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Punching Shear Reinforcing Cutoff Distance

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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, 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?


Will Haynes