two way shear on elevated slab 1

[DanGPC]

I am trying to evaluate 2-way shear in a thin mildly reinforced elevated slab. The equations I have (see attachment) reference a soil pressure factor (R). I believe it would be correct to treat this equivalent to 0 in this case. The reference comes form Lindeburg's PE review book, as I cannot find the specific ACI reference to punching shear. Can anyone tell me whether it is correct to leave out the R factor when evaluating an elevated slab, or where ACI references punching?

Please forgive the naivete, I do not typically design slabs/footings...

 

[KBVT]

No attachment.

 

[DanGPC]

Here's the attachment...for some reason it didn't upload properly the first time.

 

[OCI]

ACI318 has 2 way shear covered. I don't know the exact section, the back of chapter 11 I think.

 

[KBVT]

Two-way shear in an elevated slab is a different animal than a footing. In the design of a footing you can reduce the amount of shear required across your two-way plane by the bearing pressure within the two-way critical perimeter. In a slab, you wouldn't reduce the shear required by the loads applied around the column. You also need to account for unbalanced moment increasing the punching shear on one side of the column.

 

[slickdeals]

"In a slab, you wouldn't reduce the shear required by the loads applied around the column"

I am intrigued. Why wouldn't you take the shear at the bo perimeter?

I agree that unbalanced moments need to be accounted for.

 

[BAretired]

I am intrigued too. The load within the critical section does not contribute to shear, so the total shear may be reduced by that amount.

BA

 

[KBVT]

Technically, you could remove the load applied within the critical shear section but how many people account for no live load within the actual column envelope. My point of not disregarding the applied load within the critical shear perimeter was not of engineering principles but of ease of calculation. When I calculate my column loads, I don't remove 4 sq ft of live load because I have a 24x24 column.

 

[BAretired]

The point is that two way shear in an elevated slab is no different than in a footing. You could deduct more than 4 square feet of both live and dead load for a 24x24 column. The area deducted would depend on the effective depth of slab.

Whether anyone bothers to do it is another matter.

BA

 

[steellion]

The punching shear equation is located in Chapter 11.12.2 in ACI 318-05. Vc = 4 * sqrt(f'c) * bo * d.

If you are calculating punching shear failure, there are several options:
-drop panels (PS make the extra slab depth = a true common wood size, the contractor will appreciate it)
-column capitals
-shear rails in the slab

 

[KBVT]

BA - agreed.

 

[KBVT]

One note on the drop panel forms, the contractor appreciates it even more if you make it a common 2x size + 3/4" for his plywood.