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RE: Drop Panels

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Scott,

Let my expand/correct myself a little...of course there needs to be
reinforcement steel in the area of the drop panel.  This would be the slab
flexural steel (and possibly shear steel, but that is a whole other ball
of wax) that would required whether there is a drop panel or not.  This
would include the typical negative moment steel due to flexural strength
issues, continuity steel for both top and bottom steel, etc.

My point was that typically drop panels are added to increase the
resistance to punching shear.  The drop panels are typically used to
increase the concrete (Vc) resistance in shear if the "base" slab
thickness is not sufficient to provide adequate punching shear resistance.
In this type of scenario, then additional steel (other than what is
already required for the "base" slab itself) is not required.

Now, there are also provisions to add shear reinforcement to slabs to help
prevent punching slab failures.  This shear reinforcement can take the
form of stud rails, reinforcement bars, and shearheads.  This type of
solution is not typically used in low seismic zone since it is typically
more cost efficient to either thicken the whole slab or add drop panels.
Shear reinforcement _IS_ used in higher seismic zones, especially after
the Northridge quake.  There were several buildings in LA that were made
of cast-in-place two-way slabs with perimeter moment frames.  The thought
was that the moment frames would take all the lateral load and the rest of
the two-way slabs would "go along for the ride".  The problem was that the
additional moment at the two-way slab columns that arose due to the quake
did cause some punching shear failure of the slabs at some columns.  This
was largely due to the drift that the building was required to sustain.
This discovery has added to impetious to research and produce procedures
for providing shear reinforcement for a more ductile punching shear
response in two-way slabs.  Some this is being put into recommendations by
ACI committee 421 as well as ACI committee 352.

HTH,

Scott
Ypsilanti, MI


On Fri, 5 Jul 2002, Haan, Scott M. wrote:

> Forgot to say why.  Bottom steel through the column core is a prescriptive
> requirement to prevent a sudden punching failure.  See ACI 318 13.3.8.5.
>  
> Scott M Haan P.E. 
> Plan Review Engineer 
> Building Safety Division 
> Development Services Department 
> Municipality of Anchorage 
> http://www.muni.org/building <http://www.muni.org/building>  
> phone:907-343-8183  
> fax:907-249-7399 
> mailto:haansm(--nospam--at)ci.anchorage.ak.us <mailto:haansm(--nospam--at)ci.anchorage.ak.us>  
> 
> -----Original Message-----
> From: Haan, Scott M. [mailto:HaanSM(--nospam--at)ci.anchorage.ak.us]
> Sent: Friday, July 05, 2002 4:19 PM
> To: 'seaint(--nospam--at)seaint.org'
> Subject: RE: Drop Panels
> 
> 
> If you have negative moment you need steel to resist it.  Also you need at
> least 2 continuous bottom bars in the column strip through the column core.
> Also see ACI 318 Fig. 13.3.8.  Tell me the buildings you guys are designing
> without reinforcement in the drop panels so I can stay on the top floor.
>  
> Scott M Haan P.E. 
> Plan Review Engineer 
> Building Safety Division 
> Development Services Department 
> Municipality of Anchorage 
> http://www.muni.org/building <http://www.muni.org/building>  
> phone:907-343-8183  
> fax:907-249-7399 
> mailto:haansm(--nospam--at)ci.anchorage.ak.us <mailto:haansm(--nospam--at)ci.anchorage.ak.us>  
> 
> -----Original Message-----
> From: Lutz, James [mailto:JLUTZ(--nospam--at)earthtech.com]
> Sent: Friday, July 05, 2002 2:20 PM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: Drop Panels
> 
> 
> If there is no drift, and the moment stays negative over the panel area,
> then the drop panel is always in compression, and you shouldn't need steel
> in the drop panel. 
>  
>  -----Original Message-----
> From: Davin Weinhardt [mailto:yellowhumpy(--nospam--at)hotmail.com]
> Sent: Wednesday, July 03, 2002 4:16 PM
> To: seaint(--nospam--at)seaint.org
> Subject: Drop Panels
> 
> 
> 
> Just out of curiosity, how many would say that a
> 4" drop panel below a 9 1/2" two-way slab needs
> reinforcement? What if I added that there is no
> drift issues? If you still say yes, please state
> why (references would be great)!
>  
> Thanks in advance,
> Davin
> 
> 


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