View Full Version : Torsional first mode
I would like to have your opinion on that.
I solve structural scheme of the building and the mode 1 its not in transverse or longitudinal direction but its in torsional direction.
Whereas mode 2 and mode 3 are respectively in transverse and longitudinal direction.
Building have 7 stories (reinforced concrete) and I am using Sap2000.
Is there any limit value when first mode is in torsional direction?
I really would appreciate if you would suggest me any ideas, articles, books ect.
First by serial number or mass participation? Upload the analytical mass participation of modes!
Upload periods too!
In brief because I have no idea of your model. If you want upload a sketch.
In such high building is not a good idea to design without diaphragms!
Since you have used dynamic analysis I suppose that your building is located in a seismic region.
This (http://www.designtoeurocodes.com/forums/content.php/172-Modal-analysis-and-seismic-design) article describes why torsional first modes should be avoided.
As for your building, provided that the first mode activates the greatest percentage of the total mass, is torsional sensitive... This means maximum behaviour factor q=2.00 (it depends on the regularity of your building) which will lead to structural elements with bigger cross-sections.
For bibliography check here (http://www.designtoeurocodes.com/forums/showthread.php/42-Bibliography-for-seismic-design-of-structures).
If he was designing with diaphragms he would have 3 degrees of freedom per storey and the model would be less difficult to understand... Am I right?
It's not only about the understanding issue but mostly about the realistic face up to proper seismic design.
Anyway, there is a design problem here. A sketch or 3d model would clarify the picture.
So when percentage of the total mass in torsional sensitive exceed 50% (or even less - how might be?) should be avoided.
It's not the 50% the problem. Even 30% would be, the problem still exists. There is a wrong design or/and simulation problem here. But as long as we don't know your model, we only guess.
Btw, I repeat (as a picture) my 1 1/2 old answer in civilea for the same issue:
I agree with rigid_joint! There is no such limit according to Eurocodes... Provided that the dimensions of your slabs are adequate, simulate them as diaphragms, run the dynamic analysis again and post the results of the modal analysis (the same two files you uploaded before) together with a 3d view of your model.
so when percentage of the total mass in torsional sensitive exceed 50%(or even less.how might be?) should be avoided.
And there is another great "hidden" problem in high torsional participation that has to do directly with the software.
Are you sure that your software designs columns due to bending-shear-torsion? (surprise :)).
That's why, even 30% or 20% are huge numbers.
Provided that the dimensions of your slabs are adequate, simulate them as diaphragms, run the dynamic analysis again and post the results of the modal analysis (the same two files you uploaded before) together with a 3d view of your model.
I agree with Ion. Make this test and upload the results in order to compare them!
These are the results for diaphragms.
I don't know how etabs works, but do you have restrained rotation for x, y axes?
Otherwise, what matrix do you use for analysis?
Rx, Ry results had to be near zero and not near 100%.
In my opinion something happens with your model or/and with the analysis type that you use.
Results for diaphragms are making no sense.
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