MASS PARTICIPATION

[Er Aayush B]

My mass participation ratio in mode 3 is less than 65%, which corresponds to irregular mode of oscillation.
but when I remove the basement wall which is of 2 floors, the mass participation is more than 65%, why is this happening. I'm using ETABS for 9 storey building analysis.

 

[HTURKAK]

@Er Aayush B (Civil/Environmental),
Not sure if the structure is regular or not. You do not provide a lot info. here. But your statement implies you have modelled the two storey rigid basement with upper flexible portion.
I just want to remind that the rigid basement storeys will mobilize at higher modes .
Alternatively you may perform Two-Stage Analysis Procedure.

Pls look ASCE 7-16

12.2.3.2 Two-Stage Analysis Procedure. A two-stage equivalent lateral force procedure is permitted to be used for structures that have a flexible upper portion above a rigid lower portion, provided that the design of the structure complies with
all of the following....

You may prefer to provide more info. to get better responds.



According to the grace of God which is given
unto me, as a wise masterbuilder, I have laid the foundation, and another buildeth thereon. . . .
I Corinthians 3:10