ACI 318-19 8.6.1.2 Asmin Punching Shear

[bookowski]

Can anyone point me to a worked example of calculating the min concentrated steel for punching required in 318-19 per 8.6.1.2. Is the intent to provide Asmin at each face (for each bslab direction), or is this the total? The commentary does not clarify although it does explain the origins of the derivation so I could maybe work it out, but clarity would be useful.

 

[bookowski]

Bumping my own post from May. Does anyone happen to have a worked example/reference of this 318-19 calc that clarifies the intent of Asmin per my previous post?

 

[Celt83]

It is intended to be As,min crossing each vertical face of critical section width, the check only considers the direct shear stress from the vertical column reaction so uniform stress on the perimeter (no moment transfer). Have been finding drop panels at transfer slab conditions need to get a little bigger to avoid needing (40) bars to satisfy this requirement at the face of drop, last sentence of the commentary says the check applies at the drop face. The Bo Bslab term tends to get huge at the drop face.

 

[KootK]

I would have to think that it would be referring to only the rebar on the tension side of the slab. Rebar on the compression side, while providing some measure of dowel action, would actually serve to prevent clamping of the shearing surface in the compression zone.

 

[bookowski]

Yes, definitely on the top. By each face I meant each of the 4 column faces.

Being lazy here not going to attach an image. Let's take: 23ft sq. grid of 8" slab, 15psf superdead, 40 live = wu = 202psf gives a Vu = 107k. Take a 16x28 column, all f'c = 5ksi. Bo of 115" gives a vu = 138 psi which exceeds the limit.

Based on the above, Asmin = 1.8 sq. in within the reduced column strip. This would be (6)-#5 top bars within a reduced strip = 40 inches (16" + 2 x 1.5 x 8") in one direction and (6)-#5 over 52 inches in the other.

I'm just making sure that I am understanding the intent correctly. Trying to automate some of this and making sure I'm getting this new provision down and compare it to what we've been doing. Based on some examples like this it looks like we'll need to revise how we concentrate bars to be a bit tighter.

 

[Celt83]

A_s,min should be calculated for each direction so for:
b_slab = 40 in --> A_s,min = 1.763 sq. in or (6)#5
b_slab = 52 in --> A_s,min = 2.292 sq. in or (8)#5

A_s,min = (5 v_uv b_slab b_o) / (Φ_v α_s f_y) [ACI 318-19, eq. 8.6.1.2]

 

[bookowski]

Right, thanks Celt. Was doing that as I typed it in here so missed adjusting the 2nd direction. I've got it. Not to try and automate the checks. Thanks.

 

[Celt83]

just be careful when drop panels start getting into the mix, I think the committee didn't put enough effort into the specifics for this formula as well as the new shear formulas so certain conditions can balloon the As,min requirement.