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RE: Shear friction methods in masonry

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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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