Due to the limitation of mode numbers of staad pro for dynamic analysis, i could not run a large model to reach the required frequency and mass participation factor.
Based on the superposition of modes method: Once a set of modes has been calculated for a system, the response at any frequency (within certain bounds) in response to many inputs at many points with different time histories can be calculated by superimposing the result from each mode. This assumes the system is linear.
therefore, i will run the time history with a serial of frequency range as following:
Run a time history analysis for entire building model.
CUT OFF MODE SHAPE 2800
CUT OFF MODE SHAPE 1000 shift FREQ 19
(19 hz is the last frequency from the first run)
To check if the total mass participation factor reach 95%. Continue run the next frequency range if not.
Results sum all results from first run, second run and .......last run.
please let me know if anyone will see a problem for this method.
STAAD.Pro allows only modal superposition method of TH- it is definitely a linear TH analysis. There is no other option available in STAAD.Pro. If you are facing issues in extracting high number of modes, you should use CUT OFF FREQUENY command.
i can use cut off frequency to calculate the response for different frequency range and then superposition the results, is it correct?
Yes, it should solve the problem
to correct, i used "cut off mode 700 shape shift freq 7.8 " to divide the range for each individual run, and superposition the results from each run.
thank you so much for the confirmation