Torsional Irregualrity in Building 3

[meistersd]

Hello!

I analyzed a fairly regular and symmetric building in ETABS and I was checking the results of Modal Analysis as per IS 1893 (Part 1): 2016. Code says that "the fundamental lateral natural periods of the building in the two principal plan directions should be away from each other by at least 10 percent of the larger value". My modal participating mass ratios are coming out to be okay but this required difference between the natural time periods is not being fulfilled. Is this a cause for concern? What is the physical significance of having this difference between the fundamental lateral natural periods?

 

[HTURKAK]

Will you post the excerpt showing the subject statement ?
I suspect this could be a requirement for vertical irregularity , limitation for closely spaced modes..

 

[meistersd]

I have uploaded a screenshot combining both the Codal Clause and the results in my ETABS Model!! The difference between the time periods of the first two modes is less than 10% of the higher value of time period in my model, which seems contrary to the codal requirement!

 

[HTURKAK]

This is due to vertical (mass ) irregularity . It is not allowed for ZONE 4 and ZONE 5..

Will you post the deflected shape for modes 1 to 4 ?..

 

[meistersd]

Here it is! Deflected Shapes for first 4 modes!!

 

[HTURKAK]

So the first mode on X direction, 2 nd Y direction, 3rd is torsional mode. 4th mode again X direction.

The 1st mode 0.639 sec. the nearest mode on X direction is 4th mode 0.204 sec. so , there is no problem..

Assume , if the structure fully symmetric ( the structural elements also ) the periods on X and Y directions will be similar. With your assumption, fully symmetric structures Should not be constructed ....

 

[Blackstar123]

Assume , if the structure fully symmetric ( the structural elements also ) the periods on X and Y directions will be similar. With your assumption, fully symmetric structures Should not be constructed ....

A ground motion of certain duration might or might not contain the frequency equals to (or even closer to) the natural frequency of the building, but it's a good design practice not to have fundamental periods too close to each other in principal directions, in order to avoid merging responses from two peak modes.

Following is an excerpt from a good book by an Indian author which might shed some light on the proposed value of 10%.

 

[HTURKAK]

I downloaded the subject code and looked to that requirement which is mandatory requirement for Seismic Zone 4 and 5. If so, the OP has some alternatives , probably the simplest , the use of rectangular columns in one of the principal direction.

I just want to remind that, there is no such requirement at international codes ( at least i did not hear ), and more over, the symmetrical plans , symmetrical structural systems encouraged.

If the EQ records looked in detail in NS and EW , one can see that there is a primary vibration direction .

The following excerpt from The Seismic Design Handbook (by Bruce A. Bolt )

6.2.4 The Optimum Seismic Configuration

• Low height-to base ratio
-Minimizes tendency to overturn
• Equal floor heights
-Equalizes column/wall stiffness
• Symmetrical plan shape
-Reduces torsion
• Identical resistance on both axes
-Balanced resistance in all directions
• Uniform section and elevations
-Eliminates stress concentrations
• Maximum torsional resistance
-Seismic resisting elements at perimeter
• Short spans
-Low unit stress in members
• Redundancy
-Toleration of failure of some members
• Direct load paths, no cantilevers
-No stress concentrations


I would like to see opinions of other engineers on this subject ..

 

[hardbutmild]

This might be related to the way response is combined, i.e. if SRSS method is used. CQC method should definitely be used here.
Blackstars comments is very interesting and I like it. It however usually means that strengths are not the same in both directions of the structure because strength and stiffness are closely related to one another. I'm not entirely sure what this would imply, but it seems to me that in that case (providing different strengths in two main directions) you could simply reduce the required ductility in the direction where higher capacity is achieved (of course, keeping at least some of it).
Anyway, I think structures can be designed both ways, but CQC method should be used if modes are closely spaced.