Global torsional instability issue in the building

[abbaskhan2294]

The RCC structure below was checked in etabs for UBC-97 stability checks. It passed all the design and stability checks except torsional stability check i-e the ratio of the average displacements of the extreme corners were really high, the highest being 2.4. What can be the possible solution to overcome this issue. The structure consists of basement and G+3 stories.

[UmarMakhzumi]

Abbas,

I would suggest the following:

1) Add additional shear walls as marked.

2) Add spandrels 

Thanks.

[abbaskhan2294]

19 hours ago, UmarMakhzumi said:

Abbas,

I would suggest the following:

1) Add additional shear walls as marked.

2) Add spandrels 

Thanks.

Thanks umer for your suggestion. But let me share what I did to overcome this problem. 

I figured that the topmost grid in x axis is too much stiff as compared to the bottom grid in x direction with shear walls. These shear walls were not in architectural planning but I had to add them for structure stability. Also I was not allowed to make the bottom grid concrete throughout again due to architectural issues. So I thought to decrease the stiffness of the topmost grid in x axis by reducing the length of the shear wall and luckily it worked to overcome the torsional instability in seismic zone 3.

Below are the images of the remedial methods adopted.

[UmarMakhzumi]

There is another reason for which I was suggesting additional walls. You have a single moment frame in transverse direction for each bay. That is not a good practise as system has no redundancy. I would suggest that you reconsider your framing or design it using over-strength factors as system is not redundant. 

Thanks.

[abbaskhan2294]

3 hours ago, UmarMakhzumi said:

There is another reason for which I was suggesting additional walls. You have a single moment frame in transverse direction for each bay. That is not a good practise as system has no redundancy. I would suggest that you reconsider your framing or design it using over-strength factors as system is not redundant. 

Thanks.

Yes that was a major issue and i did consider over-strength factors for the design of the structural members. 

[Hamza Ahmad]

Sir, i have a question ? is shear wall the solution for controlling torsional Irregularity ? or Can it be controlled by adding additional column grid in X or Y direction ? 

[abbaskhan2294]

On 7/17/2019 at 4:54 PM, Hamza Ahmad said:

Sir, i have a question ? is shear wall the solution for controlling torsional Irregularity ? or Can it be controlled by adding additional column grid in X or Y direction ? 

In some cases "yes" and in some cases "no". Torsional irregularity is due to more stiffness at one corner as compared to the other corner. Illustration is given in the first figure where grid#9 had a full panel of shear wall where as in grid # 5 the length of the shear wall was restricted due to architectural issues so as grid#9 was more stiff as compared to grid#5 , the structure gave severe torsional irregularity.

The solution I came upto was to either increase the stiffness of walls along grid#5 (which was possible to achieve by either increasing length of the panel or increasing its thickness or increasing the strength of concrete) or by decreasing stiffness of wall along grid #9 which was a more feasible option in my case. you can see the second figure i posted to control the torsional irregularity.

Edited July 18, 2019 by abbaskhan2294