torsional vibration as mode 1 or 2 1

[luungoclinh]

Hi all,
Could anyone explain to me why we need to avoid having a building where the torsional vibration is mode 1 or 2? In case that is unavoidable, what should we do?
Thanks
Lisu

 

[luungoclinh]

properly, I got my answer, it is because of the buildings with low torsional stiffness experiences large
deflections, causing distress in both structural and nonstructural components. However, I dont know how to delete this thread.
Anw, thanks for reading (if there is someone)
Lisu

 

[PicoStruc]

What is the problem as having torsion mode for vibration as fist mode ? Absolutly None !

If you account for mass distribution in 3D, you will of course have vibration mode of torsion or combination of lateral mode with torsion in one of the first 3 mode of vibration.

A lot of engineer don't have torsion mode because they concentrate the mass at center of rigidity as point mass in their model. By doing that your won't obtain torsion behiavor and it's wrong !

 

[PicoStruc]

If you have large displacement, increase the torsion stiffness of your lateral resisting system.

Locating SFRS components at exterior perimeter or using core (tube) wall help to increase torsion stiffness.

 

[luungoclinh]

thanks Picostruc, I read somewhere in this forum people saying that we should avoid having the building where torsional mode is the mode 1 or 2. I think it is properly because people want to use static equivalent force procedure that only considers the transnational modes.

 

[luungoclinh]

please correct me, if I am wrong. Thanks

 

[PicoStruc]

1) Having First mode as torsional don't mean that your building is sensitive to rotation a.k.a irregular in torsion
It all depend of your inelastic displacement amplitude due to your torsionnal stiffness and vibration frequency vs spectrum.

2) By definition, Static equivalent force procedure is a 2D analysis and neglect torsion mode of vibration.
Building code generally require to check torsion sensitivity to verify if 3D dynamic analysis is required.

3) Event in 3D analysis, if the engineer doesn't define the distribution of mass in 3D or doesn't include the mass of inertia in the mass matrix (generally done automatically), no dynamic torsional mode of vibration will be considered or found in the analysis.

4) Considering torsion dynamic properties generally soften (reduce) the GLOBAL lateral dynamic response but might increase the LOCAL shear due to torsion in Lateral Resisting System components. It really vary for each project.

 

[luungoclinh]

thanks for your comprehensive explanation. Do we think that if we have modes 1 and 2 as transnational modes, mode 3 is torsional mode, I can conclude the building is not sensitive to torsion?

 

[PicoStruc]

No, Torsional sensitivity is a check that need to be done by checking the displacements responses (generally Max/average).

Check your local building code.

 

[luungoclinh]

I think these are related. Building code asks for checking displacements responses because it is easier to obtain than conduct modal analysis considering torsional modes.
I have another question regarding this issue. The Code requires us to calibrate the base shear values from modal response spectrum analysis with the base shear values from static equivalent force procedure. My question is that we should calibrate the base shear from modal response spectrum analysis with the model considering torsional modes or not considering torsional modes.
Thank you.

 

[PicoStruc]

Your last question is really good. This is where Engineering Judgment is important.

My personnal opinion is to calibrated with a locked 2D-model (To compare 2D with 2D) instead of 3D shear component

But some engineers might disagree with that.

 

[luungoclinh]

thanks for your quick response. You mean that you use locked 2D model to calibrate the base shear (obtain scaling factors), then you apply that scaling factor to 3D model to get the design values or the design values is from calibrated 2D model?

 

[asixth]

Sometimes it's unavoidable and I often get torsional mode as the 1st or 2nd mode in the low-rise buildings (up to 10 stories).

In my town the general building systems are reinforced concrete cores as the primary lateral system and outriggers if the building gets really tall. There seems to be a trend lately particulary with office buildings to push the lift's to one side of the building to optimize floor space much to the dislike of structural engineers.

There was a building in Christchurch which collapsed in an Earthquake in 2011 and when you look at the floor plan it is just ugly, the primary lateral system is a reinforced concrete core which is located outside the floor plate and only supporting on one-side. When the earthquake hit and the building twisted the concentration of shear at the slab-core interface disengaged and it came down.

Apparently torsional mode of vibration wasn't considered because it was only analysed using a 2-D ETAB's program which I guess was just called TAB's or AB's back then.

 

[johnbridge231]

Place more shear walls as far as possible from the center of rigidity of the plan of the building.

Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html

 

[slickdeals]

Place more shear walls as far as possible from the center of rigidity of the plan of the building.

And symmetrically (if possible) on either side of the CoR.