center of rigidity

[rubyk]

Hello.
I'm designing a 20 storey building with an applied rigid diaphragm to each floor. All floors are equal, however the centre of rigidity varies in each floor, when it shold be the same position in each floor. I can't find an explanation for this and i don't know what to do.
Thanks in advance

 

[WillisV]

The center of rigidity concept is slightly different for 3 dimensional buildings, as there is a coupling effect vertically between floors. If you remove and only analyze a single floor, the results should match your hand calculations, but with twenty floors the center of rigidity of each floor is partially dependent on the stiffness of the floors below. See

https://wiki.csiberkeley.com/display/etabs/Center+of+rigidity

 

[sur123654]

I have read in this forum that rigid diaphragm was required in the old version of Etabs. In the version 2006 onwards diaphragm is not required to be rigid. Under the above circumstance I am not able to visualise as to why rigidity has been assigned to the diaphragm by Rubyk. Please advise.