Question on punching shear

[Lion06]

Let's say we have a really thick slab with a column on it. It is a structural slab, not a S-O-G. Let's say the column has uplift on it. The rebar is fully developed into the slab. Is there still concern of a punching shear issue for this situation? If so, would the critical perimeter be measured at d/2 to the INSIDE of the column, and not the outside?
I haven't convinved myself that it is even an issue yet, but I'm getting a little resistance to my thinking. My thinking is this. The 45 degree shear plane would have to radiate toward the center of the column, not away from it as is typical for punching shear. The reason for this is the uplift and the direction of principle tensile stresses. Because of the direction that the shear plane wants to go, it will be crossing the tension rebar, which, in my thinking, negates that failure.
I guess my thinking is similar to this. Say you have a concrete beam with a stirrup sticking out of the bottom and you pull on that stirrup with a significant force. The shear crack wants to start right at the stirrup because the shear is highest just to either side. The stirrup which is delivering the load is also acting as the shear reinforcement. The load can never be larger than the bar can support in tension, and, by default, the shear is not an issue. I guess the only difference is the difference in phi factors - tension is 0.9, shear is 0.75.

Any thoughts?

 

[msquared48]

If I understand you correctly, you are concerned about a punching shear failure in uplift. Seems to me any failure like that would have a basically conical shape with the point of the cone at the bottom of the slab, irrespective of any shear reinforcing consideration.

Mike McCann
MMC Engineering

 

[hokie66]

I would still consider the critical perimeter to be at d/2 from the face of the column. As the uplift is resisted by the column reinforcing, which is anchored near the bottom of the slab, that is where the truncated pyramid starts.

 

[beton1]

I tend to agree with Mike, that the 'tendency' or mechanics are the same. As this is a load reversal, is the reinforcing going to act as you say, or do you need something additional. I guess it depends on the loads. If it is OK in gravity direction (and it is a thick slab), how large is the uplift to warrant this concern. And it's five o'clock somewhere...

 

[Lion06]

The uplift force is around 800k. That's not a typo, it's 800k. It is a 3'-0" thick slab. Can someone explain the difference between this situation and a footing with uplift. We wouldn't check a footing for 2-way shear in the uplift case, right?

 

[msquared48]

With a load that large, why not? The direction is immaterial here.

Mike McCann
MMC Engineering

 

[Lion06]

I've never heard of anyone checking punching shear for uplift on a footing and I've never seen a text example. I guess I'm just failing to visualize the failure mode.

 

[msquared48]

What is resisting this load other than the dead weight of the footing? Soil above or a grade beam?

Mike McCann
MMC Engineering

 

[BAretired]

Each vertical bar develops its own stress cone if the bars are far enough apart. If they are not, stress cones overlap and cause a reduction in shear area available.

I have never seen such a large uplift on a footing, but I believe that anchorage should certainly be checked using the provisions of the code.

BA

 

[hokie66]

If you want to be conservative, you could use the shear perimeter as d/2 from the column bars rather than from the column face.

 

[kslee1000]

Same as checking column anchorage in tension.

 

[BAretired]

For uplift forces of that magnitude, I would not be averse to drilling a hole below the footing deep enough to develop the vertical bars in bond and with a diameter equal to the diagonal dimension of the column. Then you could use the "stub column" below for peripheral shear.

BA

 

[hokie66]

BA,

SEIT didn't say his slab is a footing. He introduced the footing into the discussion as a comparison.

 

[kslee1000]

Same principle. The cone is the limit that may develop stress required to resist the tension.

 

[BAretired]

hokie,

Sorry, chief I got carried away. I suppose another approach is to weld the vertical steel to a baseplate within the slab. It can be made as large as you like in order to satisfy the punching shear criteria.

BA

 

[Robbiee]

One of the differences between a beam hanging from a column and a beam sitting on a column is that the critical shear is at the face of the column for the first case and at d/2 from the face of the column for the latter case. Your case is similar to the first case and the critical shear is at the perimeter of the column.

 

[Lion06]

I am asking how the breakout cone forms. Doesn't satisfying the development length of the bars into the slab preclude any breakout failure?

 

[hokie66]

Ailmar,

For a beam sitting on a column, most codes actually allow consideration of the shear at d from the column face, provided that "diagonal cracking cannot take place at the support or extend into it" (AS3600).

However, the OP's question relates to punching shear of a slab around a column, which is a different situation. For the column in compression, the design model in most codes provides for taking the critical perimeter at the averaged d/2 from the face, which is a simplification of the truncated pyramid section. For the column in tension, the codes are not as specific. That is why differing opinions have been expressed here.

 

[slickdeals]

Maybe I am not understanding this rather clearly, but how is this different from a footing subject to uplift or a anchor rod in tension?

Would it not be a truncated pyramid like failure?

 

[kslee1000]

For uplift of such magnitude, once the breakout stress exceeds concrete tensile strength (which does not have much)along the column foot print, I think drop pannel, or capital, should be utilized to develop the column reinforcment, considerations are similar to anchorge design/check. Also, additional reinforcing in the form of embeded beams would enlarge shear resisting area around column-slab interface.

Just curious, what causes such uplift. Say the column has a tributary area of 400 sf, it results in 2 ksf uniform pressure, an intensity even rarely seen even in gravity load applications.