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RE: Drop Panels

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I certainly was not attempting to suggest that additional stresses due to
lateral drift/moment tranfer should not be designed for.

Part of my point (which was not well articulated) is that many times the
additional punching shear stresses due to the lateral drift/moment
transfer have still be traditionally handled by just providing additional
concrete depth either just near the column (as in a drop panel) or through
out the slab.  As a result, traditionally the inclusion of drop panels
_does not_ change the reinforcement in the slab from what it would be
_without_ the drop panels.  Typically, the drop panels have only been
added to increase the Vc component that resists the punching shear.

In more recent years, it has been discovered/learned that this form of
resisting the additional punching shear stresses due to lateral
drift/moment transfer is not a very ductile manner to handle situation.
As a result, it is becoming more common, especially in moderate and high
seismic regions, to try to resist the additional punching shear due to
lateral drift/moment transfer in a more ductile manner by using shear
heads, stud rails, and rebar for shear.   There is, however, not to much
that has made "official" appearances in the 318 code.  In many ways, the
stud rails have been considered a "favorite", but since they are a
proprietary system, it makes it difficult to include in a national
standard such as ACI 318.

The point is that you can design a two-way slab system with drop panels
that don't have any "extra" reinforcement that will calculate out as being
fine, but recent research has show that you can still have some problems
with such a system largely because it is still a very brittle system (you
are strictly relying on the concrete to handle all the punching shear,
both standard punching shear AND punching shear due to lateral
drift/moment transfer).

The end result is that _IF_ your building is in a low seismic region and
the two-way slab will not have to resist much in the way of lateral drift,
then there is _NO_ reason to provide any extra reinforcement in the drop
panels other than what would normally be required for the same two-way
slab without the drop panels.  This can even be true in a moderate to high
seismic zone if the lateral drift is control in some manner that the
additional punching shear stresses due to lateral drift/moment transfer
are minimal.  If, however, the additional punching shear stresses due to
lateral drift/moment transfer are significant, then it is strongly
advisable to provide a more ductile system that include either shear
reinforcement (take a look at Figure R11.12.3 in the ACI 318-02),
shearheads (see Figure R11.12.4.7 in ACI 318-02), stud rails, or some
other system, even though it would be likely be permissible under the
ACI code just add additional concrete depth to increase the Vc enough to
just "overcome" the additional punching shear stresses with concrete
alone.

HTH,

Scott
Ypsilanti, MI


On Mon, 8 Jul 2002, Haan, Scott M. wrote:

> The 97 UBC 1633.2.4 "Deformation Compatibility" says non-lateral system
> elements need to be designed for internal forces caused by the greater of
> DELTAm displacements or .0025 times the story height. Similar requirements
> were in 1994 UBC 1631.2.4, 1991 UBC 2337(b)4 and 1988 UBC 2312----. 
> 
> The shear from moment transfer should be included in the punching shear
> check.  It may not have been enforced or designed for but increased punching
> shear stress from deformation compatibility moment transfer has been in the
> UBC since 1988.
> 
> -----Original Message-----
> From: Scott Maxwell [mailto:smaxwell(--nospam--at)engin.umich.edu]
> Sent: Friday, July 05, 2002 6:35 PM
> To: 'seaint(--nospam--at)seaint.org'
> Subject: RE: Drop Panels
> 
> 
> 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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