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RE: Two-way Shear Through Wide Beam

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There is no single answer to your question and the correct answer
depends on the specific geometry of your condition.

Here is a generic description of the approach that I've sometimes taken:

If my beam is a lot wider than the column into which it frames I add
additional cantilevered "in beam" brackets (or corbels) perpendicular to
the main beam that poke off each side of the column. This creates sort
of a T-bracket configuration to which the main beam connects. If you
have a beam that is 7 feet wide that is framing into a column that is 2
feet wide, then the cantilevered beams poking out of each side of the
column are each cantilevering 2.5 feet (2.5 + 2.5 + 2 = 7 feet beam
width). If you have a beam reaction of 210 kips, then the uniform load
on each of the two cantilevered beams would be 210k / 7 ft = 30 k/ft.
Design the cantilevering beams/brackets/corbels for that load plus the
appropriate torsion due to moment transfer into the column. Make sure
you detail the reinforcing steel in your brackets to properly develop.
The exact details of the rebar would vary depending on the geometry of
your framing.

Be careful how you approach this issue. Your question used the term
"punching shear". Punching shear implies two-way action and implies that
you are trying to use Vc=4*(f'c)**0.5. You must have transverse
reinforcing steel and you must have a beam to column connection that is
engaging 3 full sides of the column before you can even consider the use
of "punching shear". I always use the more conservative Vc=2*(f'c)**0.5
when checking shear at these conditions. Also, I believe (I am sitting
in my basement at home snow bound, and I don't have my ACI 318 handy)
that ACI 318 requires you to use Vc = 2*(f'c) as soon as you add shear
reinforcement for 2-way punching shear.

Also, depending on which part of the country your project is located in,
there is a good chance that the additional "corbel-type" reinforcing
steel may not be detailed by the rebar detailer, and if it is detailed
on the reinforcing steel PLACING DRAWINGS, it still may not be properly
placed in the field unless it is CLEARLY AND PRECISELY DETAILED ON THOSE
PLACING DRAWINGS. This is because the additional rebar we are talking
about here is not your "typical stuff". Of course, there will be an
increased probability that this additional reinforcing steel will be
properly detailed and installed if it is clearly and properly shown on
the contract documents. (You can read between the lines as to some of
the issues that have been irritating me with regards to cast-in-place
concrete construction lately...any thoughts or comments on this "salvo"
that I just fired off?)

Cliff Schwinger


> 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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