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First mode torsional

C

Civilboy

Civil/Environmental
Apr 19, 2025
3
In my ETABS model, the first mode appears as a torsional mode. The structure is symmetric in both directions. There are two opposing U-shaped shear walls located at the center of the building, connected by a coupling beam. What could be the reason for the first mode being torsional?
 
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There is no way to know based on the information provided. I like to look at the mode shapes graphically.... It helps me to understand what aspects of the structure are affecting them.

FWIW, it ALWAYS comes down to either mass or stiffness.

By stiffness, I mean that some structural elements may not be connected properly or their stiffness properties are not defined properly. In a way that emphasizes the torsional mode.

By mass, I mean that the loading used for mass could be defined in a way that emphasizes torsion.
 
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