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Re: Reinforcing Masonry Shear Walls

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Nels,

You are correct that the UBC and the MSJC don't really "break" appart the
contribution from masonry and reinforcing when you design by WSD.
Basically, if you want the masonry to take it you design it as
unreinforced which requires meeting one set of allowable values.  If you
want other than the just the masonry to take it, then you design as
reinforced and have a different set of allowable stress values.  This
values for reinforced walls in the code are not a function of the the
level of reinforcing, IF you provide the minimum reinforcing required by
code (but if below that, then you would use the unreinforced values).  In
otherwords, if you exceed the minimum reinforcement requirement outlined
in the WSD method, you get no benefit.  I would assume that the allowble
values for reinforced wall contain contributions from the reinforcement
_AND_ the masonry...it is just "smushed" together.

Now, when Michael gave his suggestion to use strength design, I have admit
that at first I kind of thought "Here goes a wise guy (PG version of what
I thought) that is pushing that new fangled stuff" (and I am still a young
punk...but one that has been brought up on WSD in masonry at least, which
is really true for just about everyone since Strength design in masonry is
still in its infancy).  BUT, then I looked at the codes (1997 UBC and 2002
MSJC), and HE IS RIGHT ON THE MONEY FOR A RECOMMENDATION.  The strength
provisions in the 1997 UBC and the 2002 MSJC (which are basically the UBC
provisions that were taken and modified in the 2000 IBC which were then
taken and modified to become the MSJC provisions) DO in fact break the
contribution of the masonry and reinforcing similar to what is done (in
theory at least) in concrete.  Take a look at section 2108.2.5 of the 97
UBC and, in particular, section 2108.2.5.5.  You determine the nominal
shear capacity which is Vm + Vs and then it goes on to specify what Vm and
Vs are.

HTH,

Scott
Yspilanti, MI


On Thu, 28 Aug 2003, Nels Roselund, SE wrote:

> The 1997 UBC rules for reinforced masonry shear walls provide allowable
> shear values for two conditions: masonry designed to resist all of the
> shear; reinforcing steel designed to resist all of the shear.  Rules for
> adding the masonry capacity  and the reinforcing capacity are not provided.
> What is the reason for this?
>
> I can imagine that the rules for designing reinforced walls include, hidden
> in the numbers, the masonry capacity; or else, there was uncertainty in the
> minds of the code writers concerning how to add the capacities.
>
> I am designing the strengthening of existing unreinforced masonry shear
> walls.  Based on testing, I can establish the shear capacity of a URM wall
> on this project -- it is not enough, so I want to supplement  its shear
> capacity with reinforcing.  In the past, I have done this by applying
> portland cement stucco with welded wire mesh reinforcing, based on some
> research done in the 1960's for strengthening URM public school buildings in
> Los Angeles.  My current project is on a historic building for which it is
> desirable to avoid adding finishes to the walls.  For that reason, I am
> looking into the Cintec system, which installs steel rods into the walls,
> embedded in cementitious grout injected into holes that are drilled the
> length of the walls.  Every piece of reinforcing is a big deal because of
> the drilling operation, so I want to do whatever is reasonable to minimize
> the amount of  reinforcing that needs to be installed.
>
> What will be sacrificed by adding the shear strengths of the masonry and the
> reinforcing?
>
> Nels Roselund
> Structural Engineer
> South San Gabriel, CA
> njineer(--nospam--at)att.net
>




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