Slab-On-Grade thickening to support new concentrated loads 3

[jochav52802]

Good Day!

I'd appreciate some help with the following topic.

I've got an existing 6" slab that needs to support some new large concentrated moments and vertical loads. Unfortunately, the slab is not thick enough to provide adequate post-installed anchor embedment to resist the loads. As a result, I've proposed to demo an area of the slab so that it can be thickened to a depth sufficient to provide enough anchor embedment, while also utilizing dowels to maintain the slab's prior moment capacity/continuity. Please see my attached PDF sketch for my approach.

Questions:
1) Does this approach make sense, and is it aligned with common practice for similar situations?
2) After the existing slab area is demoed, will specifying the remaining perimeter area of concrete to be roughened to 1/4" amplitude be enough to re-engage the one-way & two-way shear capacity of the slab, or will the new interface create a weak-spot that I won't be able to overcome?

As a side note, if you're wondering why my sketch is showing hairpins/anchor reinforcement, our loads were so high that Section R17.2 of ACI 318-14's requirement to provide a rigid base plate was leading to an unreasonably thick base plate. As a result, we chose to bypass that requirement by providing anchor reinforcement/hairpins, in order to eliminate the brittle concrete breakout cone load path and replace it with a ductile anchor reinforcement load path, (we sized the base plate sufficiently to keep it's stress below allowable.) I confirmed with ACI's technical support that this is in line with the intent of Section R17.2 of ACI 318-14. A downside of eliminating the need for a rigid base plate is that I now need a thicker replacement section in order to make room for the anchor reinforcement's development length on each side of the anchor breakout cone.

I appreciate your time and comments!

 

[jochav52802]

Morning JLNJ,

The existing slab can easily carry the new concentrated moments, however, it is not thick enough to develop the anchors.

Therefore, I'm demoing and re-constructing a thickened area of the slab, while also providing a full lap splice into the existing slab and thickened section so that it operates as continuous slab again.

With that said, it appears the part you're not understanding is that the 3'x3' thickened section is being lap-spliced into the existing slab, which renders the 3'x3' a continuous part of the slab rather than operating as an independent footing.

 

[RWW0002]

It is difficult to comment without knowing your loading, but I agree that it sets off bells and whistles to have an overturning moment that would require a 10x10 spread footing, but "easily" works as a 6" slab.

You might try meeting half way and design the thickened footing for the full moment but take advantage of some portion of slab weight as "mobilized" dead load to resist overturning (for stability and bearing checks at the thickened "footing"). This way the slab and doweled connection does not have to resist the full moment but only a portion relative to the amount of slab mobilized.

 

[r13]

Increased depth of the spread footing is the main cause for the 10x10 requirement (M = V*d), the OP is minimizing the lever arm to keep the rotation in check, that's why I backed off from my earlier suggestion.

 

[Celt83]

jochav52802:

how are you analyzing the 6" slab with the applied loads to get that it works for the moment?

What happens when in the future someone trenches thru the slab to run a new utility line or other form of maintenance does that create potential for failure?



My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[r13]

The OP is proposing 2' concrete pad under the load.

 

[Celt83]

r13:
OP is only proposing that so the anchors don't get embeded into dirt.

The existing slab can easily carry the new concentrated moments, however, it is not thick enough to develop the anchors.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[jayrod12]

I'm in the camp with the others wondering how you can go from only needing to improve anchorage to a 10x10 pad for teh same moment. The math just doesn't check out.

But, I'm coming with a potential option. I would be comfortable using the slab as a lateral restraint and then counting on the passive pressure on the side of the footing to rectify the overturning. That way, regardless of how thick the footing is, your moment arm would still be only to the top of slab.

 

[r13]

Okay, you guys are all correct. This simple problem is similar to a pond has formed by the dripping faucet

It all starts with embed anchors are required to resist the additional loads, the OP propose 2' concrete pad to accommodate the anchors, and calculate the 6" slab at the perimeter is adequate to resist the resulting shear at the interface. However, the OP find out the need to apply over-strength factor, then resulting in inadequate concrete breakout strength. So, what is the best solution? I suggest the OP to provide the loads to get better suggestions.