Pure Torsional Mode as 1st Mode of vibration in tall building 1

[Usman3301]

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.

 

[r13]

I think you need to double check your model, as the torsion is consistent throughout the modes.

 

[Agent666]

Retired13, the first mode is clearly the dominant torsional mode at 44.3% mass participation for rotation, the next highest torsional mode is ~3% mass participation. Hardly consistent through all the modes.

Your structural system is very torsional, a 5 second torsional first mode period is miles ahead of your principle translational modes at 2.4-2.9 seconds.

This suggests you need more stiffness if you want to affect this fundamental behaviour. Stiffening the perimeter moment frames or thickening your core walls.

I'm not aware of anything that states you cannot have a torsional 1st mode. However, what you don't want is a ductile response that is torsional. For example you don't want lateral structure on one side of your building forming some ductile mechanism, the associated loss of stiffness means you can get unconstrained torsional deformation/drifts which is highly undesirable.

The only thing you can practically do is to ensure the orthogonal lateral load resisting system remains elastic providing sufficient resistance to the possibility of these unconstrained drifts or providing multiple lines of resistance so if the outermost lines form a mechanism then the remainder can still remain elastic or offer up stiffness to resist the loss of stiffness in adjacent lines of resistance.

Also with a torsional mode, you pay for it in both orthogonal directions of loading. You will pay for it later on in a real design unless its well thought through at early design stages.

 

[r13]

Your structural system is very torsional

, that's what I meant, look at all modes. And as I suggested, "This suggests you need more stiffness if you want to affect this fundamental behaviour."

 

[r13]

Did you check mass distribution of the model?

 

[Usman3301]

Thanks Agent666 and retired13 for valuable comments.
I did increase the stiffness of perimeter beams which lead to drop in fundamental time period from 5 sec to 4.1 sec. However, for second iteration, increasing shear wall stiffness didn't have any significant effect on torsional rigidity of structure. I have attached plan view of my structure; it's a 20 story building with 3 basements. It's evident that structure shape (rectangular) is playing its role in enhanced torsional effects(among common shapes, rectangle is the shape most suspectible to torsion). Kindly suggest me a solution to overcome this. For now, I believe that by introducing symmetrical shear walls might be a good solution.

(In this image, hatched section except for columns represents shear walls)

 

[Tomfh]

Torsion makes sense for that floor plan. Put some walls near the ends to stop it spinning.

 

[r13]

I will first to find the eccentricities between rigidity center and load center, then make necessary modifications to reduce the incidental torsion. Your option is limited, as you might not able to override the architect's wish. Shear walls do help in beefing up resistance though.

 

[Usman3301]

@retired13
I have already done that. During initial design, these eccentricities(b/w center of mass and center of rigidity) were causing a lot of torsional moments, and I had to adjust for these. I am past this stage and have already made my building compliant to drift ratios, accidental eccentricities and other important checks. Only when I performed dynamic analysis, this appeared which totally makes sense to me now. As @Agent666 and @Tomfh suggested, I consider putting shear walls at the end of building as better way to control this torsional behavior.

 

[r13]

Another thing to keep in mind is, torsional effect is maximum at the middle of the long edge of a rectangle shape, so stiffen the structure elements accordingly will help.

 

[Blackstar123]

Agree with Tomfh, on how to control the torsion mode.
Also on a side note, the minimum number of modes that should be considered, must be greater than or equals to 3 times No. of Stories. Since you've considered less number of modes, sum of mass participation in each principal direction is less than 90%.

 

[Usman3301]

@Blackstar123
You are quite right in your observation. I did notice that before and changed modal analysis from "EIGEN" to RITZ analysis, and this proved to be a game changer. Ritz analysis successfully captures higher modes. I was able to achieve 90% modal mass participation (translational) at 9th mode. Significant difference between Eigen and Ritz analysis!
@Retired13
I am right now working on adding stiffness elements. I'll keep you updated of changes I am making in my model to control significant torsional effects.

 

[Agent666]

Another thing to keep in mind is, torsional effect is maximum at the middle of the long edge of a rectangle shape, so stiffen the structure elements accordingly will help.

