Cracked Slab Shear Capacity
Cracked Slab Shear Capacity
(OP)
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.
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.






RE: Cracked Slab Shear Capacity
RE: Cracked Slab Shear Capacity
It is also my belief that the Vc value that we calculate based on the code equations is a post-cracking capacity. Otherwise, it would make no sense to combine it with Vs as we do.
In summary, I don't believe that you've lost any of your Vc capacity. Of course, there must be some crack width at which shear capacity would diminish. I've yet to hear or see that quantified however.
In a member that has partially cracked due to flexure, the lion's share of the shear transfer occurs in the compression block. It sounds as though you've probably still got that working for you.
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.
RE: Cracked Slab Shear Capacity
RE: Cracked Slab Shear Capacity
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.
RE: Cracked Slab Shear Capacity
RE: Cracked Slab Shear Capacity
So the shear cracking is due to punching shear around the columns?
It is possible to epoxy inject AFTER FRP install, assuming the spacing of FRP strips (or pultruded rods) is adequate to still visually see the existing crack, and therefore no fire protection has been applied. Unorthodox, but possible without removing the already installed FRP.
RE: Cracked Slab Shear Capacity
RE: Cracked Slab Shear Capacity
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.
RE: Cracked Slab Shear Capacity
RE: Cracked Slab Shear Capacity
So it is a RC flat plate with very small diameter columns judging by the photo. Are there caps under the slab, for additional punching shear capacity?
So the FRP is acting as additional top flexural strengthening reinforcement, not direct punching shear reinforcement?
There was recent research into FRP used for direct punching shear strengthening via threading/weaving up-and-down small diameter vertical drill holes in slabs.
RE: Cracked Slab Shear Capacity
At the end of the day, it may not matter whether the true nature of the cracking is shear or flexure. As this article describes, they are really two sides of the same coin: Link
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.
RE: Cracked Slab Shear Capacity
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.
RE: Cracked Slab Shear Capacity
RE: Cracked Slab Shear Capacity
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.
RE: Cracked Slab Shear Capacity
RE: Cracked Slab Shear Capacity
If Vc were not post-cracking capacity, and Vc is zero once the slab cracks, then ordinary flexural steel would be useless!, as the slab would fail in shear the moment it cracks in bending.
RE: Cracked Slab Shear Capacity
Recesses were cast into the slab to facilitate the installation of the steel columns. Instead of pushing the top steel to the side of the recess, the top steel was pushed below the recess. When flexural cracks developed over the columns en route to redistributing flexure to the positive bending regions, those cracks telegraphed up through ~4" of cover to form the scary pattern show in the photos. I believe that the slab has a flexural problem rather than a punching shear problem. And that makes your FRP repair pretty spot on.
Did your rebar scanning indicate depth to top steel?
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.
RE: Cracked Slab Shear Capacity
RE: Cracked Slab Shear Capacity
I don't believe we have the rebar depths. I just don't know if I can buy that it is only flexural cracks. I believe that there is some kind of combination of flexural and punching shear cracks. Either way, there really isn't any way for me to prove 100% that it is only flexural. So, I think that I will be going down the expensive, yet safest route of reinstalling the FRP after epoxy injecting the cracks. I don't see any other way around it.
structSU10:
I have not looked into that in any kind of great detail. They are wanting an answer to this pretty quick and I don't have the time to really learn and understand everything entailed with that before submitting an answer. I also don't really know if it will end up helping in this case, though it does look like really good information.
Tomfh:
I would agree that Vc should be post-cracking, but I am worried that if in the worst case something does happen with this slab and I didn't do anything, I won't have anything that is in a code or research paper or anything that says that my assumption is correct. Maybe it is just common sense and any engineer in my situation would agree with the Vc not being zero, however, there is probably a point where the crack size does take away from the concrete shear strength and without knowing what that may be, I am stuck in a bad situation.
RE: Cracked Slab Shear Capacity
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.
RE: Cracked Slab Shear Capacity
RE: Cracked Slab Shear Capacity
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?
RE: Cracked Slab Shear Capacity
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.
RE: Cracked Slab Shear Capacity
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.
RE: Cracked Slab Shear Capacity
RE: Cracked Slab Shear Capacity
RE: Cracked Slab Shear Capacity
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.
RE: Cracked Slab Shear Capacity
RE: Cracked Slab Shear Capacity
"#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.