waffle slab drop panel allowable shear

[wubwub]

I am currently analyzing an existing two-way waffle slab for the proposed installation of new equipment and am running into a problem on how to calculate teh shear stresses at the drop panel face. We are having a debate on how to calculate the allowable shear at the face of the drop panels in a two-way waffle slab. Some are saying that the allowable shear would be phi*2*sqrt(f'c)*b*d where b would equal the width of a rib times the number of ribs framing into the drop panel. Others are syaing it should be treated as "punching shear and the equation should be phi*4*sqrt(f'c)*bo*d. Essentially this would give twice the allowabel shear. I know ADOSS uses the equation with the "2", not the "4", but when the equation with a factor of "2" is used, the slab seems to exceed it's shear capacity by about 20% with just the original loads applied.

In both cases, I have multiplied the allowable shear by 1.1 per ACI since it is two-way joist construction.

I was wondering if anyone had thoughts no the matter and does anyone have a definitive source on what the allowable shear at the drop panel face should be. Unfortunately our sources in our office to not address this issue.

Thanks in advance to anyone who responds.

 

[abquad]

The ADOSS calculation of shear for drop down panel is right.