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Re: Flat Slab Punching Shear Reinf.

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I've left the office now so I can't tell you exactly how long they
are, but if I remember right they are around 6 ft total length. They
are called out as "shear bars". For example, they call for (4)-#3
shear bars each way over the top of the columns. They are in addition
to the longitudinal column strip moment resisting bars. The 45 degree
segment of the "shear bars" fall in the area of where a punching shear
failure would occur.

The slab system I am analyzing has approximately 20 ft square bays so
the shear bars are a good bit shorter.

WH

On Fri, Jun 20, 2008 at 5:38 PM, Scott Maxwell <smaxwell(--nospam--at)umich.edu> wrote:
> Are they specifically called out as "shear bars"?  How long are they...the
> middle section as well as the end sections?  That shape looks like standard
> shapes used for flexural bars (i.e. at the top for negative moments at the
> supports and at the bottom for positive moments in midspan).  Your
> implication is that they are relatively short bars JUST at the column, but
> it is not 100% clear if that is really the case.
>
> In response to your question...I honestly don't know if they are
> "acceptable".  It could somewhat depend on the shape/length of the bar.  In
> theory, if the 45 deg slope portion feel at the right place, it could resist
> shear at a shear crack.
>
> If you clarify, I might be able to see what I can find out from my sources.
>
> Regards,
>
> Scott
> Adrian, MI
>
> -----Original Message-----
> From: William Haynes [mailto:gtg740p(--nospam--at)gmail.com]
> Sent: Friday, June 20, 2008 4:13 PM
> To: seaint(--nospam--at)seaint.org
> Subject: Flat Slab Punching Shear Reinf.
>
>
> I am analyzing an existing slab for a change in occupancy load. The drawings
> are about 40 years old and have a 9" thick flat slab with 15" square
> columns. Over the columns they call for shear bars that are bent
> longitudinal bars but there are not any stirrups. The "shear
> bars" called for look something like this   ______/========\______.
>
>
> I think I have seen bent bars like these in concrete joists to resist shear
> in older drawings before. In my case, they are straight at the bottom, then
> bend upwards at 45 degrees, are flat along the top over the column, then
> bend down at 45 degrees and are straight again along the bottom of the slab.
>
> Can you use bars like this under ACI today to resist punching shear in a
> flat slab? ACI talks about longitudinal bars AND stirrups but the shear bars
> they call for on these drawings are just several bent bars like I described
> over the columns.
>
>
> WH
>
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