Punching shear at edge column with slab overhang
Punching shear at edge column with slab overhang
(OP)
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?
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?






RE: Punching shear at edge column with slab overhang
RE: Punching shear at edge column with slab overhang
RE: Punching shear at edge column with slab overhang
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.
RE: Punching shear at edge column with slab overhang
RE: Punching shear at edge column with slab overhang
RE: Punching shear at edge column with slab overhang
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)).
RE: Punching shear at edge column with slab overhang
Punching shear is scary, and I want to always be on the conservative side.
RE: Punching shear at edge column with slab overhang
RE: Punching shear at edge column with slab overhang
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.
RE: Punching shear at edge column with slab overhang
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.
RE: Punching shear at edge column with slab overhang
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