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Re: Slab/Column Connection and Detailing

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Hui Li,

As all others have answered, a proper zero moment pin connection is not possible. And at the lower floors, the axial effects will dominate and the columns will be in full compression and uncracked and will attract the full moment based on elastic sections. It could actually be worse than this because the slab will crack while the columns will not so they will be relatively stiffer possibly attracting more moment. The slab must be analysed with the full column stiffness and the slab column connection designed for the full moment attracted to it. You cannot redistribute from a brittle punching failure.

The other point I will make is that in attempting to minimize the column sizes, you are creating differential support shortening problems. The shear walls will be relatively lightly stressed under vertical loads as they have been dimensioned for earthquake effects. The columns will be highly stressed under vertical loads because you are making them as small as you possibly can based on vertical loads only. When you consider long term effects (you should even though most do not), you will find that the creep shortening of the columns will be far higher than the creep shortening of the shear walls. This will create extra stresses in the column slab connections and, if the building is not symmetrical, will cause severe sway deflections (this is under vertical loading only and is a permanent condition). Make sure you take this into consideration when dimensioning your columns.

I would hope that you were not considering monolithic pouring of slabs and columns. I do not think this is ever advisable. But there will still be a moment connection at the column/slab interface if they are poured separately.

I would have thought for an earthquake designed structure, the use of ties in a thin slab for punching shear would be "as useless as ---- -- a bull" in terms of making the connection ductile.


At 04:26 AM 29/07/2004, you wrote:
Gentlemen,

Thank you for your input and time.

When modeling the whole building, we use pinned at slab/column joints so
that the shear walls will resist all the lateral loads. We agree that the
roof slab should be fixed due to the fact that vertical bars are developed
into the slab but this is not the case for all intermediate levels that is
why we are wondering if pinned connection is advisable in the design of both
PT and conventional slabs. Additionally, construction sequence does not
allow monolithic pouring of columns and slab and sometimes of different
compressive strengths. The program we use considers the drift of the columns
and we use the provisions stated on ACI Section 21.9 "Frame Members not
Proportioned to Resist Forces Induced by Earthquake Motions".
Is it acceptable to assume a reduced stiffness for the column and
complimented by detailing the column connection to the slab with additional,
closely spaced ties to minimized the width of cracks which may develop at
this junction (PTI Design Fundamentals of PT Concrete Floors p.2-15).
Our ultimate goal why we asking these questions is to reduce column sizes or
slab thickness due to punching shear problem. Any suggestions?
Any input will be greatly appreciated.

Hui Li
Project Engineer


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Regards  Gil Brock
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