with a high mass participation, is it still reasonable to add dominant mode signage?

This is a matter of some engineering judgment. If I've got a mode with 60% mass participation and no others that are above maybe 20% then I think of this as a fairly "dominant" mode. Though I have to remember that if I use this flag, then I may see some odd deflected shapes or such.

* what are some general ideas or adjustments I can make to designs to achieve less evenly distributed mass participation results?

This is a concept that I used to talk about in the RISA-3D training course on dynamics. I called it "discretizing" the mass. Which means rather than relying on self weight for the mass definition for a Response Spectra analysis, I would recommend lumping the mass at locations that still capture the spatial distribution of mass, but which tend to limit local modes.

Attached is a (somewhat sloppy) writeup that I created for that class that talks about how to discretize the mass of a structure to reduce local modes. This was done maybe 10 years ago (probably longer). So, it will be a little dated if you're looking at the RISA interface. But, the general concepts should remain the same.

https://files.engineering.com/getfi...3-b7fb-24edd21f3608&file=Discretized_Mass.pdf

 

[goodbusiness]

@JoshPluSE this is invaluable information for me, thank you! I will have to play around with this technique and understand it more before implementing it, but this concept of lumping mass in additional BLCs is certainly not something I would have been able to discover on my own.