Punching shear check of mat when thickness changes away from column 2

[NewbieInSE]

Hello Dear Engineers,
Would you check the mat punching shear and one way shear (if governs) at the locations where thickness becomes the minimum?
Pls see the attached figure for understanding my question.

The figure is taken from previous projects.

Thanks a lot.

 

[lexpatrie]

It depends, I'd say, on how you sized the thickened portion, if the thickened portion is sized so that the remainder of the slab is unnecessary, then shear at that interface wouldn't be needed to be safe?

 

[JoshPlumSE]

I think you check punching at both. You've got a d/2 perimeter from the face of the column based on the larger thickness (4'4"). Then you've got a second punching shear perimeter around the "drop slab" area. This would be based on the d/2 of the thinner slab (3').

I know that this is a matt foundation with a thickened area. But, it's pretty close to the behavior of an elevated slab with drop panels at the columns. I'm sure there are some examples for drop panel slabs that you could look up to give yourself a little more confidence about how you should be doing it.

 

[NewbieInSE]

The thickened portion is sized for two considerations
1. Make punching shear ok at d/2 distance away from colukn face
2. Make flexural reinforcement reasonable.

Let's say the shear force is 800 kips. when the punching perimeter is at d/2 from column face into the thickned portion,the punching shear capacity is 900 kips. But at the section of reduced depth, the punching shear capacity is 600 kips considering the higher pumching perimeter but lower depth.
To me, it seems the reduced thickness is nkt enough thick for punching.
Am I right in my understanding?

 

[NewbieInSE]

Thanks lexpatrie and JoshPlumSE.
I think I'll check a problem of flat slab with drop panel. You are right.

Please note that the data I mentioned are not mathematically based on the figure I posted.