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RE: Slab/Column Connection and Detailing

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I agree 100% with what Craig is saying.

Read chapter 11.12.6 (on page 16-10) of the ?PCA Notes
On ACI 318-02?.? If you don?t have a copy, here is
what it says?

"Two-way slab systems are usually fairly ?forgiving?
in the event of an error in the amount or distribution
of flexural reinforcement; however, little or no
forgiveness is to be expected if shear strength
provisions are not fully satisfied.?

I challenge anyone to quote a more blunt warning about
any aspect of structural design (concrete, steel,
timber, masonry, etc.) that they have seen published
anywhere.

If you want to see what can happen when a punching
shear failure occurs, read this:

http://www.eng.uab.edu/cee/reu_nsf99/reu_nsf00/carlos/Harbour%20Cay%20Condominium%20Collapse.htm

?I guess the engineers who designed this condo thought
they knew everything there was to know about
structural design since they were retired rocket
scientists - literally! They lost their PE licenses as
a result of ignoring punching shear.

When explaining this topic to young engineers
designing flat plates for the first time (and doing so
under close supervision), I always warn them, "Don't
f--- with punching shear."  

Making the slabs as thin as possible is a good idea.
In my opinion, there is not much economy to be
realized in making columns unreasonably small. On the
contrary, larger columns with minimum vertical
reinforcing steel are very economical. If the
architect complains about the columns being too big
(which they always do - so what else is new) tell them
they might want to look into using this concrete:

http://optics.org/articles/news/10/3/10/1

On second thought, maybe it would be best if we didn?t
tell architects about translucent concrete!

Cliff Schwinger



--- Craig Leech <cleech(--nospam--at)nyc.rr.com> wrote:
> 
> It seems almost dangerous to detail the columns with
> pins.  What could be
> the advantage?  All you will do is reduce the
> overall redundancy of the
> structure.
> 
> Even if you detail the reinforcing as a pin, the
> accumulated gravity load
> will most likely mean that the column may never see
> tension (depends on
> building height etc).  Thus moment will be delivered
> to the column and slab
> whether you want it or not.
> 
> You should look at the 2005 draft recommendations
> for slabs not proportioned
> to resist lateral load.  It follows the IBC 2003
> section 1908.1.6.  This
> will govern the balance between column size and slab
> thickness.
> 
> Craig
> 
> -----Original Message-----
> From: Hui LI [mailto:hli(--nospam--at)maengineering.com] 
> Sent: Wednesday, July 28, 2004 2:27 PM
> To: seaint(--nospam--at)seaint.org
> Subject: Slab/Column Connection and Detailing
> 
> 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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