Not sure I follow your logic as it seems opposite to my experience. The shorter end sees the maximum torsional effects as it is furthest from the centre of rigidity, similarly the best bang for buck to resist the torsion will be had putting stiff elements as far as possible from the centre of stiffness (I.E. On the shortest face, opposite to what you are suggesting).

 

[r13]

Agent666,

I was surprised when I learnt this phenomenon in college. You can do some research yourself to verify it.

 

[KootK]

1) I'm not sure where I've seen it but, at the least, I know that I've seen a recommendation against having a torsional first mode if not an outright prohibition on it.

2) Certainly, in ASCE7, torsional properties like this will see your design punished pretty hard in the "irregularities" department.

3) Some additional thoughts on layout below.

4) In reality, I suspect that your columns, acting as slab frames in concert with your slabs, will iron out the extreme torsional behavior that your model has predicted. While I'd personally be comfortable leaning on slab frame for this, somewhat secondary, purpose they are generally frowned upon for use in high seismic areas and it's probably not worth the effort of trying to justify using slab frames in this way to your AHJ.

5) An interesting phenomenon related to this is that, in a dynamic load situation, a building configuration like this will posses a significant torsional inertia that will go a long ways towards ironing out your torsional issues without adjusting the layout. I found out about this myself by working through some of the NCSEA seismic design manuals for an examination long ago. In the examples, they took advantage of this concept to, effectively, ignore torsion in the distribution of load to shear walls. I'm not recommending that you rely on this in this situation, or any other for that matter, but the concept is definitely interesting food for thought.

 

[Usman3301]

@KROOTK
Thanks a lot for detailed recommendations and explanation. In the diagram, I missed details of coupling beams. Coupling beams have already been added and the shear wall system is in closed-ish shape. I added shear walls, at similar location, just like you and others have mentioned, and it definitely worked. But later on, I had to adjust location of these extremely located shear walls in Y-direction, because of architectural reasons. I had no other choice but to utilize column space for placing shear wall at left. Length of shear wall placed at left side o building is around 4', which is not enough but right now, I don't have another option. I'll have to run the analysis to see how it effects mode shapes, for now it seems to me that this will contribute to "rotation" because of unsymmetrical placement of shear walls at extreme ends. Maybe I'll have to look for another option.

Can you please shed more light on how for such configuration of building, torsional inertia will contribute in minimizing torsional effects. This seems quite interesting to me. If you have any relevant research paper or data related to this, kindly do share.

 

[KootK]

You're most welcome Usman, it's an interesting problem.

...I don't have another option.

Not even moment frames as shown below? I feel that would be an elegant solution to this. If your sketches are anywhere near scale, you've already got gobs of capacity for direct shear. you just need a little perimeter resistance to iron out the torsion. If I held my rightful place as boss of the universe, I would dictate that all building had a nominal moment frame system about there perimeters.

I'll have to run the analysis to see how it effects mode shapes, for now it seems to me that this will contribute to "rotation" because of unsymmetrical placement of shear walls at extreme ends.

I'd bet a thumb and a six pack that doesn't wind up being the case. Massively increased torsional stiffness is going to greatly outweigh the asymmetry thing.

 

[KootK]

If you have any relevant research paper or data related to this, kindly do share.

I got nothin' that I haven't already mentioned.

Can you please shed more light on how for such configuration of building, torsional inertia will contribute in minimizing torsional effects.

I'll try to explain it intuitively. Which of the two systems shown below do you think will be easier to accelerate torsionally? A big old slab is a bit like the system on the right wherein significant mass is located far from the point of rotation.

 

[r13]

Usman3301,

Seems you already have a good handle on your situation. Below listed are for your information only.

- Remember that ETABS lists mode based on descending order of T and not mass participation

- Generally, the first mode of vibration is the one of primary interest.
• usually has the largest contribution to the structure's motion
• period of this mode is the longest
• shortest natural frequency and first eigenvector
- Significance of a mode is indicated by Mass Participation

Below is your original report with three modes that having high mass participation ratios redlined. The rank of importance thus is M3-M2-M1.

 

[Usman3301]

@KOOTK
I'll go with first case, since mass is concentrated at center, applying force provided moment arm larger in first case as compared to second, this will definitely help me in easily generating torsional acceleration.