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FLOATING COLUMN

 
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nabeel_musthafa
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PostPosted: Sat Jun 13, 2015 6:52 pm    Post subject: FLOATING COLUMN Reply with quote

Respected professors and structural engineers,

I have quite a few doubts regarding the behaviour of floating columns.

I am doing my M.E. Thesis on the very same topic.

I have modelled a 5 storey regular building in ETABS to understand how the floating column works. I have introduced a floating column at the first floor, i.e., a column running from the 5th floor is stopped at the1st floor.


To my surprise, when i checked the axial force transferred from the floating column to the transfer girder, it was almost negligible.


The axial forces from the column are never transferred to the transfer girder at the first floor.

And practically speaking, when i introduce a floating column at the 1st floor, there shouldn't be any effects on the beams in the upper floors.


But the analysis from ETABs has shown that all the beams above the floating column are affected and carry huge shear forces and moments and the floating column does not transfer its axial forces on to the transfer beam considering it has to transfer all the forces of the above storeys to that beam.

I have attached the model for better understanding. Please do check the axial forces and shear forces in the model. Something tells me the analysis done by ETABs is wrong.


Pls throw some light on dis topic. Thank you



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paresh_patel
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PostPosted: Sun Jun 14, 2015 6:33 am    Post subject: Reply with quote

dear brother ,

i have checked your model ,
column size is 450x450 and its rest on transfer girder of size 230x300 , is it ok?
just increase the beam size eg. 450x750 at  1st storey ( only 2 beam on which floating column rests )    and run analysis , you will get what you want , best of luck.

regards,
paresh patel
surat, gujarat.
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nabeel_musthafa
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PostPosted: Sun Jun 14, 2015 10:51 am    Post subject: Reply with quote

paresh_patel wrote:
dear brother ,

i have checked your model ,
column size is 450x450 and its rest on transfer girder of size 230x300 , is it ok?
just increase the beam size eg. 450x750 at  1st storey ( only 2 beam on which floating column rests )    and run analysis , you will get what you want , best of luck.

regards,
paresh patel
surat, gujarat.



Dear Paresh Sir,

Yes, I have not thought about the sizes of the beam and columns while running analysis. I assumed the sizes had significance only during design.
I have tried the model once again with larger sized transfer girder and it worked pretty fine.

Thank you for sparing your valuable time and clearing the doubt which gave me nightmares for quiet some time.

regards,
Nabeel M
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yadavenkatesh
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PostPosted: Mon Jun 15, 2015 8:52 am    Post subject: Re: FLOATING COLUMN Reply with quote

nabeel_musthafa wrote:
Respected professors and structural engineers,

I have quite a few doubts regarding the behaviour of floating columns.

I am doing my M.E. Thesis on the very same topic.

I have modelled a 5 storey regular building in ETABS to understand how the floating column works. I have introduced a floating column at the first floor, i.e., a column running from the 5th floor is stopped at the1st floor.


To my surprise, when i checked the axial force transferred from the floating column to the transfer girder, it was almost negligible.


The axial forces from the column are never transferred to the transfer girder at the first floor.

And practically speaking, when i introduce a floating column at the 1st floor, there shouldn't be any effects on the beams in the upper floors.


But the analysis from ETABs has shown that all the beams above the floating column are affected and carry huge shear forces and moments and the floating column does not transfer its axial forces on to the transfer beam considering it has to transfer all the forces of the above storeys to that beam.

I have attached the model for better understanding. Please do check the axial forces and shear forces in the model. Something tells me the analysis done by ETABs is wrong.


Pls throw some light on dis topic. Thank you



Dear Friend,

You need to consider the following points to get the appropriateforces in the floating columns and transfer beam.


<if>1.      <endif>Increase the stiffness of the transfer beams to3 to 4 times of it.  ( Generally usestiffness modifiers of transfer beam equal to 1 when other beams modifiersequal to 0.35 in the ultimate model)


<if>2.      <endif>Run construction sequence analysis in ETABS


<if>3.      <endif>Cross check with manual tributary area loads.


<if>4.      <endif>Check requirement of transfer beam size approximatelyby manual calculation ( Strength and mainly deflection ( L/800 to L/1000)) andprovide the transfer beam size in ETABS in the initial run itself.


<if>5.      <endif>It may be required to run static non-liner analysisif you notice tension force in floating column.


Hope with this you will get correct results.

See attachment for further guidance.

Venkatesh



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Abhishek_Singh
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PostPosted: Sat Jun 20, 2015 6:16 am    Post subject: Reply with quote

This happens and without looking at your model I can say that it is correct (model-wise speaking).

Due to the multiple frame action, the frames adjacent are holding up your column. In other words the column is actually hanging from the frames above and not supported on the transfer girder.

This happens because the structure suddenly came into existence in the model. All the floors suddenly came into existence. But in actual, every floor gets constructed slowly slowly so practically, the force gets time to settle on the respective members supporting it. At site, this floating action will not be there but there will be considerable incoming force from your "floating"column onto the transfer girder at the first floor.

If you do sequential construction in ETABS, such a behaviour will not occur.

Hope this helps....

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Thank you,
Abhishek Singh

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