Hallo AhmedR,
thanks for your helpful reply. My problem is now that Etabs takes the 10th mode for y-direction and that results in an enormous base shear which is beyond a designable value! As I have some Null Lines moving in the animated view I guess this could be the reason for the high mass...
I have generated my static base shear in Etabs with "Time period Ct(ft)Programm calculated" and the result are different base shears in x- and y-direction. Befor that I have used "Time period Method A" and the result were same base shear values for both direction. Does anybody know why I get...