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punching shear

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

Connected how? What is the magnitude and area of the applied load? Provide a sketch.

BA

RE: punching shear

No. Unless you are going to pour them monolithically (i.e a 1000mm thick pad) then they act as two separate entities.

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

I think for punching shear, (2) 500mm pads are equal to (1) 1000 mm pad.

DaveAtkins

RE: punching shear

Punching shear breaks at the smallest (initial) contact area, continues sequentially until the substance is through the whole plate, and only theoretically cuts through the entire substance in one "chunk" ..

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

I fear of stress concentrations developing both between the bottom block and the soil and at the interface between the two blocks. There's bound to be som imperfections in the faces of the blocks and the excavation base. I just don't like it, if he's got the kind of forces that require a pad larger than 20" deep than I would be mandating it as cast-in-place.

RE: punching shear

I might look at it this way:

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

If it were me, I would find a way to spread the load out through a base plate, cribbing, a pedestal, or JAE's method. Just not worth the time to do anything fancier.

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

(OP)
hi! Correction on the statement above:the pads are "not" connected. Thanks for the replies. Yes they are precast concrete. But the are already available. So I don't have the option to pour it as one. Magnitude of load is 5700kN on a 6mx4mx500mm concrete pad.

RE: punching shear

The behavior is somewhat dependent on how well the two pads fit together. If the two abutting pads fit snugly together, the failure surface would be the truncated pyramid described by JAE. Even with minor deviations in the surfaces, the behavior would be essentially the same; the lower pad benefits from the larger perimeter provided by the upper pad.

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

So stack the two on top of each other, but apply a layer of grout between the two to assure 100% contact.

RE: punching shear

Grout is a good idea if it can be done properly. I considered grout, but 6m x 4m is a pretty big area to get a good grout job; and it can't be inspected when it's finished. If the surfaces appear pretty flat, I think I would be inclined to skip the grout. A thin coating of sprayed or brushed on tar might help to smooth our minor roughness.

BA

RE: punching shear

I wouldn't add the strength of the two panels, I would look at the bottom panel only assuming the larger critical perimeter obtained from teh bottom of the top panel. I think this is what JAE suggests.

RE: punching shear

Not sure why I didn't mention it before, if they're pre-cast why not talk to the manufacturer. 6mx4mx0.5m seems like an extremely large pad with not much depth. What is in them for reinforcing? I would be inclined to think that flexure and one way shear would begin to govern at the sizes you are talking about.

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