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Re: Masonry Shear walls

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As others have alluded to, this is highly dependant on how the CMU backup
is connected to the structural frame.  It also depends on which loads you
are talking about.

Obviously, you want the CMU backup wall connection to be detailed such
that out-of-plane loads (mainly wind but also seismic inertial forces) can
be "taken" into the structural steel frame and distributed to the lateral
system (moment frames, braces, shear walls, etc).  In low rise
construction, this can be done with dove tail anchors (or something
similar) that allow for the out-of-place loads to be taken into frame
while still allowing differential vertical movement between the CMU wall
and building frame.  This is common for a "stacked" CMU wall (i.e.
building is short enough that all the masonry sits on the foundation and
is stacked from there).  For higher buildings, where stacked walls are not
feasible, then the out-of-plane loads can also be taken by detailing
connections for where the masonry is supported of the frame (on the edge
of a slab, a lintel, or some such), along with some cirsumstances of using
a dove tail anchor system.

For taking in-plane lateral loads to the frame or vise-versa, this becomes
a much more complicated process for CMU backup walls.  Many times when CMU
walls are used as shear walls, they ARE NOT just backup walls but also
bearing walls.  The use of a bearing wall as a shear wall make the
conneciton details to be able to get the in-plane loads into the shear
wall much easier.  If you have a CMU backup wall that you also want to use
as a shear wall, then the connection details become much more complicated.
As others have pointed out, YOU (or someone else in your firm) need to
detail those connections such that they can take the lateral loads from
the building into the lateral system (the shear walls).  As someone else
has pointed out, this needs to be done in a fashion that still allows for
potential vertical differential movement between the frame and the
shear/backup wall, especially if the wall is stacked.

The end result is that I would recommend looking at another type of
lateral system rather than trying to use the CMU backup walls as shear
walls.  While it can be done, it will end up being a major pain the butt.

Regardless, I will offer some other advice since it seems that you might
be new to the wonderful world of masony.  Be very careful with your
details on the brick and CMU walls.  While the brick tends to "fall" under
the architects purview, you should still review some of their details.  In
particulal, pay close attention to detailing to accomidate movement of the
masonry.  Brick will tend to expand (mainly due to moisture, also due to
temperature and other factors) while the CMU will tend to shrink.  This
means that details such as joints below relief angles for the brick, below
windows, and at paraphet caps need to be reviewed to make sure that enough
movement space is provided.  If not, then best case scenarios are walls
bowing or windows sticking, while worst case scenario can be brick walls

One last little note for very careful using the title of
Structural Engineer.  Since you are an EIT, in some states you may be in
violation of the PE Act.  In some states, the PE Act is written such the
use of the title of Engineer is only permitted to licensed PEs.  And in
some states that have an SE act (California and Illinois for sure, but
also likely Washington and others), the use of the title of Structural
Engineer is reserved (right or wrong) to those with an SE license.  I
would hate for you to start off your career by getting dinged by the state
licensing board!


Ypsilanti, MI

On Mon, 21 Oct 2002, Terry Triest wrote:

> This question pertains to CMU walls used for back-up to masonry brick.
> How much capacity does CMU back-up distribute to the structural frame?
> Does anyone have information on the testing of this assembly?
> I use RAM and was wondering if (how much) I can rely on the CMU for lateral
> resistance.
> Terry Triest, EIT
> Structural Engineer
> P: 814-940-4041
> F: 814-949-2644

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