ETABS Dynamic Analysis Mode 1 translation, Mode 2 Rotation, Mode 3 Translation

[wisnu666]

Hei guys

I have a bit problem here. I analyzed 8 stories building (include basement and ground water tank). But i have a problem in mode shape when i analyzed in dynamic. The right result should be mode 1 and mode 2 are translation. But i am facing a problem where mode 1 translation and mode 2 rotation and mode 3 translation. Any one can help me to figure this out? Maybe a sugestion in paramaeter or design section.

I analyzed with etabs 9.7.2

Really glad and looking for your help guys

 

[wisnu666]

the 3 first mode result

Mode/ Period/ UX/ UY/ UZ
1/ 1.525007 / 56.4795/ 0.0007/ 0
2/ 1.361121 / 0.4548/ 0.5129/ 0
3/ 1.31204/ 0.0014/ 57.1468/ 0

 

[Roy88]

This is to do with poor structural framing? Do you have an eccentric core? Try to look at the difference between the centre of stiffness and centre of mass. If the eccentricity is huge, then it is not surprising the you have torsional as your dominant mode.

 

[saadpervez1]

The mode with the longest period will be listed first by ETABS, if your structure is flexible in torsion, that mode will be listed first. As Roy88 said, check your core.