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Re: Flat Slab Punching Shear Reinf.

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Thanks Scott, I really appreciate it.


Will

On Sun, Jun 22, 2008 at 3:16 AM, Scott Maxwell <smaxwell(--nospam--at)umich.edu> wrote:
> 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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