punching shear
punching shear
(OP)
I am designing a concrete pad to support purely vertical load. I only have 500mm thick pad available and it will fail in punching shear. Can I just stacked 2-500mm thick pad to double the effective depth? One pad will be on top of another but they are connected. thanks.






RE: punching shear
BA
RE: punching shear
I'm assuming you are limited because they are going with pre-cast pads? I always reject that idea, the soil prep never leaves a matching surface to the bottom of the pad and so you end up with stress concentrations. Why don't you pour them in place? Or specify to the pre-caster that they need to include some reinforcing to prevent the punching shear (although more depth is always a more effective and more economical option)
RE: punching shear
DaveAtkins
RE: punching shear
Think of a press pushing a 1 inch dia solid rod into concrete. The failure is at the edges of that rod coming in contact with the circumference of that circle. (This is, after all, exactly how a punch gets pushed through a steel plate to make a hole.)
For concrete, you've got to explain (draw a sketch!) how that shear load gets loaded onto the upper concrete surface and find a way to spread it out uniformly across more contact area. Once the surface has cracked through, the adding more depth to the slab won't prevent failure.
RE: punching shear
RE: punching shear
Check for the punching shear capacity of the upper 500 mm slab based on the initial load application (base plate, etc)
If it works then the lower slab isn't required.
If it fails, then I would assume that the "punched" out portion of the upper slab would be larger than the uppermost base plate. i.e. the slab would fail in a truncated pyramid form where the load application on the lower slab would be over a bit larger area. This might then work as you have a higher perimeter shear area (bo in ACI).
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RE: punching shear
I am, however, interested in this from a theoretical standpoint. I don't think that it is correct to assume that the two pads act as one monolithic pad unless horizontal shear transfer is established between the two pads (bonding agent, dowels, etc). The two pads should not be considered composite for shear unless they are also composite for flexure. I think that shear and flexure assumptions go hand in hand here. That being said, the answer that I'm going to pitch is basically P/2 to each slab with some fudge to account for uncertainty -- same as you'd get my assuming monolithic construction save some differences in how "d" is determined.
To visualize this, I considered the two pads separated by hinged rigid links. It make's things clearer for me.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: punching shear
RE: punching shear
From the standpoint of moment and one way shear, the pads must be considered as separate entities, each with an effective depth of less than 500 mm.
BA
RE: punching shear
RE: punching shear
BA
RE: punching shear
RE: punching shear