Punching shear in basement slab

[Woody1515]

Hello everyone,

I have a situation where a residential basement layout is changing. Long story short, we need to add a new telepost to support the floor above. I would like to avoid putting in a new footing. When checking the slab for punching shear, is it best to check the slab as an unreinforced concrete slab? I have no information regarding the rebar (if any) was used. Or a more general question - is it correct to assume a reinforced slab will be stronger in punching shear resistance than an unreinforced slab?

Thanks in advance!

 

[Woody1515]

I guess my question boils down to this - when I’m finding the area to divide the telepost load by to get the shear stress, do I use the full slab thickness (h) or the effective depth (d)?

 

[KootK]

ACI has a section for plain (reinforced) concrete. There's a section there for punching shear I believe and I'd use that.

 

[oldestguy]

I'd take a masonry drill and measure slab thickness. Above a selected thickness I'd use no special footing.

 

[Woody1515]

I agree, I think I would check it as an unreinforced slab. I guess I’m looking for some good reasons to support doing this. I’m having trouble explaining my reasoning to a colleague.

 

[JoshPlumSE]

If it works as an unreinforced slab per KootK's suggestion, then just do that. If not, then why not get a guy out there to scan the slab for rebar. It shouldn't cost all that much.

 

[Woody1515]

If there is reinforcement (I assume there is, just can’t cofirm), does that usually make the slab stronger in puncher shear? The slab is only 4” thick.

 

[JAE]

How thick is the slab? ACI, at least, limits "footing" thicknesses to 6 inches I think.

If the basement is generally unused, we sometimes simply construct a footing on top of the slab.
If the basement is used, and a footing block on top isn't desired, we would simply cut out a section of the slab, dig down and construct a new footing.

Even if you get the theoretical punching shear to work, the variations in slab thickness, actual concrete strength, etc. would concern me.
I guess the upside is that if it did indeed punch through, you'd have warning signs ahead of any sudden collapse.



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[KootK]

If there is reinforcement (I assume there is, just can’t cofirm), does that usually make the slab stronger in puncher shear? The slab is only 4” thick.

- In this scenario, I would say no.

- If you had appreciable reinforcement crossing the punching shear frustum, that would have to help in some fashion via dowel action. In a 4" res slab, however, you've probably got wire mesh at 6" - 12" or, far less likely, widely spaced rebar. And the mesh is probably laying on the ground.

- Even if the rebar is there in force, I don't know of an accepted method for accounting for it.

- In elevated slabs, you get some serious available shear stress based on "d" because it's assumed that the slab is flexing up a storm and that, as a result, the compression zone will be highly stressed and therefore capable of significant aggregate interlock. In a slab on grade, even with the reinforcement, you'll not have the flexing.

 

[KootK]

I definitely second JAE's practical proposals for addressing this.

There is this method of assessing SOG point load capacity:Link The capacities are high though and some have expressed concerns about the validity of the method.

The last time I looked into this, TR34 seemed to be the state of the art. Unless you've got an uncommonly high design budget for this, however, it might be best to skip right to JAE's recommendations.

 

[Ron247]

Most basement slabs I have excavated had at best, wire mesh on the bottom of the slab. I rarely find residential 4" slabs with wire mesh in the middle. I doubt rebar was used at all unless it is standard practice in your area. The problem with using the unreinforced slab on grade, is they may require you to allow 1" for spoilage if the concrete was not kept from getting contaminated during the pour. As already recommended, measure the thickness where you want the post to be. I still see contractor's using a 2x4 as their depth guide, claiming that 4" of concrete is like 4" in a dressed 2x4. Unless your load is very small, you probably need to err on the side of putting in a footing.

 

[PEinc]

I have used this reference many times.

www.PeirceEngineering.com

 

[MotorCity]

If you are following ACI 318, note that you are not permitted to use the nominal slab (footing) thickness for the design of plain concrete. You must deduct something like 2" or 3" from the nominal thickness to get your design thickness. I believe it is intended to account for high spots in the subbase or thinner areas of the slab.