torsional vibration as mode 1 or 2
torsional vibration as mode 1 or 2
(OP)
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
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






RE: torsional vibration as mode 1 or 2
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
RE: torsional vibration as mode 1 or 2
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 !
RE: torsional vibration as mode 1 or 2
Locating SFRS components at exterior perimeter or using core (tube) wall help to increase torsion stiffness.
RE: torsional vibration as mode 1 or 2
RE: torsional vibration as mode 1 or 2
RE: torsional vibration as mode 1 or 2
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.
RE: torsional vibration as mode 1 or 2
RE: torsional vibration as mode 1 or 2
Check your local building code.
RE: torsional vibration as mode 1 or 2
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.
RE: torsional vibration as mode 1 or 2
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.
RE: torsional vibration as mode 1 or 2
RE: torsional vibration as mode 1 or 2
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.
RE: torsional vibration as mode 1 or 2
Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis
http://www.engissol.com/cross-section-analysis-des...
RE: torsional vibration as mode 1 or 2