SAP2000 CQC Mode Combination

[esap2014]

Hi all,

So, I have a complicated building structure that I've modeled in Sap2000 v15. I'm trying to determine the shear forces acting at each concrete wall in my model. I’m checking my model by comparing the sum of the reactions at the base joints to the base shear output as given in the table. When I run the static analysis this works; the sum of the reactions at the base joints matches the base shear perfectly.

However, when I run the dynamic analysis (which I am required to do per code), the sum of the reactions at the base joints is about twice as much as the base shear output. I'm assuming this is because the modal/dynamic analysis shows the worst case forces, so my "envelope" reactions are much greater than my base shear. Is there a way to determine the actual forces acting at each wall? Are there specific modal or dynamic analysis settings that I can change so that the sum of the base reactions approximately matches the base shear?

Per CSI Tech Support:
"If you were to calculate the base reaction from the results, you will need to multiply the modal reaction values from the "Base Reactions" table, by the response spectrum modal amplitude values in the local U1direction "U1Amp" shown in the "Response Spectrum Modal Information" table. After combine the calculated response spectrum reactions you will obtain the same result as shown on the "Base Reactions" table."

I'm not quite following this, but it seems to imply that I need to recombine my modes by hand using the CQC method, and then combine the forces using SRSS. Does anyone have a practical method for CQC combination of the all of the modes?

 

[Guest262653]

You need to use the section cut tool at the base of each shear wall. The force integration is done properly in this way. Play with the definitions using quadrilateral or group selection to get a good grip of the program features. You'll notice that you need to be very careful with the force output location.
If you try to integrate the forces at each joint manually, as CSI support already told you, you'll be done by next year only...

 

[esap2014]

Thanks for your help. After more discussions with the people at CSI america, I've started using section cuts.