You can use it but I think the width of the drop panel (and not the overall column strip width) is what I would use as the width of the "beam" in determining [φ]Vc.
Eventhough, this requirement is used to calculate the reinforcement at the drop panel, but should also be used to calculate the shear capacity of the section. And I am referring to ACI-318, 13.3.7.3.
Don't you check column+middle strip (L2) width against shear and not just the column strip?
I tried finding an ACI provision for this and as usual I could not. I agree the drop attracts more load, but if the one-way shear worked before the drop was added, it seems like it would work after. Meaning that if you just use the width and depth of the drop and shear doesn't work, but works for the entire trib without the drop, then I would THINK it works anyway. (I don't see why adding more concrete and "d" at part of the slab would penalize your overall shear capacity).
I guess it could be that it fails first at the drop and then dynamically at the rest of the slab trib. So maybe you should stick with just the drop contribution.
haynewp - this is similar to a shear question that we once tossed around - about concrete pan joists.
For pan joists, the individual webs have tapered sides (1:12) so the question arose as to what was the "b" in the shear equation.....the width at the bottom, at the top, or the average? Also - would you count on any of the flanges as contributing to the shear area which defines [φ]Vc?
We used the average web width for "b" but never considered the flange as helping as it "seemed" much too flexible in proportion to the main web.
Similarly, a two way flat slab with drop panels (is that redundant?) also sort of has a flange (the slab beyond the drop panel) and a web (the concrete within the width of the drop panel) - just the difference in relative rigidities isn't as pronounced as in my pan joist example above.
I guess you could check [φ]Vc based on the shallower d and full width of column strip and then also check it based on the deeper d (which would include the drop panel depth) and the limited width of the drop panel - and use the lower value?
In any case, I think using just the drop panel width and depth seems more rational.
This same thought came to me this morning on the way to work. You wouldn't count the flanges for a T-beam as resisting shear so you shouldn't count the "flanges?" of the rest of the slab outside the drop panel to resist shear either.
