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Fundamental Mode - Torsion in ETABS

Fundamental Mode - Torsion in ETABS

Fundamental Mode - Torsion in ETABS

To All,

In one of the structure, the fundamental mode is a torsional mode.

When i asked ETABS to perform static method as per UBC code. I assigned X direction as well Y direction. The first three time periods are

1). T1 = 1.963 sec - Rotation
2). T2 = 0.85 sec - X translation
3). T3 = 0.65 sec - Y translation

ETABS picked up the corresponding time period T2 & T3 in the calculation of base shears. It avoids fundamental time period T1. Is this correct?

Your input will be appreciated.

Murali G

RE: Fundamental Mode - Torsion in ETABS

ETABS is correct in that it is finding the modes for each earthquake direction which have the largest modal participation factors in that direction.

RE: Fundamental Mode - Torsion in ETABS

Hi Willis V

You are correct. But the modes are really coupled with respect to both the translations & rotation.

Say, T1 = 1.963 (33% - UX, 29% - UY & 17% - RZ)
T2 = 0.896 sec (1% - Ux, 10% - Uy & 30% - Rz)

Certainly we need to go for response spectrum method to capture the realistic distribution of force.

This question comes exactly in the scaling the foces with respect to static base shears. i.e scale factor.

Which time period would be realistic to calculate the static base shears?

Hope you got my point

Murali G

RE: Fundamental Mode - Torsion in ETABS


For the static method, the code is assuming a force distribution based on a perfect translational mode shape.  Technically you can just use the approximate period equation in the code and use that to calculate the base shear, even if you do have the actual modal response of the building.  I would feel fine with using the period values ETABS gives as having the most translation modal participation in that particular direction and ignoring coupling effects.  Obviously the code approximate method ignores them.  

RE: Fundamental Mode - Torsion in ETABS

Hi Willis

You are correct.

In this case, as per ETABS, the demand would be 35% more than the base shear calculated from approximate period (coupled mode).

My opinion is that the structure would behave as a coupled mode before it reaches a perfect translational mode.

ETABS automatic calculation of time period & base shear (as per static method) needs some engineer judgement when it comes to irregular building.

Murali G

RE: Fundamental Mode - Torsion in ETABS

i have a question what coupled mode mean?

RE: Fundamental Mode - Torsion in ETABS

A mode shape involving both twisting/torsion and direct lateral translation.  

RE: Fundamental Mode - Torsion in ETABS

More specifically and simply: you apply a force or excitation in the X direction, and you get resulting forces in the X, Y and Torsional directions -> you have coupled modes.

Exciting a simple building in 1 direction will result in reactions in that direction.

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