Different static base shear in x- and y-direction 1

[TomKC]

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 differt results for my base shear and which is the correct way of generating the base shear?

 

[NewImhotep]

that is expected, cuz u r using the period from model analysis, and Etabs taking the period which have the maximum mass participation in that direction

for example
1st mode 2.5 sec with UX=70% UY=10%
2nd mode 2.0 sec with Ux=10% UY=12%
3rd mode 1.7 sec with UX=9% UY=60%

the period which is etabs taking to calculate the static base shear in X& Y direction will be:

Tx= 2.5 sec
Ty=1.7 sec

 

[TomKC]

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 activation in the 10th mode. I'm going to check this. Did you have similar effect anytime?

 

[NewImhotep]

Hi TomKC

10th mode is not reasonable to me , usually 1st 3 modes are govern. If there is no problem in the model ,I guess you structure have is separated or flexible or slender. if it is ,so it is better to use response spectrum analysis.

for static analysis you may take one of the 1st 3 modes which has the larger mass participation in each direction and apply it as user defined period in each direction ,or use method A static analysis is approximated method anyway

regards