Hello,

I have 15 stories steel moment resisting frame exposed to Dynamic Nonlinear Analysis (FNA) in ETABS.
When I defined Ritz modal, I have problem with the number of Ritz modes that I should define.

Large differences in results have been obtained when I change the number of modes.
In ASCE 41-17, they say that (2.5 times the number of nonlinear degrees of freedom) is a rule for specifying the number of required modal modes as shown in attachment.

If I use this ASCE41-17 rule on a 15 stories steel moment resisting frame, the number of Ritz modes will be very high, and sometimes the response is steady by less number of this and vice versa.

Noted that I reach 90% mass participation ratio in 50 mode and I always capture 100% static load participation ratio, the changes accrued in dynamic load participation ratio
I reach 50% in 400 mode
and 70% in 500 mode
and 90% in 825 mode

Can you kindly explain a rule for choosing a number of required modes rather than trials?

• https://files.engineering.com/getfile.aspx?folder=06123bfd-1766-445e-9eaf-0