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Re: Two-way Shear Through Wide Beam

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Michael,

I would likely only account for Vc and ingnore the effect of Vs.

If you really want/need the effect of Vs, however, then I would say that
you need to determine if the stirrups will really be in a "position" to be
effective.  By this, I mean...how big is the load application when compared
to the wide beam? When compared to the stirrups?  In otherwords, take a
look at where your "failure" plane of the punching shear "cone"/truncated
pyramid intersects the stirrups and then determine if there is enough of
the stirrup steel still outside of the area of "punched" concrete to fully
allow the stirrups to be developed.  That is...if the load application
area is bigger than the beam in a direction or the same size in a
direction or just a little bit smaller in a direction, then there would
not be enough of the stirrup left in the "good" concrete (i.e. outside the
punching zone) for the steel to get developed allowing the steel to become
fully or even partially effective.  The point is that there must be enough
of the steel bar embedded outside of the punching zone for an adequate
development length so that the bar can actually do its "work".  If not,
then the bar is not really going to do much.  (See also the first line of
the commentary for section 11.12.3 of ACI 318-99)

You should take a look at section 11.12.3 of the ACI 318-99 code.  This
section deals with the use of shear reinforcement of bars and wires for
punching shear.  If you look at it, you will first notice that you may not
get too much benefit from the steel reinforcement.  The maximum Vn can be
with such reinforcement is 6*sqrt(f'c)*b0*d, while the maximum Vn can be
without such reinforcement is 4*sqrt(f'c)*b0*d.  Thus, at best, you can
only really get a 50% increase.  But then also notice that section
11.12.3.1 limits the Vc portion of that 6*sqrt(f'c)*b0*d to only
2*sqrt(f'c)*b0*d.  Thus, to really get to the maximum Vn with reinforcing,
your reinforcing needs to contribute the other 4*sqrt(f'c).  The end
result is that more than likely you are just better off not bothering with
trying to allow for the stirrups to contribute, especially since it is
rather unlikely that they would be anchored well enough to really do much.

HTH,

Scott
Ypsilanti, MI

On Wed, 4 Dec 2002, Michael Bryson wrote:

>
> Quick question: if I want to check punching shear through a wide beam with
> stirrups, can I count on the stirrup reinforcing to fully contribute to the
> punching shear strength Vn=Vc+Vs ? Or should I only consider one-half of Vs,
> or zero of Vs ? I am guessing use one-half of Vs is reasonable.
>
>
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