Baseplate (with axial & moment) on unreinforced slab on grade

[StrEng007]

Hello everyone,
Any recommendations on applying a baseplate (1 anchor at each corner of a square baseplate, (4) total) with a vertical point load and overturning moment from a column, on an unreinforced slab on grade? My gut reaction is to provide an explicit shallow foundation to resolve this. A major concern of mine is whether or not sawcuts are going to be present in the location of the baseplates. However, for the exercise (ACI 318-14):

1. Check punching shear on the slab (ACI section 14.4.3.4.1)

2. Estimate/iterate a plan area of concrete below the footing to resolve the combined P/A +- M/S:

a. Determine the soil-bearing reaction (iterate for allowable limits).
b. Check tensile flexure in footing, along the compressive soil side, using Mn=5λ√f'c Sm.
c. Check one-way shear in footing, (ACI section 14.4.3.3.1).
d. Resolve overturning tensile side with post-installed anchor bots.

i. Will the load path at anchor tension warrant a local check of the slab adjacent to the anchor bolts. I'm thinking tension induced from "lifting up" the slab​

If an explicit shallow foundation/spread footing is provided by saw cutting, removing the slab, and pouring back a reinforced footing, would you dowel the old slab into new footings? This assumes top of slab = top of new spread footing.

 

[phamENG]

I think your process is more or less correct...but put in a footing. Too many variables that you have no control over and no way of knowing. Don't forget, you have to deduct 2" from the thickness of plain concrete cast against earth for moment capacity. Unless you're dealing with one serious slab on grade, that'll probably kill the whole exercise.

 

[StrEng007]

phamENG, I'll probably end up putting in a footing.

Thanks for the heads up on the 2" rule. Does that still apply even if you have a moisture barrier. I'm thinking yes, due to the language "unevenness of excavation" which is independent of moisture protection.

 

[dik]

How thick is the existing slab? What is the strength? Is it in good condition? Are the loads and moment small? Are any anchors in tension?... bunch of questions.

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

-Dik

 

[phamENG]

I'd say that could be a judgement call, but I can't really think of a situation where I would assume I could use the whole thickness. I might make an exception for new construction where I have some control of inspections and observations, but then it's probably still easier (and safer) to just add an inch or two of concrete.

 

[dik]

For anything except the lightest loads or moments, I like a new patch of concrete... the top of the footing flush with the top of the slab.

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

-Dik

 

[RabitPete]

Curious if anyone is using isolation joints when cutting in a new patch of concrete? Any code reference to when isolation joints are required? I see them added around interior column pedestals sometimes, while in many other cases slabs are poured right against pedestals.

 

[dik]

It depends on the new foundation. If it is a new footing and I don't rely on the adjacent slab, I use flexcel, else if it's reinforced,I roughen the adjacent slab edge and use the shear friction and existing rebar to help resist any overturning... often do small jib cranes that way.

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

-Dik

 

[MSUK90]

StrEng007, can you share more data for the same?
Like magnitude of compression force, uplift and moment?
What type of structure is it?
Etc..
Also, I have rarely seen an unreinforced slab on grade.
Generally they provide a wire mesh here for every grade slab.

 

[phamENG]

MSUK90 - 90% of the time the wire mesh is on the ground. Contractors love to 'lift' it into place as they pour the concrete, but it doesn't work. If the concrete is workable enough to pour a slab, it's probably not stiff enough to support the wire mesh - especially with laborers walking on it as they 'pull it up'. If there are no chairs, it might as well not be there. It's usually intended for crack control more than strength, and it's not great at that anyway. If you're going to rely on existing mesh for a critical structural application, it would be prudent to have cores taken to verify its presence.