Cracked Slab Shear Capacity 4

[Stenbrook]

Does anyone know where you can find information on the shear capacity of a cracked reinforced flat plate slab? I don't believe there is any capacity with no shear reinforcement, but I just wanted to see if anyone had more knowledge on this than me.

Criteria:
Existing 10" slab
30'-0" x 30'-0" bay
#6 transverse reinforcement (top and bottom grid @ 12" o.c. max)
Cracks don't extend all the way through the slab. Actual depth has not been confirmed.

 

[KootK]

@Stenbrook: I sense that you've chosen your course and I respect that. It sounds like the contractor's on the hook for this so the path of least liability may well be the best path for you. I'm trying to be helpful rather than argumentative. So permit me just one more volley. I own a copy of this paper: Link. It presents a straight forward method for evaluation of punching shear based on strut and tie methodology which is, by definition, post cracking. You may be able to hang your hat on this if you choose to.

Frankly, I don't see great value in the epoxy injection other than aesthetics and possibly corrosion resistance. You're just filling existing cracks in the tension zone that will likely just have future tensile cracks right next door.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[mike20793]

I don't know this for sure since I have never designed with the Eurocode, but I heard that it requires Vc to equal zero because the member is assumed cracked and loses any shear capacity from the concrete. I don't know if that is true so I apologize if that is incorrect. The ACI provisions on shear are vague and confusing and, at times, somewhat contradictory and I've heard from ACI that it is simply because most of the original academics that did the research studies simply aren't around to ask for clarification or further the study. There are supposedly some changes in the works for the next code cycle or the following cycle.

 

[Ingenuity]

Stenbrook,

From a calculation perspective, does the slab satisfy the applicable code for negative flexural capacity and punching shear capacity over the columns?

Or is the strengthening using FRP in response to existing cracking only?

 

[rapt]

Stenbrook,

I would go with Ingenuity's question. Does the slab design meet code requirements for flexure, shear and deflection.

With the amount of reinforcement that you have said is present, and assuming the drops provided are simply drop caps for punching rather than full drop panels, I would think that the slab design does not work for any of those. A 10" slab spanning 30' RC concrete simply does not work according to my calculations, especially with the reinforcement you have said is provided, no matter what the loading.

 

[Tomfh]

Stenbrook,

I agree that you would be in trouble should something happen with this slab. The cracking is pretty ugly. I'd want to know why it's cracking like that. My comment about Vuc was just a general comment about RC slab design which assumes flexural cracking.

What do the numbers say about its capacity?

It would be good to know the real depth to reinforcement.

 

[Stenbrook]

To answer your questions, the slab does not meet the capacity required, and that is why we are installing the FRP. We based our design off of the code required cover and went from there. Since that capacity was not enough, any situation in which the cover was increased would only make it worse.

 

[Tomfh]

What do the numbers say regarding the punching shear capacity?

 

[Ingenuity]

Since that capacity was not enough, any situation in which the cover was increased would only make it worse.

I think it is prudent that you do some limited-scope site investigative work to determine the as-built conditions. You have a contractor on site doing the FRP so with some judicious supervision you can directly observe/instruct them to cautiously remove cover concrete at the top of slab adjacent to the pipe column location, and coupled with GPR scanning and a pacometer determine where the top steel is located with some level of accuracy. Then see where your calcs take you based upon this as-built condition.

 

[hokie66]

I think the cracking is flexural, and agree with rapt that, as described, the slab is off the scale.

 

[rapt]

You have said the reinforcement is

"#6 transverse reinforcement (top and bottom grid @ 12" o.c. max)"

Do you mean that the top reinforcement is equally spaced over the whole width, rather than a concentrated strip over the columns with less between? My calculations would show that you need about 2-3 times (or more depending on loading)this reinforcement concentrated over the columns in a column strip. If it is equally spaced, then yes, it would be cracked badly in flexure and be very under capacity in punching shear.