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FLOATING COLUMN

 
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nabeel_musthafa
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Joined: 13 Jun 2015
Posts: 3

PostPosted: Sat Jun 13, 2015 6:45 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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R_Shenoy
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Joined: 20 Mar 2012
Posts: 8

PostPosted: Sun Jun 14, 2015 5:06 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
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amol_acharya
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Joined: 13 Aug 2013
Posts: 39

PostPosted: Mon Jun 15, 2015 7:44 am    Post subject: Reply with quote

Hi,
Good to see young engineers like you are not flatly relying on the software output. To make the floating system work you need to understand - The feasibility of transfer system, Stiffness of the Transfer beam/supporting columns and Usage of releases to get the load path correct (if required).


Probably only you know that the columns at the corner grid D-4 should take down all the forces to the transfer beam and the transfer beam should act like a cantilever transfer beam. However any modeled system's response (without getting much into details) is based on the shortest path, constraints/releases and the stiffness assigned to the modeled elements to carry a certain percentages of the loads to the base.

Looking at the element sizes and spans in the model it appears to me that the system that you modeled was not a floating column system. Rather on each floor the beams were actually supporting the floor and the column at the corner (D-4). Since you are doing a thesis I suggest to play with the model a little bit to understand how the model respond to changes in stiffness and releases. Use appropriate stiffness (I suggest you to design the transfer beam using hand calcs/load take down first and assign the required size of the beam/column in the model) for the transfer beam and column, use appropriate beam releases to see how the system responds. I am sure you will find the expected behavior of the structure very soon.

Regards,
Amol Acharya
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