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.

 

[Usman3301]

@retired13
Yes, I have developed a better understanding of this now with help of you guys. Problem here is that I don't want to be a prey of torsional irregularity. I want to avoid it, and I achieved my goal but architectural reasons forced me look for another solution. I'll try to strengthen my moment resisting frames at parameters and see how much it contributes to minimizing torsional irregularity along with shear wall at extreme ends of shorter side. I hope this works.

 

[r13]

I would try add shear walls along the long sides too, if possible.

 

[KootK]

I'll go with first case...

Me too. You can't get a shear wall in as shown below, huh?

 

[Agent666]

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

The behaviour of an uncracked beam under torsion has no real direct correlation to the OP's problem regarding addressing global torsional effects in a 20 storey building. If the structure was a 20 storey solid rectangular piece of concrete maybe your point would be relevant, but it isn't is it.

Kootk, irrespective of the actual distribution of mass and alignment of the centre of rigidity with the core I guess you still have to deal with accidental eccentricity (0.1b stuff) which won't be insignificant for a floor plate of this aspect rstio. But at least if you can align the centre of mass with centre of rigidity its your best bet to minimise the effects of this. But I still think it warrents a very detailed look, you don't want to get into the realms of an unconstrained torsional response if one side of the core yield and loses stiffness.

But having a single core in this scenario in a 20 storey structure concerns me. The core will get hammered by the torsion, and if I'm understand correctly the slab is a flat plate/column deal which won't be able to take much drift before becoming susceptible to a punching shear type of failure. Sure you can kind of make that a ductile failure of sorts, but your best bet is to lay down the law with the architect, get a wall(s) elsewhere and start to control the interstorey drifts. You need structure sometimes and they just need to accept that.

Perhaps if you're stuck with the configuration, then getting into your performance based design and appropriate non linear seismic and/or wind analyses is your best bet to demonstrate things work. Use of linear elastic based analyses have their limitations.

 

[KootK]

But I still think it warrents a very detailed look, you don't want to get into the realms of an unconstrained torsional response if one side of the core yield and loses stiffness.

I never, for second suggested otherwise. I was just runnin' through the gamut of what's in play for these sorts or things. As an aside, the rotational inertia business is not dependent on the center of mass being aligned with the center of rigidity. Obviously, that alignment only helps matters but the inertia stuff is not predicated upon that.

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.

But having a single core in this scenario in a 20 storey structure concerns me.

Me neither. On a separate front, taller buildings of such configuration are candidates for the separate, but related phenomenon of torsional bucking under gravity load alone. Based on my past investigations into this, it takes a good deal more axial load than will be present here but it might be something worth considering. It's a check that isn't on many people's radar because the possibility is remote and codes never seem to have anything to say about it.

 

[Agent666]

Yeah I know its based on the distribution of mass (mass moment of inertia if you like) and not as simple as simply aligning centroids of mass and stiffness, but that's an obvious starting point for anyone venturing down this rabbit hole...

Yeah I guess if the core is the only thing restraining the columns through the slab. So you could see the columns buckling over several storeys if things went really pear shaped. Kind of similar in a way to how a megabrace and the secondary restraining system might work in a steel structure.

One other thought for the OP, if architect won't let you have walls, what about some form of brace/megabrace on the two ends. Just spitballing here.

Me neither? Does that mean it doesn't concern you in the context of my comment?

I kind of feel going into a concept with something 'difficult' means as the design progresses it generally heads towards 'impossible'. Start out 'simple' with well thought out structural systems and if you're lucky you end up only reaching 'difficult'. Since this is only for educational purposes at least you learn that lesson in an environment that means the job isn't pulling money out of your pockets....

 

[KootK]

Me neither? Does that mean it doesn't concern you in the context of my comment?

Let's change that to me too. It concerns me for the reasons that you mentioned and some additional ones to boot.

 

[Usman3301]

@Agent666 @KOOTK
I can't change the location of extreme shear walls to where you guys have mentioned. But I did try (for sake of understanding) similar shear-wall placement as mentioned by you guys and it worked. Also, I can't add braces at either ends (architectural reasons again!) Problem here is that these short sides are sort of face of building (entrance) and architects won't compromise on that.
You are quite right in suggesting, "starting out with well-thought structural systems." I have realized that things get complicated with time and little intricacies become major issues.
I'll be following configuration for placing shear wall for now.

If things don't work out well, it'll be interesting to see how pushover analysis exposes inability of structure to overcome torsional stresses. Though relative loss of stiffness of core seems to me as a major issue and shouldn't be taken lightly.

 

[r13]

Can you treat the entrance area as architectural component, and leave it out of the main force resisting system? Just thinking.

 

[Trenno]

Is this a real project or simply an educational exercise?

 

[Usman3301]

@Trenno
I have recently started working in a structural design firm after completing my Bachelors. Considering my interest in dynamics (I did my FYP in structural health monitoring and successfully developed a Finite element model of historical monument) , my boss thought it would be a good exercise for me to develop understanding of how things work out in real life. He tells me about certain limitations (architectural, construction related) I must follow. I have learnt a great deal from this project and wish to perform performance based design.
@retired
No, I don't think architect (hypothetically) would let me do that considering the timeline I am in.