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Dynamic analysis of structures, Mass participation Regarding

 
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Bindu Sunil
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PostPosted: Tue Apr 06, 2021 9:50 am    Post subject: Dynamic analysis of structures, Mass participation Regarding Reply with quote

Request valuable suggestions from everyone.

We are designing a G+27 storied building with approx plan area 30m x 32m.
It is a Shear wall building, with parking in GF,FF and second floor. Third floor is a podium level. Apartment floors start from 4th floor, walls are rcc walls, floating of these rcc walls are done using transfrer girders at 4th floor.
We have modelled in Etabs.
We are not able to achieve the required stiffness to avoid Soft storey generation and 65% mass participation in first 3 modes, simultaneously in the model.
We have tried some 20 models with slight changes in properties, orientations , diagonal bracings etc, which we can do within the architectural serviceability requirements.
The closest we've achieved is 80% minimum stiffness ratio ( ie, no soft storey generation) plus 55% mass participation in first 3 modes in seismic excitation.
When mass participation increases ( by making the structure more flexible), soft storey is formed with stiffness ratio around 70-75 %


What is the absolute minimum stiffness ratio required to consider no soft storey ?
If at all soft storey is detected, can we design it with additional shear component etc, as was in earlier 1893 code?
Is it absolutely necessary to have 65% mass participation in first 3 modes itself?
With first 5 modes, we are getting  65% and more.

Request the valuable suggestion, opinions from the experienced structural engineers here.
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