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!

 

[r13]

My only concern is the u bars must have adequate development length above and below the intercepting crack plane assumed.

 

[Ron]

If your sketch is to scale, the isolation footing is not large enough to handle much moment from the column. You'll be transferring a lot of load to the 6" slab if your moment is high, essentially like an overturning moment.

I would not use the U bars at all. Intersect the crack plane with linear bars and call it a day. I would put flat washers on the end of the anchor bolt to spread the load.

 

[dik]

I tend to avoid flat washers because they can develop a plane of weakness if you have a large number of anchor rods terminating at one place... a normally heavy hex head will develop the rod in about 20 or 24 diameters (AISC indicates about 18 dia)... and containment reinforcing will look after the rest.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik

 

[r13]

The u bars/hair pins are supplement to the headed stud for tension breakout concern. Hardened washers may help to an extent. Dik has valid concerns.

 

[dik]

Manitoba Hydro for their large tower anchors had specially fabricated anchors with a helical plate, just to avoid this plane of weakness... sort of like a tension 'screw pile' in concrete.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik

 

[dik]

not normally required... some references suggest the 'cantilever' distance should be limited to twice the washer thickness.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik

 

[jochav52802]

Thank you all for your responses!

It appears my second question still remains to be weighed in upon; is it reasonable to call for the perimeter of the demoed area to be roughened to an appropriate amplitude in order to achieve full monolithic slab shear capacity once the thickened portion has been poured and cured? Or will the boundary between the existing and new concrete be a weak spot of lesser shear capacity even with the roughening of the existing concrete?

Many thanks again for your contributions!

 

[r13]

Your call is what I will do, so no comment there. But ideally, the location of the cut should be kept out of the high shear stress area around the column.

 

[dik]

If you are resisting the load by the soil, a normal sawcut should be sufficient... if relying on the slab, then roughen the existing slab cut surface so you can use a higher shear friction coefficient and use this as the load on the concrete and see how much of the slab is needed to resist this load... then treat the slab as a cantilevered plain concrete slab using a concrete tension strength of about 7 or 8 x sqrt(f'c)... give you a ballpark idea of the stength... and I'd throw a couple of epoxied dowels in to help.


Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik

 

[Ron]

Your dowels will transfer the shear load if the shear-friction of the roughened slab edge is insufficient.

@dik...I agree with your plane of weakness comment if the anchors are closely spaced. With the proposed depth, the termination of the anchors can be staggered; however, I wouldn't trust a contractor to get that right. My approach would be to use round plate washers, not the large square ones shown in many details. This helps to keep the shear cone failure mode of the anchor intact.

 

[KootK]

It appears my second question still remains to be weighed in upon; is it reasonable to call for the perimeter of the demoed area to be roughened to an appropriate amplitude in order to achieve full monolithic slab shear capacity once the thickened portion has been poured and cured? Or will the boundary between the existing and new concrete be a weak spot of lesser shear capacity even with the roughening of the existing concrete?

In my opinion, this would definitely be a plane of shear weakness. So much so that I'd be inclined to not rely on shear capacity there for anything serious. My concerns:

1) With the weird access, weird angles, and thin receiving member, I feel that there's pretty high potential for local cracking spalling at the dowel installs.

2) I seriously question whether shear friction is an appropriate design for a situation like this where:

a) the member is very narrow in the direction parallel to the shear force and:

b) if there's a meaningful tension face, the dowel will be pretty far from it.

These thing tend to push you further from the intent of the shear friction provisions and the testing upon which they are based. I'd be willing to call this a nominal shear connection and base that on true dowel action rather than shear friction if discrete capacities were required.

As ron intimated, I would be inclined to ensure that the new footing could deal with all of the incoming forces, except perhaps lateral sliding, without aid from the SOG.

 

[dik]

concur... but easier and cheaper to shear off a chunk of BAR... you need some thickness that regular washer don't have and you have to fabricate the round ones normally. It's very seldom that I use plate washers, anyway...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik

 

[jochav52802]

Thank you all for your responses.

After further research into this topic a colleague referred me to page-10 of Hilti's below resource, which utilizes HIT-RE 500 V3 adhesive with doweled flexural reinforcement to the ACI 318 lap splice depth as well as coupled with shear friction reinforcement per ACI 318 to carry the shear load.

https://view.joomag.com/product-tec...talled-rebar-guide/0604970001570657600?short&

Since my existing slab is 6" thick it only has (1) layer of reinforcement, I plan to use that layer as both flexural and shear friction reinforcement across the crack plane, between the existing and new slabs. This is specifically addressed and allowed by Section 22.9.4.6 and R22.9.4.6 of ACI 318-14.

From what I can tell, this addresses all of my concerns in this post, but feel free to let me know if you have any additional input.

Thank you!

 

[JLNJ]

I would also make the cutout and replaced slab large enough to do all the work without any help from the slab.

Trying to justify using a doweled detail into such a thin slab seems to be more wishful thinking than solid engineering.

Additionally I’m a little confused how you can have such large loads that you need special anchorage details yet the required footing size is so small.

 

[jochav52802]

Thanks JLNJ,

1) I agree that would be ideal, however, I've got to have a really good reason to justify that more costly option. We have a strong construction team and I don't think doweling into a 6" is impractical for them. I'll check with them and adjust course if needed.

2) I appreciate your point.

3) The large moments are coming from some new cantilevered column pipe supports for some very large piping. The anchors are being designed based on the ASCE 7-16 Overstrength load combinations, so the seismic loads for the anchors are higher than the regular load combinations that the slab is designed for. Tying into the existing slab engages a large area of the slab, while also keeping the slab unity ratio very low. I'd like to add bracing to the pipe supports to reduce the concentrated slab moments, but there is no physical space to do so.

Thanks again!

 

[r13]

JLNJ has valid concern over the bearing capacity of the proposed footing. You may consider the detail below, if necessary. (I would also provide ties, not shown)

 

[jochav52802]

Thank you r13,

Bearing capacity of the foundation isn't a problem, as once I tie into the overall existing slab, the concentrated moments get spread enough to keep us well below our bearing capacity.

I appreciate your sketch, yet I would need the footing to be upwards of 10'x 10' in order to keep our overturning safety factor to 1.5, which is a lot more demo and construction to deal with compared to just locally thickening a new pad for the anchors and then providing moment and shear continuity via drilled lap-splices.

Thanks again!

 

[r13]

Got you.

 

[JLNJ]

You seem very confident, but I remain perplexed.

You say you need a 10'x10' footing if it were an isolated footing, but you can get away with a small (3'x3'?) footing if you "tie" it into a thin slab on grade?

On its face, this doesn't make sense.