Upside Down Drop Panel - Retrofit?

[bookowski]

I have a 6" roof slab, flat plate from the 70's. Due to a new slab opening and some increases in loading I have a column that does not meet punching shear requirements. There is no access to reinforce from below but I have complete access from above.

I have seen that Hilti has a post installed shear reinforcing system but they do not sell it in the US.

Has anyone done or seen a post installed upside down drop panel? We would roughen the existing slab. Then install some type of reinforcing, possibly drilled and epoxied diagonal bars... not sure of this detail yet. We would then pour a rectangular "drop" on top of the slab.

I have several concerns about doing this but my main one is developing rebar from the new upper panel into the existing slab.

Any thoughts on this?

 

[Ron]

Offhand, can't think of a reason that it wouldn't work (assuming it doesn't greatly affect your moment reversal in bending...shouldn't but check anyway), though I would probably make it larger than necessary to prevent influence from edge effects or debonding.

If the hole is the only reason for the problem, why not reinforce the face of the hole with channel or other?

 

[bookowski]

That's another argument that we've been having. I don't see how reinforcing the face of an opening with steel actually helps. Punching shear checks are based on not allowing you to exceed a max allowable stress at any point around the shear perimeter. Adding steel around an opening would not necessarily do anything to assure this. That's a different question though.

Back to the first question - what about developing the bars in the upper "drop" into the original lower slab?

 

[Ron]

Lap length would need to be longer and I would dowel in as well. Would be great if you could dowel at angle for development, but hard to do.

 

[hokie66]

You may have seen this discussion about an upside down drop panel.

thread507-307810

This was for new construction, so the upstand to be cast monolithically. Developing the horizontal shear in a relatively small area between the new and old is problematic.

 

[bookowski]

Yeah saw that. For a monolithic pour it seems reasonable. I'm not completely sold that I can do it as a retrofit though. Hilti has a post installed system of diagonal reinforcement (Hilti HZA-P) but it doesn't seem to be used in the US. Part of my problem is that I only have access to the area from above.

 

[hokie66]

I would lean toward assuming the new cap and the existing slab as non-composite. You indicated that the problem is solely punching shear, so you could make the new cap thick enough on its own, then "hang" the existing off the new cap. The design of the hanging bars and the development thereof would then be your main issue.

 

[Ron]

hokie66...I agree with your idea; however, it will still be exhibit composite action since the new will be doweled to the old to "hang" the old.

Slabs don't have to be "concrete bonded" to exhibit composite action...the doweling will do that....but then, composite action in this case, though not really required, won't hurt either.

 

[BAretired]

I agree with the concept of assuming the new cap to be non-composite. Any composite action you do get, as Ron said will not hurt, in fact it will help.

BA