Modal Participation factor and SRSS

03-Analytical/modelling capabilities
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mindlessflip
Posts: 8
Joined: 27 Apr 2021, 10:51

Modal Participation factor and SRSS

Post by mindlessflip »

Hi dear all,
I have a question about using the modal participation factors and mode shape values. As I want to use the method of SRSS to combine the response spectra for the first two Mode shapes the formula gives me:

=SQRT(SUM(r_i*phi_i*S_d,a,i)^2)

- From an Eigenvalue Analysis I got for my 2 dimensional RC-frame with one bay (4m, storey height 3m) the fallowing Participation Factors:

r_1=1,71 and r_2=0,51

-From the deformed shape Viewer I read out the relative displacements of the two storeys which seem very small with roof displacement of 0,0007 and first storey displacement of 0,0004.

I am a bit confused now what value to use for my different phi_i, since in the help is written that phi_n is equal to 1?

Thank you very much and would be pleased for a quick response!
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seismosoft
Posts: 961
Joined: 06 Jul 2007, 04:55

Re: Modal Participation factor and SRSS

Post by seismosoft »

We are not sure what you are trying to calculate.
The modal shapes can be normalised to any value, since what is really of interest is the relative (rather than the absolute) value between the nodal deformations.
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mindlessflip
Posts: 8
Joined: 27 Apr 2021, 10:51

Re: Modal Participation factor and SRSS

Post by mindlessflip »

Hi @seismosoft,

First of all, thank you very much for your reply. I solved the problem by myself. I originally wanted to calculate the participation function which is defined by:

phi_i x r_1

I used the participation factor given from SeismoStruct and multiplied it with the deformed shape value, also given by SeismoStruct. I just had to correct the deformed shape value from 0,0007 to 0,7. So the equation is: 0,7 x 1,71 = 1,197. Although I have not found the reason why the deformed shape value given by SeismoStruct is 10^-3 smaller than it should be.

Greetings
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seismosoft
Posts: 961
Joined: 06 Jul 2007, 04:55

Re: Modal Participation factor and SRSS

Post by seismosoft »

Since the mode shapes can be normalised in different ways, the absolute magnitude of the modal participation factor and the modal shapes have in effect no meaning, and only its relative magnitude with respect to the other participating modes is of significance.
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