perform a spectrum analysis with spectral values on 3 directions
perform a spectrum analysis with spectral values on 3 directions
(OP)
Hi everyone,
i'm trying to perform a spectrum analysis with ansys apdl 13; i've seen several tutorials about performing that analysis with a single response spectrum, but i have to combine spectrum values on three directions (x,y,z); in this special case, does anyone knows if the combination of modal shapes has to be done on post-processor fase or i can simply define the three spectrum and combine everything with a single SRSS solution command?
thank you all,
regards
i'm trying to perform a spectrum analysis with ansys apdl 13; i've seen several tutorials about performing that analysis with a single response spectrum, but i have to combine spectrum values on three directions (x,y,z); in this special case, does anyone knows if the combination of modal shapes has to be done on post-processor fase or i can simply define the three spectrum and combine everything with a single SRSS solution command?
thank you all,
regards






RE: perform a spectrum analysis with spectral values on 3 directions
The 1997 UBC and older codes allowed an SRSS combination of the results. The newer ASCE code, for some reason, insist that you use a 100%+30% combination method for the two horizontal directions. I've seen this extended out to 100%+30%+30% for spectra applied in all three directions. However, I believe the nuclear codes have now moved towards a 100%+40%+40% combination method.... though I am not positive about that because it has been a long time since I worked on a nucler job.
RE: perform a spectrum analysis with spectral values on 3 directions
i think i will use the SRSS combination; however, i don't understand how i can do this procedure in ansys: i found some tutorial about spectrum analysis, but i can't find anything about combination of results for each sysmic input. Any ideas?
RE: perform a spectrum analysis with spectral values on 3 directions
If they have any post-processor type Macros that you can build then you could run three separate analysis and build some Macros to export the results to Excel for combination... or something along those lines.
RE: perform a spectrum analysis with spectral values on 3 directions
RE: perform a spectrum analysis with spectral values on 3 directions
We analyzed the structure for acc. in one direction, reanalyzed for missing mass correction, and wrote a post processor to combine the two results to get forces in one direction. This was repeated for other two directions. After getting the forces in three directions, they were combined using SRSS 100 - 40 - 40 method. The seismic response was then combined with gravity to get the final design forces.
We also wrote a post-processor in 'C' language for design of concrete elements per ACI - 349. We had to get the approval for the post-processor from the review board.
With SRSS the signs are lost. So all the six forces either could be positive or negative for load combination purposes.
RE: perform a spectrum analysis with spectral values on 3 directions
I assume that you can get plotted results in Ansys for the response spectrum analysis in one direction, right? What do you think of the results that you're plotting?
The reason why I ask is because response spectrum results involve a statistical combination (SRSS or similar) of individual modal results. While the results for each mode make sense. You get positive and negative deflections and forces, you look the positive / negative sign convention when you do a statistical combination of the individual modes.... So, each numerical value is a valid maximum design value. However, you don't know whether the sign is positive or negative. That can cause problems for plotted contours and such. So, you frequently have to turn to the spreadsheet results anyway. At least for the final data.
Just something extra for you to think about....
RE: perform a spectrum analysis with spectral values on 3 directions
RE: perform a spectrum analysis with spectral values on 3 directions
RE: perform a spectrum analysis with spectral values on 3 directions
Combination of modal participations is done using SRSS/ CQC/ 10% or other methods.
Combination of multi-direction responses is done using 100-30-30 or 100-40-40 methods.
Best to be clear about the difference between modal combination and directional combination.