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.

 

[jayrod12]

I would still think there should be a nominal shear capacity from aggregate interlock. I guess since there is developed bars on either side could it be consider shear friction?

 

[KootK]

In my opinion, any given section needs to possess both shear friction and diagonal tension capacity. It's not the case that it's buffet style and one or the other will do.

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.

 

[jayrod12]

I knew if I put shear friction in my response Koot would be quick to add his 2 cents to the discussion.

 

[KootK]

I've created a beta, iPhone, Eng-tips app that makes my phone dance whenever anybody mentions shear and concrete in the same sentence.

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.

 

[Stenbrook]

Our problem is that if we say that the concrete shear capacity has not been reduced, we need to provide some sort of factual evidence and prove that that is the case. We are not having very much luck in finding something that says this or gives any guidelines for it. We are dealing with an existing slab that is in the process of placing FRP reinforcement around the entire building and the contractor failed to fill and epoxy some large cracks around the columns before placing the FRP. If we can't prove that the shear capacity has not been diminished, we will have to tell them to rip up a lot of FRP and redo it costing $100K+. I would like to avoid that situation if possible....

 

[Ingenuity]

...and the contractor failed to fill and epoxy some large cracks around the columns before placing the FRP.

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.

 

[Stenbrook]

From the cracking that we have observed, it is obvious that it is punching shear cracking. However, the FRP that is installed is pretty significant.They have essentially overlapped the FRP and completely covered the existing concrete for the most part. There really isn't any opportunity to inject the epoxy without removing the FRP. If it were just strips of FRP I would agree that injecting the epoxy could work, but with upwards of 5'-0" + strips of FRP I don't think it would work.

 

[KootK]

Ah. Punching shear makes things a good deal scarier. Are these flexural cracks that originate at the top of the slab? Can you post some photos.

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.

 

[Stenbrook]

Take a look at the picture attached. (We won't talk about how the contractor likes to leave heavy equipment right next to a column where cracks have formed.......). This picture is at a location they haven't covered with FRP yet. There are however 9-10 locations that the FRP is installed.

 

[Ingenuity]

Stenbrook,

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.

 

[KootK]

Ughhh... I don't like the look of that at all. To be honest, I have a hard time distinguishing between flexural and punching shear cracks over top of columns. It looks as though you might have some of both. It would be instructive so see an underside photo as well.

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.

 

[KootK]

This might be a punching shear improvement system that plays nice with the FRP: 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.

 

[Stenbrook]

There is actually a 14"x14" wide concrete column below the slab with the pipe column above. There are drop caps as well at the columns. I don't have the dimension of those at the moment, but I seem to recall them being around 3' to 4' square. I've attached a picture of the underside as well.

 

[KootK]

Huh. The underside looks right as rain. I wonder if this might be a flexural problem arising from a recessed base plate connection on the pipe column. That often results in making questionable changes to the disposition of the top steel over the column. Do you have access to the base plate detail?

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.

 

[Stenbrook]

I do not have access to the base plate detail. There are no existing drawings on the building that we know of.

 

[Tomfh]

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.

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.

 

[KootK]

Hypothesis:

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.

 

[structSU10]

ever look into compression field theory for shear? Link according to that article, for over 100 test CFT has experimental to predicted ratio of 1.01 with a COV of 12% versus the current ACI shear with a ratio of 1.47 and COV of 46.7%. Don't know if it helps here, but its a different way to look at shear.

 

[Stenbrook]

KootK:
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.