Signing CQC modal cases

[Santiago91]

Hello, up untill recently I used SAP2000 and I always let it do its CQC for dynamic analysis.
Recently I have been using ROBOT; which asks for the principal X and Y directions to recover the lost signs due to the cuadratic combination (CQC)

To where it got me thinking... How does SAP2000 does this if there is no option for the user to input the main X and Y directions?
Do SAP2000 has an algorythm to detect it?
I hope it doesn't automatically assume it's Mode 1 and Mode 2. (This auestion came up to me as I'm working in a very asymetrical building where 1st mode is torsion)

I would appreciate if someone could answer this? or ask CSI technical support (I can't since they only support paying users and I'm not using SAP2000 at the moment/anymore?)

Thanks in advance!

 

[Rahul Leslie]

Presume ROBOT is trying to do Dominant Mode Signage, where, after doing the SRSS/CQC method, gives for each quantity (BM, SF, etc.) the sign obtained for that quantity at that location for a single mode analysis -- the analysis for the Dominant Mode.

Packages with this feature (not SAP2000) usually askes you to suggest two modes as the dominant ones, one for each direction. Maybe ROBOT is more automated.

Rahul Leslie

 

[Santiago91]

Hello
Rahul Leslie, thank you for oyur response.
in ROBOT I can manually select the dominant mode.

My question is what is SAP2000 doing? There is no option to manually select the dominant mode nither to automatically select the dominant mode. I can't find any information oh how is SAP2000 working. does anyone know?

 

[mronlinetutor]

The SRSS and CQC take the square root of riri and rirj respectively. And rirj should be the absolute value. So the sign direction of the maximum response does not matters.

disclaimer: all calculations and comments must be checked by senior engineers before they are taken to be acceptable.

 

[Rahul Leslie]

<< My question is what is SAP2000 doing? >>

SAP2000 doesn't do this, because it hasn't included the 'Dominant mode signage' (DMS) option in it: Probably because Prof. Edward Wilson -- the developer of the original 1973 SAP-IV, and still an associate of the SAP2000 research team -- hasn't yet approved of it.

Please note that this DMS method isn't mentioned in any code of practices, as far as I know. I guess some technical paper came up with this idea, probably as a solution to the issue of signage loss in column design forces; and some of the packages embraced it offered as one of its options. I'm personally in favour of the option.

The paper found at...

https://communities.bentley.com/cfs...00-28-12-25/Technical-Paper-_2D00_-Sanjib.pdf

...gives a nice demonstration of the DMS methodology in STAAD.Pro (though there is a slight mess-up in the paper, which I can discuss, if anyone is interested)

Leslie