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Re: Punching Shear Reinforcing Cutoff Distance

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Kevin,

This reduction is technically correct if the floor loading is a pure UDL as is assumed for design.

In actual fact the floor loading is never uniform. There are often concentrations of load in one area and no loads in other areas. One of there concentrations of load could be immediately outside the perimeter with no load within the perimeter. For this reason, unless I had a load that I know was fixed within the perimeter (say a column load from above) then I would not do the reduction as you suggest.


At 01:48 PM 15/03/2007, you wrote:
Will, I agree with you.  The shear calcs must take account of the moment.  Actually, I wonder why the examples you will find in most concrete manuals don't also take account of the floor loads (dead and live) that fall within the shear perimeter where you want to discontinue the shear reinforcement.  Probably because of the added complexity.  But once you develop the spreadsheet to take account of these loads, (they reduce the calculated shear at the perimeter), it is no longer complex.

The shear around the perimeter is basically caused by the floor loads over the tributary area around the column (plus the moment in the column).  For the calcs at d/2 from the face of the column, virtually all of that floor load causes shear at the perimeter.  But if you extend that perimeter to some distance from the column, then some of that floor load is now within the shear perimeter, and is not transmitted to the column by shear at the perimeter.

KB


On 3/12/07, Will Haynes <gtg740p(--nospam--at)gmail.com> wrote:
I have a 2-way slab that I am designing for punching shear that will require shear reinforcing. I am looking at using stud rails or stirrups and need to determine the distance which the shear reinforcing may be discontinued.

In ACI 318-05 section 11.12.6.2(b), it states that "The shear stress due to factored shear force AND MOMENT shall not exceed phi*2*sqrt(f'c) at the critical section located d/2 outside the outermost line of stirrup legs that surround the column". The termination distance calculated in the example from the document "Shear Reinforcing for Slabs"-ACI 421.1 R-99 appears to follow the above statement by combining moment and shear stress beyond the column to find the shear reinforcing cutoff location.

However, I have a copy of PCA notes on ACI 318-99 and on page 18-35, there is an example where the stirrup termination distance is determined by the direct shear only, and ignores the shear produced by the moment! It appears to me that PCA has it wrong here, but I was wondering if there is something I am missing?

thanks

Will Haynes

Regards  Gil Brock
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