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RE: Shear friction methods in masonry

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Joe,

I have designed some CMU walls in your part of the world that required a 16" thickness. I was worried that 16" blocks were not available, and that the contractor would build a multi wythe wall exactly as you described. So I made the bottom few feet of the wall 12" concrete, and put 12" block on top of it. I don't care for it, and I doubt the contractor did either, but I expect it will be built correctly. Are 16" blocks readily available in Northern AZ? If so, I won't worry about specifying 16" CMU walls, with a very large note that says double 8" blocks may not be substituted for 16" blocks.

TIA.

Dmitri Wright, PE
Portland, OR



From: "Joe Grill" <jgrill(--nospam--at)swiaz.com>
To: <seaint(--nospam--at)seaint.org>
Subject: RE: Shear friction methods in masonry

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
=20




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