modal and seismic analysis

[mecheil2000]

as i know in etabs for spectral analysis for seismic which is based on modal analysis there are two factors
1- Mass participation
2- scale factor

if mass participation > 90% the analysis is acceptable

How we can control these two factors in robot to avoid getting wrong results from modal analysis ( lateral drift of tower = zero)

[Tomasz.Fudala]

You can check a mass participation in the Dynamic Analysis Results table.
The menu option: Results->Advanced->Modal Analysis
If 90% is not reached then you need to increase the number of modes in modal analysis parameters and run analysis again.
And continuing this procedure until appropriate number of modes is found.
It can be also done automatically when using Seismic mode or Seismic (Pseudo mode) mode in Modal Analysis Parameters and specifying Participation masses as active – in such case Robot will automatically increase the number of modes until the demanded participation percentage is reached.

[mecheil2000]

You can check a mass participation in the Dynamic Analysis Results table.
The menu option: Results->Advanced->Modal Analysis
If 90% is not reached then you need to increase the number of modes in modal analysis parameters and run analysis again.
And continuing this procedure until appropriate number of modes is found.
It can be also done automatically when using Seismic mode or Seismic (Pseudo mode) mode in Modal Analysis Parameters and specifying Participation masses as active – in such case Robot will automatically increase the number of modes until the demanded participation percentage is reached.

really you are a great person, thanks for your effective help

I have one more question
for modal analysis there are 3 analysis options
option1 :modal
option2:seismic
option3:seismic (pseudo)

can you explain to me which type i have to use and what is the difference between each one

[Tomasz.Fudala]

When this first option is selected then The modal analysis of the structure will be conducted and n vibration modes will be found.

When you select Seismic - Only modes will be set, which are important to seismic analyses, therefore those which are characterized by a large mass participation factor.

And when you select seismic pseudo modal - then Spectral and seismic analysis will be conducted using the pseudo modal method. This should be used only when applying classic analysis methods based on modal decomposition results in a very long structure analysis time.

[mecheil2000]

When this first option is selected then The modal analysis of the structure will be conducted and n vibration modes will be found.

When you select Seismic - Only modes will be set, which are important to seismic analyses, therefore those which are characterized by a large mass participation factor.

And when you select seismic pseudo modal - then Spectral and seismic analysis will be conducted using the pseudo modal method. This should be used only when applying classic analysis methods based on modal decomposition results in a very long structure analysis time.

thanks tomasz for this valuable information
but can you show me by steps what I have to do to solve a concrete tower
because what I exactly did is I solved a tower with option 1: modal analysis
so I got zero drift because mass participation very less
the time taken by robot it was about 2 hours and the number of equations 200 000
so what I did I delete the result and solve model as a second trail but I used option2 for modal anlysis (seismic) I see the program try to solve by using ritz vector and prepared 200 modes of solution and the program need 23 hours to finish the solution.
can you advice me
Thanks tomasz so much
Next subject , can you help me to get only reaction of tower and export to a new model of robot to solve only raft foundation

many thanks for your good self
Mecheil

[diming]

mecheil before doing a modal analysis you have to transform some loads (permanent DL and a part of live LL) to dynamic mass.

[Structural Shaft]

Do any of you know if there is a manual for doing modal/seismic analysis in RSA?