I have been trying to check if higher mode effect is significant in the building as required in ASCE 41 in order to know whether NSP is valid or I need to accompany the analysis with Linear Dynamic Procedure. The issue I encountered is that I need to run a Modal Analysis first to capture at least 90% of the mass participation when I set the number of modes to 30 using Lanczos method the program cut off the number of modes to just 11 which will only give me 79% mass participation in the Y direction. I a missing something how to set the number of modes here?
Second question, why is the torsional mode (Rz) is not reflected in the output as this is critical if the building has torsional irregularities?
https://www.dropbox.com/scl/fi/tslgugzy ... wbamn&dl=0
https://www.dropbox.com/scl/fi/hpo9ooms ... c5ars&dl=0
Higher Mode Effect
Re: Higher Mode Effect
A follow up question which is related to the higher mode effect, ASCE 41 states that higher mode effect is significant and should not be ignored if the story shear of the first modal response spectrum at 90% mass participation exceeds by 130% than the story shear of the MRSA considering the first mode only analysis. The NSP must be supplemented with LDP analysis as required by ASCE 41.
The question is how do we check the story shear force from the modal response spectrum analysis? I cannot find any output of story shear from the modal response spectrum analysis.
I was looking for a result similar to the attached photo.
https://www.dropbox.com/scl/fi/1xg35q2x ... ssjl1&dl=0
The question is how do we check the story shear force from the modal response spectrum analysis? I cannot find any output of story shear from the modal response spectrum analysis.
I was looking for a result similar to the attached photo.
https://www.dropbox.com/scl/fi/1xg35q2x ... ssjl1&dl=0
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seismosoft
- Posts: 1197
- Joined: 06 Jul 2007, 04:55
Re: Higher Mode Effect
1) In order to increase the number of exported eigen-modes with the Lanczos method, you need to increase the number of iterations of the eigen-solver.
2) If you increase the number of modes, you will probably get the rest of the rotational mass. Note that if you have a basement the basement masses are extremely difficult to get activated, due to the lateral supports.
3) Unfortunately, currently there is no output of the storey shear in modal analysis. We kept a note of this request and we will try to include it in a future release of the software.
Seismosoft Support
2) If you increase the number of modes, you will probably get the rest of the rotational mass. Note that if you have a basement the basement masses are extremely difficult to get activated, due to the lateral supports.
3) Unfortunately, currently there is no output of the storey shear in modal analysis. We kept a note of this request and we will try to include it in a future release of the software.
Seismosoft Support