I am curious about a couple of things on the way STAAD handles response spectrum analysis, and below are my questions. I would be very glad if you can provide me some answers . I'm using build 20.07.05.15, by the way.
Due to the client's own design manual, I had to use the NEHRP provisions to come up with a response spectrum which is to be used in seismic analysis of a bridge. Just as a side note, the functions used in NEHRP's response spectrum are slightly different than AASHTO LRFD Specs. Since STAAD.Pro does not have NEHRP among the built-in codes, I entered the spectrum by hand using the editor (in time vs. spectral acceleration format).
1. Once I entered the spectrum using the editor, I switched to the GUI to check the shape of the curve by using the Edit option. Here, I was surprised to see that the numbers listed in the edit dialog are somewhat different than what I initially entered through the editor. Below are two screenshots comparing the numbers in the editor and in the GUI. No scale factor is applied, everything is in terms of "g" for now:
Why would the spectral acceleration values be different ? Is there some kind of conversion going on behind ? Most important, which set of values would STAAD use in calculations ?
2. This one is about damping. The spectral acceleration values from the maps in AASHTO LRFD (not sure about direct values from USGS though) include damping, if I do not misinterpret the code. For example, the title of AASHTO LRFD Figure 18.104.22.168-5 says:
"Horizontal Response Spectral Acceleration Coefficient for Region 1 at Period of 0.2 Seconds with 7 Percent Probability of Exceedance in 75 Years (Approx. 1000-Year Return Period) and 5 Percent Critical Damping."
Again, if I am not wrong, the design response spectrum I construct using the AASHTO LRFD maps would already be 5% damped. So, wouldn't the default 0.05 in the "Damping" box reduce the amplitudes further ? As far as I know, If I enter "0" to the "Damping" box, it would revert back to the default 0.05. Is there any way to tell STAAD not to use damping ? In other words, do I have to make sure that my spectral acceleration values are undamped before entering them ?
3. What does the "Save" checkbox under the "Combination Method" combobox do exactly ? I checked the help file, but could not find any information on this. It looks like the help file for the GUI belongs to a previous version (I assume 2006), since the screenshot in the help file is different than what I see.
4. This is out of curiosity: What I am planning to do for load combinations was plugging-in spectrums seperately into two different primary load cases for X and Z directions. For example the command for X-direction spectrum should look like this where participation factors for Y and Z are 0:
SPECTRUM CQC X "some number depending on the R factor" ACC SCALE 1 DAMP 0.05 LIN
After that, I should be able to create the required number of 100% +- 30% combinations via "Define Combinations".
My real question is, what would be the purpose of declaring X, Y, Z participations at the same time for the same spectrum ? The only thing that came to my mind is that maybe this flexibility would allow application of seismic forces from an arbitrary angle (by using the participation factors as the amplitudes of the unit vectoral components). If there is an other reason to have this option, could you please explain ?
Thanks a lot for your time !