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Re: Slab/Column Connection and Detailing

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Gentlemen,
I really appreciate all the answers I have received.
Then I would like to conclude that

1) To design concrete slab (two-way flat plate), no matter it is a
Post-tension slab or Conventional Reinforced Concrete slab, the column
boundary condition should be fixed at both ends (when the selection of a
certain percentage of fixity is not available) so that the design for slab
and column are conservative.

2)When modeling the whole building by computer program (such as ETABS), we
use pinned at slab/column joints so that the shear walls will resist all the
lateral loads, this way will give a
conservative design for shearwall.


Hui Li
Project Engineer

----- Original Message ----- 
From: "Gil Brock" <gil(--nospam--at)raptsoftware.com>
To: <seaint(--nospam--at)seaint.org>
Sent: July 28, 2004 4:07 PM
Subject: Re: Slab/Column Connection and Detailing


> 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
> Prestressed Concrete Design Consultants Pty. Ltd. (ABN 84 003 163 586)
> 5 Cameron Street Beenleigh Qld 4207 Australia
> Ph +61 7 3807 8022              Fax +61 7 3807 8422
> email:          gil(--nospam--at)raptsoftware.com
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>
>
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