Torsional Mode in Symmetrical Building

[civilita]

Dear all,

I am designing a symmetrical building with core braced frame with SAP 18. The problem is, my building has pure torsion as the first mode.
Mode 1 - T=1.318s UX=0 UY=0 RZ=77.7%
Mode 2 - T=1.168s UX=0 UY=69.3% RZ=0
Mode 3 - T=1.132s UX=70.2% UY=0 RZ=0

1. Is it ok to have modes like this? Is it not permitted?
2. Why does it have torsional as the first mode? As far as I know, the structure which has no eccentricity between the center of mass and the center of gravity is supposed to have translational mode as the first and second mode. What's wrong with my building? is my core frame to stiff or something? Just to be noted, I've remodeled it many times, to avoid any mistakes while drawing or assigning something.
3. If it is not permitted, what should I do?

I really need your help and assistance. Thank you in advance.

 

[JedClampett]

civilita, if you need help on this, you should move this question to the "Structural Engineering Other technical Topics" or "Moment Frame and shear Wall engineering" topics. Those tend to attract the engineers who can answer this.