Accidental torsion in Etabs

[luungoclinh]

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

 

[slickdeals]

ETABS automatically considers accidental torsion in the response spectrum analysis.

https://wiki.csiberkeley.com/display/kb/Accidental+torsion+in+response-spectrum+case

 

[luungoclinh]

please read my question. I did read the article that you refer. It is also a good way to consider accidental torsion, but it only considers for mode 1 (there could be some modification factors to consider higher modes showing by the way we consider the lateral load to get torsional moment).

 

[pkjr618]

I am not familiar with the algorithm of the software for earthquake torsion but one piece of information I can give is that you could look at the moment/torsional arm of the cores if they are symmetrical with respect to the orthogonal axis. Moreover, if these cores have the same mass(for flexible diaphragm) or stiffness(for rigid diaphragm) then it is most likely that they would have the same amount of force distributed to them.

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

 

[luungoclinh]

Thanks pkjr618 for your sharing. Have you ever try using "define response spectrum and inputting eccentricity ratio" to consider accidental torsion effect?. I want to confirm whether it works or not? From my test, I see that if I input (+) eccentricity and (-) eccentricity from the center of mass, I get the same internal forces in my cores.
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

 

[pkjr618]

I have not tried this approach. Can you clarify which article it is you're referring to? It seems I have not mentioned a specific article in my last post.

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

 

[luungoclinh]

thanks pkjr618 so much for your patience. My question is simply to check whether there is a bug in Etabs 3D in considering accidental torsion by going to define/response spectrum case/modify or show response spectrum/ then input ecc ratio.
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