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RE: Flat Slab Punching Shear Reinf.

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Will:

Here is what my "source" had to say:

"Designers used to bend up bars in a slab just like they did in joists, etc.
They are considered acceptable, but clearly they only reinforce one critical
perimeter around the column.  For that perimeter you can include the
vertical component of the bars as V-sub-s and add it to V-sub-c, which is
calculated  using a shear stress of 2 sqrt fc'. Outside of that perimeter,
you go back to only using V-sub-c, as governed by the current set of ACI
equations."

This is basically in line with what I was thinking (basically the 45 deg
portion helps you at THAT location of the 45 deg portion, but the rest is
useless), but wanted verification.

HTH,

Scott
Adrian, MI

-----Original Message-----
From: William Haynes [mailto:gtg740p(--nospam--at)gmail.com] 
Sent: Friday, June 20, 2008 6:47 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Flat Slab Punching Shear Reinf.


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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