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# RE: Shear friction methods in masonry

• To: seaint(--nospam--at)seaint.org
• Subject: RE: Shear friction methods in masonry
• From: Scott Maxwell <smaxwell(--nospam--at)engin.umich.edu>
• Date: Thu, 24 Feb 2005 14:19:13 -0500 (EST)

```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

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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