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Re: Reinforcing Masonry Shear Walls

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Roger,

To my understanding of the code provisions and theory for concrete, this
is not quite true.  The Vc component DOES NOT go to zero when the concrete
cracks in diagonal tension, but it IS limited.  You still get a component
due to the concrete that is non-zero due to things like aggregate
interlock at the crack interface (unlike flexural conditions where you are
basically purely transfering a tensile force so the concrete does not
contribute even if there is some aggregate interlock).  You will also get
some concrete resistance from the portion of the section that DOES NOT
crack (for shear considerations, the entire cross-section rarely has a
crack that completely crosses the depth of the section...take a beam, it
cracks up to the neutral axis, so you still have some uncracked concrete
above the neutral axis that will provide "pure" concrete shear
capacity...this is similar circumstances in shear wall).

The end result in concrete design the Vc component of Vn need not be taken
as zero (you can if you want to be ultra-conservative) and it is typically
defined as 2*sqrt(f'c)*b*d and can be greater if the member is subject to
compression (either by prestressing or "naturally" occuring forces due to
the structural system and loading).

Now, I would presume that masony would be able to operate under similar
principles because to a large degree masonry is more less like concrete
(there ARE differences, but they are VERY similar).  Thus, the strength
provisions for masonry reflect a similar philosophy.

Regards,

Scott
Ypsilanti, MI


On Thu, 28 Aug 2003, Roger Turk wrote:

> Scott Maxwell wrote:
>
> . > You determine the nominal shear capacity which is Vm + Vs and then it
> . > goes on to specify what Vm and Vs are.
>
> But, in masonry, as in concrete, we are really dealing with diagonal tension,
> not shear, and once masonry/concrete cracks, Vm (or Vc) = 0.  So, after first
> cracking only the steel can resist shear (diagonal tension) whether we are
> designing in WSD or USD.
>
> A. Roger Turk, P.E.(Structural)
> Tucson, Arizona
>
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