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RE: Shear friction methods in masonry

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Scott, that is exactly where I am heading.  What is happening, though, is
this.  I have initially assumed that the grout blocks are spaced at 24" o.c.
horizontally.  In another words, a vertical section of face shell would be
removed every 24".  With 2000 psi grout the MSJC would limit shear through
that section to 44.7 psi.  If I assume a 6" "grout block" width (going along
with comments on grout shrinkage) I get a shear of 79 psi (VQ/I gives me 474
#/in). At the moment I'm closing up that 24" dimension to see if I can bring
down that horizontal shear value.  Things don't seem to be working very well
though.
Joe

Joseph R. Grill, P.E. (Structural)
Shephard - Wesnitzer, Inc.
Civil Engineering and Surveying
1146 W. Hwy 89A Suite B
Sedona, AZ  86340
PHONE (928) 282-1061
FAX (928) 282-2058
jgrill(--nospam--at)swiaz.com
 


-----Original Message-----
From: Scott Maxwell [mailto:smaxwell(--nospam--at)engin.umich.edu] 
Sent: Thursday, February 24, 2005 2:53 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Shear friction methods in masonry

Joe,

There is no reason why the rebar dowels that you are planning for can
basically act as the ties.  And if the grout "block" where you cut the
webs out can act as header (i.e. the grout crosses at "discret"
locations), then you might be fine.

Scott


On Thu, 24 Feb 2005, Joe Grill wrote:

> Scott,
> I agree with everything you have said.  I've been looking at the section
in
> the MSJC document and the requirements for the collar joints etc.  With
one
> side already stacked (but not grouted), the joint reinforcing can't be
> installed, so I am looking at #3 or #4 dowels to provide the "ties".  By
saw
> cutting face shells I can get a "web section" between the two wythes.
> Within the web section I can provide the dowels for "beam shear".  The
grout
> flow between the wythes should also act as a "header section" and shear
> through that should be enough to provide the horizontal shear to create
the
> composite section.  I'm still not done scratching calcs yet though, but it
> seems like it should work.
>
> Joseph R. Grill, P.E. (Structural)
> Shephard - Wesnitzer, Inc.
> Civil Engineering and Surveying
> 1146 W. Hwy 89A Suite B
> Sedona, AZ  86340
> PHONE (928) 282-1061
> FAX (928) 282-2058
> jgrill(--nospam--at)swiaz.com
>
>
>
> -----Original Message-----
> From: Scott Maxwell [mailto:smaxwell(--nospam--at)engin.umich.edu]
> Sent: Thursday, February 24, 2005 12:19 PM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: Shear friction methods in masonry
>
> Joe,
>
> Nels raises a good point...you could actually end up with two shear planes
> at joints (i.e. on each side of the mortar).  This would likely reduce the
> shear friction effect (although, I think it would still be there).  The
> main thing that you would have to "adjust" form the concrete shear
> friction approach would be the coefficients of friction that they use.
> The combination of the possible double shear plane and just slightly
> different materials would make the coeficient different.  And the double
> shear plane could reduce the "clamping force" that is needed for shear
> friction to be effective (keep in mind that the bar in shear friction is
> only "there" to hold the two surfaces together).
>
> Now the other thing to consider is "ties" across a collar joint.  Take a
> look at section 2.1.5.2.2 of the 2002 MSJC (ACI 530 et al).  It specifies
> that the allowable shear stress for a mortared collar joint with ties is 5
> psi.  That goes up to 10 psi if you use a grouted collar joint.  So, you
> can do two "parallel" 8" block walls with a collar joint and ties between
> the two and have them act compositely _IF_ the shear stress between the
> two is less than either 5 or 10 psi depending whether your use mortar or
> grout respectively in the collar joint.  In effect, this section is likely
> dealing with a shear friction type effect (i.e. the ties hold everything
> together so that shear friction between the collar joint and units will
> work...in otherwords, I doubt the ties are there to hand the shear
> directly).
>
> HTH,
>
> Scott
> Adrian, MI
>
>
> On Thu, 24 Feb 2005, Joe Grill wrote:
>
> > Thanks Nels,
> > I do understand what you are saying and agree.  What I am dealing with
> here
> > is a CMU retaining wall that required a 16" thick section at the base.
The
> > 16" thick section is required to be 5'-4" above the footing and then it
> > turns into 12" thick for the remainder of the height.  The contractor
> > (without asking) stacked an 8" CMU section with the intent of stacking
> > another 8" section next to it to make the 16" thickness.  Of course this
> is
> > not the same as a full 16" block.  Rather than telling them to take out
> the
> > first "wythe" of CMU we are looking at making a composite section.  The
> > first wythe has not been grouted, so there are some things we can do.
> What
> > we are looking at is having him sawcut the face shells, vertically, at
24"
> > o.c. or maybe 32" o.c. from the first wythe and also from the second
lift.
> > This will provide a "header" section between the wythes as the grout is
> > placed, or as is easier for me to visualize a web section for an "H"
> shaped
> > section where the flanges are the two wythes of the wall.  Even though
it
> > seems that the shear through the Web is within the guidelines for
section
> > 2.1.5.2.2 of ACI 530, we are also going to ask that dowels (really shear
> > stirrups) be placed within the "web" between the wythes.  This should
> > provide the shear reinforcing necessary.  I had been thinking of "shear
> > friction" as a means to develop the shear necessary for the composite
> > section.  My thoughts have changed somewhat to a more traditional method
> of
> > providing the shear strength in using hooked stirrups within the web
> between
> > the wythes.
> >
> > Sound reasonable?
> >
> > J. Grill
> >
> > Joseph R. Grill, P.E. (Structural)
> > Shephard - Wesnitzer, Inc.
> > Civil Engineering and Surveying
> > 1146 W. Hwy 89A Suite B
> > Sedona, AZ  86340
> > PHONE (928) 282-1061
> > FAX (928) 282-2058
> > jgrill(--nospam--at)swiaz.com
> >
> >
> >
> > -----Original Message-----
> > From: Nels Roselund [mailto:njineer(--nospam--at)att.net]
> > Sent: Thursday, February 24, 2005 9:22 AM
> > To: seaint(--nospam--at)seaint.org
> > Subject: RE: Shear friction methods in masonry
> >
> > Joe,
> >
> > Much of the shear in a masonry wall would be resisted in the mortar bed
> > joints.  Concrete Block masonry is not truly monolithic.  If the grout
in
> > which the reinforcing is embedded is not tightly bonded to the cell
walls
> of
> > the units, the reinforcing may not be capable of developing shear
friction
> > in the bed joints.  Concrete block walls not built with a
> shrinkage-control
> > admixture in the grout may have grout that is not be very well bonded to
> the
> > block units.  I've seen a concrete block wall disassembled: the grout
> cores
> > had smooth surfaces that had separated very cleanly from the cell walls,
> as
> > if there were no bond.  Something to think about.
> >
> > Nels Roselund, SE
> > South San Gabriel, CA
> > njineer(--nospam--at)att.net
> >
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