dv for 2-way slab punching shear check at column

[ajk1]

"d" in Canadian Standard CSA A23.3 is the effective depth "d", but CSA A23.3 seems to be unclear as to what "d" for shear should be when dealing with 2-way flat slab slab "punching shear" at a column support. For punching shear I have always taken it as the average "d" for the bars in both directions, but perhaps some engineers use a more conservative approach and take d to the top lower layer of bars, rather than the average d for top lower layer and top upper layer bars.

Does anyone know this may be clarified in either CSA A23.3 (or ACI 318), that I may be missing?

 

[ajk1]

I see the answer to my own question, in the worked example in the Canadian Concrete Design Handbook, where they appear to use the average "d".

 

[Agent666]

If its got significant moment in addition to the axial load I usually use the d that corresponds to the moments direction if its less than the average, as the stress carried by the sides is less critical in this case. No real basis for this, but it feels a conservative approach.

 

[ajk1]

to Agent666 - ok, that seems logical. Thanks.

 

[Dowels93]

I can't find the original source with a quick look, but MacGregor & Bartlett's "Reinforced Concrete Mechanics and Design" (1st Can. Ed.) states that ACI-ASCE based their clause on Johannes Moe's work, but used a d/2 simplification where d was the average effective depth of the slab.

Based on extensive tests, Moe concluded that the critical section for shear was located at the surface of the column. ACI-ASCE Committee 326 (now 445) accepted Moe's conclusions, but showed that a much simpler design equation could be derived by considering a critical section located at d/2 away from the face of the column, where d is the average effective depth of the slab.

Johannes Moe paper: Shearing strength of reinforced concrete slabs and footings under concentrated loads. Development bulletin D47, Portland Cement Association. April 1961.
ACI-ASCE reference: Shear and Diagonal Tension, Slabs. ACI Journal, Proceedings, Vol 59, No 3, March 1962, pg 353-396.

MacGregor and Barlett present a truss model analogy for it, which shows that d in both directions would affect the capacity (through top bar tension ties and diagonal struts with a force that varies depending on d to the top bar), but this likely comes close to averaging out over a square/small aspect ratio column, although it may be less applicable with large aspect ratio and edge columns. I'll dig deeper later as well!

 

[ajk1]

to Dowels 93 - thank you for the info. Interesting.

 

[BAretired]

When top bars are specified to have the usual 3/4" cover in a slab, they are frequently placed 1/2" lower, so it may be prudent to use the lesser value of 'd'.

BA

 

[ajk1]

Thanks.