Punching shear on slab

Question

Punching shear on slab

johndeng (Structural)

(OP)

1 Oct 24 15:51

Just the questions in my mind which could be simple to your talents, but I want to know it clearly myself. Thanks
For example, the punching load on top of 8" slab,
1. if the slab horizontal reinforcement is 2" from top, what's the d value.
2. if the slab horizontal reinforcement is 6" from top, what's the d value.
3. if the slab has no reinforcement, what's the d value.
4. if the column only take moment, thru bolts with top and bottom plate on slab, (2) bolts in tension, (2)bolts in compression, what's the punching shear load? is the cross section of punching shear still d/2 from the plate?

BulbTheBuilder (Structural) · Answer 1 · 2024-10-01 21:23:55

I am curious to see what others type. For "d", I use effective depth to the centroid of reinforcement. Some argue is it conservative, I only think that's a more appropriate approach.

I am using #5 for these example
1. 8"(thickness)-2"(cover)-0.625"(mid-point of 2#5 each way) = 5.375"
2. 8"(thickness)-6"(cover)-0.625"(mid-point of 2#5 each way) = 1.375"
3. I have not done anything on plain concrete before but check ACI chapter 13. They have more info on that
4. Punching shear stress is due to both "vertical shear" and slab moment resisted by column.

Yes, d/2 from the face of column not plate, Ref fig 8.4.4.2.3 in ACI

JoshPlumSE (Structural) · Answer 2 · 2024-10-01 21:31:07

I'll emphasize that it depends on the direction of flexure. If (as for an elevated slab) the top of the slab is in tension in the location of the punching load, then I agree with BulbTheBuilder (great UserName!).

However, if you have slab on grade where the bottom of the slab is in tension, then I believe the calculation would be the opposite.

BulbTheBuilder (Structural) · Answer 3 · 2024-10-02 14:11:43

Note the direction of the flexure JoshPlum mentioned; I only considered elevated slab. Thanks, JoshPlum!

johndeng (Structural) · Answer 4 · 2024-10-02 15:22:55

@JoshPlumSE @BulbTheBuilder
Thanks! Just want to be on the same page, assume the post is on the top of the slab, tension is on the bottom.
I am still confused.

BulbTheBuilder (Structural) · Answer 5 · 2024-10-02 15:34:29

RE: Punching shear on slab

BulbTheBuilder (Structural) 2 Oct 24 15:34

Think of it like a footing/foundation

johndeng (Structural) · Answer 6 · 2024-10-02 19:24:09

@BulbTheBuilder
Got you! so your numbers are base on the 1st picture.

For the 1st picture, if the column only has moment, what's the punching load? my case is a post with 4 thru bolts (Top & bottom plates), not a concrete column

BulbTheBuilder (Structural) · Answer 7 · 2024-10-02 22:35:32

https://help.idecad.com/ideCAD/punching-shear-desi...

This is a good reference.

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Punching shear on slab