Accidental torsion in Etabs
Accidental torsion in Etabs
(OP)
Hi all,
I have an asymmetrical building with two cores. I want to consider accidental torsion in my model. I use the option to input eccentricity ratio in defining response spectrum case data in Etabs. I understand that, if I following this, I will consider accidental torsion for different modes and sum them together as the idea of response spectrum analysis.
However, If I input two cases: (+) eccentricity and (-) eccentricity, I mean, for example, I input +0.1 and -0.1 to eccentricity ration, I get the same internal forces for my two cores.
The results seem to be inaccurate since I expect to get different values from two examined cases.
Is my expectation correct?
Thanks
Lisu
I have an asymmetrical building with two cores. I want to consider accidental torsion in my model. I use the option to input eccentricity ratio in defining response spectrum case data in Etabs. I understand that, if I following this, I will consider accidental torsion for different modes and sum them together as the idea of response spectrum analysis.
However, If I input two cases: (+) eccentricity and (-) eccentricity, I mean, for example, I input +0.1 and -0.1 to eccentricity ration, I get the same internal forces for my two cores.
The results seem to be inaccurate since I expect to get different values from two examined cases.
Is my expectation correct?
Thanks
Lisu





RE: Accidental torsion in Etabs
https://wiki.csiberkeley.com/display/kb/Accidental...
RE: Accidental torsion in Etabs
RE: Accidental torsion in Etabs
Additionally, you might want to check if your structure has torsional irregularity for which the accidental torsional might have to magnified. Consult your local code for this.
Yours truly,
pkjr618
RE: Accidental torsion in Etabs
For sure if I follow your referred article's procedure I have different internal forces for my cores when I use (+) eccentricity and (-) eccentricity from the center of mass.
Thanks
Lisu
RE: Accidental torsion in Etabs
In general, you might expect to get the same magnitude of force distributed to the cores if they have the same magnitude of moment/torsional arm with respect to the centre of mass regardless of the direction of eccentricity considering the cores have the same magnitude of stiffness or joint mass and beam stiffness effect if precision is warranted. Only the direction of forces will vary.
Also, the structure that you are modelling has an inherent torsion mode already due to the asymmetry. If this torsion mode is greater than what you are specifying in the accidental torsion then technically this inherent torsion should govern the design. I don't know for sure if ETABS makes this comparison.
Maybe, a rough sketch of the framing together with the cores might help us better understand this situation. I don't think more help can be provided without some more information.
Sincerely,
pkjr618
RE: Accidental torsion in Etabs
In SAP 2000, normally we consider accidental torsion by defining a static load case with torque distributed from base to top, and we can do the same in ETabs, everything is fine, but it takes time...
Thanks
Lisu