# Punching Shear on Flat Slabs, Columns Drop Panels, and the use of d.



## PowerStroke79_PE (Oct 10, 2017)

Im a little confused on the use of d for calculating the punching shear for flat slabs @ columns. If we have a drop panel, what is the use of the extra drop assuming it is less or larger than the slab thickness? Is it to only find the required shear? How would the drop panel "d" be used for finding Vc? In using the controlling equation for Vc, the "d" used in it will ALWAYS be "d" from the slab? And never "d2" per say or "d1+d2"?  How about for bo (critical perimeter)? Do we always use "d" from the slab? 

Any help is appreciated. 

Thanks,


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## TheStructuralEngineer (Oct 11, 2017)

You use drop panel "d" while calculating punching shear for critical section at distance d/2 from the edge of column and you use slab thickness "d" while calculating punching shear for critical section at edge of drop panel?
Please see design example at https://structurepoint.org/pdfs/Two-Way-Concrete-Slab-Floor-With-Drop-Panels-Design-Detailing.pdf . 
Thanks.


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## PowerStroke79_PE (Oct 12, 2017)

TheStructuralEngineer said:


> You use drop panel "d" while calculating punching shear for critical section at distance d/2 from the edge of column and you use slab thickness "d" while calculating punching shear for critical section at edge of drop panel?
> Please see design example at https://structurepoint.org/pdfs/Two-Way-Concrete-Slab-Floor-With-Drop-Panels-Design-Detailing.pdf .
> Thanks.


But in every case, the critical section at column will control wont it? 

In calculating the controlling terms for two way shear, which is

1.(2+(4/beta))   

2. (2+ ((alpha*d)/bo)   

3. 4 

Do we ever use d2 at all? which is the depth of the slab + depth of drop panel. Is it solely used for critical section of drop panel which proves most useful when finding the allowable shear force on the column?


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## TheStructuralEngineer (Oct 16, 2017)

Hemi79 said:


> But in every case, the critical section at column will control wont it?
> 
> In calculating the controlling terms for two way shear, which is
> 
> ...


If you see page 9 of the attached example, you will see they use 12.75" (d @ drop panel) instead of 8.5" (d @ slab). It just happens they are using d as notation for both values.


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