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RE: Two-way Shear Through Wide Beam

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Cliff,

Just a slight correction to a portion of your post.

Punching shear DOES NOT require transverse steel (i.e. stirrups, or other
shear reinforcing) nor is it limited to columns that are fully engaged on
three sides or more.

Punching shear is definitely applicable to corner columns (i.e. only two
sides fully engaged).  This can easily be verified by looking at equation
11-36 of the ACI 318-99 code and the term alpha-sub-s in that
equation...section 11.12.2.1 clearly indicates to use alpha-sub-s of 20
for corner columns.

And the vast majority of punching shear situations DON'T have transverse
(shear) reinforcing steel used, especially in low seismic zones.  The only
reinforcing present will be the longitudinal steel that is present for
flexure of the slab/footing/etc.  It has become more recognized in the
recent past that shear reinforcing steel in slabs is something that is
highly beneficial in areas of high seismicity, as the added effects to
the punching shear due to the additional moment at the joint due to a
seismic event can cause the joint to have a rather unductile response.  As
a result, there is at least one ACI committee that is working on updated
recommendations for shear reinforcement for column-slab connections in
high seismic zones.

You are correct, however, that if bar or wire tranverse (shear) steel is
used and accounted for, the Vc term is then limited to 2*sqrt(f'c)*b0*d.

HTH,

Scott
Ypsilanti, MI

On Thu, 5 Dec 2002, Cliff Schwinger wrote:

> 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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