Punching shear at edge column with slab overhang 2

[cliff234]

I have a question about how to determine the critical section for checking punching shear at edge columns where there is a slab cantilever at the edge greater than d/2. The critical section for computing punching shear is d/2 from the face of a column. If I have an edge column where the slab edge extends greater than d/2 how do I determine the geometry of the critical section? ACI 318-08 is vague on this issue. ACI 318 discusses the need to consider slab openings within 10 x slab thickness from the column but it is silent on how to treat edge columns where the slab cantilevers past the column. Conservatively I could ignore the cantilever, but I would like to take advantage of the additional strength provided by the cantilever.

Here's an example: 24"x24" edge column with a slab with d=6" and with the edge of the slab 12" from the face of the column. Can I take the critical section to the edge of the slab? Conservatively I could use a U-shaped critical section with dimensions 30" on each side, or I could use a less conservative U-shaped critical section with dimensions of 39",30",39". My gut tells me that I should be able to use the larger critical section as long as it does not exceed the strength of the critical section of an interior column.

This leads to the next question. How big does a slab cantilever at an edge column have to be to consider the punching shear critical section that of an interior column?

Thank you in advance for any insight anyone might be able to share. Are there any readily available articles or publications that discuss these questions?

 

[ron9876]

I would check both conditions that you mentioned and use the most critical.

 

[tngolfer]

I've considered the same question before. I tried to tell myself that the crack has to extend to the edge of the slab, thereby increasing my perimeter. (I'm still not convinced it isn't the case.) CSi's SAFE computer program stops the perimeter as if the slab edge is closer which will give you conservative (maybe actual) results... take that FWIW.

 

[PicoStruc]

In canadian code commentary, it's specified that you cannot account for overhanging concrete if the column exterior face is at less than 5d from the edge of the slab.

If the column exterior face is at more than 5d from the edge of the slab, it's an interior column.

See comment 13.3.3.4 of CSA A23.3-04 for edge column.

 

[dcarr82775]

The crack clearly has to extend around the exterior edge of the column and there must be some strength obtained from that failure surface. If I needed just a little more I would assume some distance beyond the column, but if I needed a fair amount of extra strength I would just put in studrails(or similar) and ignore the cantilever.

 

[PicoStruc]

Another clause (13.3.3.3) suggest a 'U' shaped critical section with an extension in the overhanging slab up to one 'd'.

 

[nac521]

Ref: ACI318-08
R11.11.6 addresses the critical section near free edges along with figure 11.11.6(d). This shows the critical section extending to the free edge of the slab. The moment transfer in slab-column connections also increases the punching shear stress (11.11.7) and is especially important if there are no perimeter beams (Fig R11.11.7.2(b)).

 

[hokie66]

As nac521 noted, the shear stress is not constant at edge columns. You have to consider the effect of the moment.

Punching shear is scary, and I want to always be on the conservative side.

 

[rapt]

You should calculate the perimeter both ways, first with the sides extending to the edge of the cantilever, and second as an internal column. The option that results in the largest shear stress controls (smallest perimeter if axial loads only). For ACI code with moments applied using ploar moments of inertia, it would be necessary to actually calculate the stresses in both cases. For other design codes that provide a way of merging axial and moment effects, the smaller perimeter controls.

 

[cliff234]

Thanks for all of the responses! Yes I am considering the moments. My concern is whether I can legitimately consider a U-shaped critical section with the vertical legs of the U extending to the slab edge beyond d/2 from the column face. The d/2 critical section permits the use of the 4 x square root f'c punching shear strength. If a portion of my critical section is outside that d/2 zone can I still use 4 x square root f'c?

I've thought about what "rapt" said. Compute both critical sections with the strength being governed by the one with the highest stresses (axial plus flexural stress). That makes sense. I wish ACI 318 was more explicit on this - especially since punching shear is such a critical limit state.

 

[rapt]

cliff234,

I think where ACI possibly falls down on the 4 root f'c rule is that Beta should be related to the dimensions of the perimiter, not the column. Then you would not need to worry as beta would allow for the effect of a longer extent by reducing the allowable stress!

Figure R11.12.5(c) shows what happens at a free edge. But you must make sure that the free edge controls by checking it as internal as I mentioned above.

 

[BAretired]

The theory is by no means clear and hokie is correct when he says that variation in shear due to moment has to be considered. I have always used an approximate method as follows: assume the load from the interior span to the center of the exterior column is doubled...then treat that load using a 30x30 critical section.

This would be tantamount to using a "U" shaped section of 15, 30, 15 with the actual load (excluding cantilever load).

The column may be considered an interior column when the cantilever moment is equal to the interior span negative moment, i.e. there is no imbalance at the column.

BA