Pure Torsional Mode as 1st Mode in Tall Building

[Howard Roark]

Hi there,

I am currently working on a 20+ story building (For educational purpose only) having dual resisting system (Moment resisting frames and concrete shear walls located in center). I intend to perform Non-Linear Static Pushover Analysis to capture non-linear response of the structure and to study Demand/Capacity ratios for in-elastic response of structure. However, when I performed Response Spectrum analysis, I got 1st mode of vibration as "Torsional mode", which doesn't make sense to me. Story-drifts are restricted as per code's limitations. Also Torsional irregularity checks are also okay (Considering eccentricity between center of mass and center of rigidity). Base shear for both, Equivalent Lateral Force Procedure and Response spectrum analysis also matches (approximately equal). 

Since, Pushover analysis is applicable to structures whose response is dominated by its first mode of vibration (Which isn't the case here), although my first mode appears to be in rotation, my gut feeling says its translational. I am attaching Modal participating ratios, any sort of help is appreciated.

 

 

Edited June 21, 2020 by Howard Roark

[Waqar Saleem]

Hi Howard,

Go through this video lecture but you will need somebody who knows Urdu.

 

[Badar (BAZ)]

It would have been better had you had shared you structural system as well. If you do not want the torsion as first mode, then adjust it so that center of mass (CM) and center of rigidity CR are closer.

Time period of first mode looks abnormal to me. It is way more than the time period of second mode.

[Howard Roark]

Since rectangular shape is most susceptible to torsional stresses, for given plan of building, 1st mode as torsional mode now makes sense to me. To counter this, I added shear walls at extreme end of shorter side of building, and my fundamental mode switched from torsional to translational mode (as expected). But later on, had to shift the the position of shear walls in Y-direction (Lx= X axis, Ly= Y-axis), due to architectural reasons and that's when torsional irregularity kicked back in. If there is any other better alternative, kindly do suggest. I am all ears. 

Eccentricity between centre of mass and centre of rigidity is well within limits of ASCE-07.  

 

 

 

 

Edited June 22, 2020 by Howard Roark

[UmarMakhzumi]

On 6/23/2020 at 2:11 AM, Howard Roark said:

counter this, I added shear walls at extreme end of shorter side of building,

This is the solution. Try to convince your architect. You never want your first mode to be in torsion even if it is within the allowable limit. 

[Howard Roark]

23 hours ago, UmarMakhzumi said:

This is the solution. Try to convince your architect. You never want your first mode to be in torsion even if it is within the allowable limit.

This is not possible in this case. I can't provide shear walls along extreme short sides. Previously, I wanted to perform Pushover analysis but that's not an option anymore. Since Pushover analysis takes into account only the fundamental mode, assuming it to be predominant response and does not consider higher modes, my analysis based upon Pushover won't provide a reliable estimate of deformation demands. (It is recommended to have modal mass participation around 70% for 1st mode) to effectively estimate deformation demands. 

Now, I am thinking of performing Time history analysis. It will be great if you can recommend me some valuable research papers, books or lecture series on this topic. 

 

Edited June 28, 2020 by Howard Roark

[UmarMakhzumi]

I don’t have experience with time histories for seismic. I have a done a few for rotating equipment so I won’t be able to recommend you anything.
 

Thanks.