column rotation between 2 consecutive floors

[structural87]

Due to architectural reasons, the column of a multistory building rotates from one floor to another as shown in the attached pic.
The design is based on ACI318-11.
I have 2 interrogations:
1 - The bearing strength is verified versus the axial force carried by the upper column and a drop panel is introduced to develop the bars of the upper column.
Any other special consideration for moment transfer ?
2 – if the bearing strength of the common area of the 2 columns is not enough, can I make use of the drop panel and consider that the supporting area is wider on the 4 sides and therefore, use a higher bearing area according to section 10.14 ?
Thank you in advance.

 

[racookpe1978]

The hidden line view is opposite what I'm used to, but appears to work if you are looking "up" and not from a plan view.

What is the proposed rebar pattern from lower column )through the pad through the floor slab ??) to the upper column?

Are there any beams or horizontal restraints other than the slab?

 

[KootK]

1 - The bearing strength is verified versus the axial force carried by the upper column and a drop panel is introduced to develop the bars of the upper column.
Any other special consideration for moment transfer ?

Simply providing development length for the upper column bars is not necessarily sufficient to guarantee moment transfer between the upper and lower columns and between the slabs and columns. If you want to be certain of moment transfer, it's probably a job for the strut and tie method. Another, less labor intensive approach is to count on only the bars common to both columns to transfer flexure. In these situations, I'll often perform the flexural design of the slab assuming that the columns are pin connected members precisely because I'm not overly confident in moment transfer through the joint. Punching shear evaluation may require a different approach.

2 – if the bearing strength of the common area of the 2 columns is not enough, can I make use of the drop panel and consider that the supporting area is wider on the 4 sides and therefore, use a higher bearing area according to section 10.14 ?
Thank you in advance.

The procedure can be applied. However, it is not the bearing area that is increase but the allowable bearing stress. It's an improvement gained by way of confinement.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[PicoStruc]

Note that you also have to consider the difference in concrete resistance in the joint.

Usally, column are poured with higher f'c that the slab. Calculation of effective compressive strength of the concrete of the joint is required and dowel might be required to increase axial resistance.

With column rotation, the common area for the joint is even smaller and more dowel might be required to acheive adequate compression resistance.

 

[structural87]

Thank u for your replies.
I didnt understand why the fact that if the bars are fully developed the moment transfer is not 100% achieved.

 

[KootK]

I didn't understand why the fact that if the bars are fully developed the moment transfer is not 100% achieved.

The space between columns is a structural joint composed of rebar and concrete. In providing the development lengths, you've only addressed the rebar part of the equation.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[structural87]

Ok and what is the problem with the concrete part kootk ? Thanks a lot by the way

 

[KootK]

Ok and what is the problem with the concrete part kootk ? Thanks a lot by the way

You're very welcome. There may not be a problem with the concrete. However, until you do something to check the concrete, there's really no way to know. In my opinion, the most rigorous way to design a joint like this, with moment, is the strut and tie method. That's a pain in the butt though, of course.

A common alternative that I see is to design the portion of the slab/drop common to both columns (overlap) as a vertical mini-column that may or may not require reinforcing. Where significant moment transfer from column to column is needed, one would need to design the min-column for that and run the rebar dowels far enough into both columns such that both could be considered to be flexurally spliced to the mini-column.

The one thing that would usually be exceedingly difficult to do, in my opinion, would be to create a joint capable of transferring the full moment capacity from one column to the other. This might be possible where there is a large degree of overlap between the columns.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.