Appendix D Supplementary Reinforcement - fu versus fy

[JAX91]

ACI 318-08 D.5.2.9 allows the use of supplementary reinforcement to develop concrete breakout. It says to use a strength reduction factor of 0.75, but does not specify to what this 0.75 factor applies. In the past, I have used equation D-3, (Ncb=n*Ase,N*futa), and used a phi-factor of 0.75 to calculate the tensile strength of the rebar. I have come across several design examples put out by ACI which uses the rebar strength as equal to 0.75*As*fy. For grade 60 rebar, the latter equation gives 2/3 the capacity of the former. I don't see why the tensile strength of rebar should be calculated using a different equation than the tensile strength of an anchor. I think equation D-3 could be used, but the design examples say otherwise. The code does not really say either way. What equation have other people used?

 

[slickdeals]

I have previously used 0.75 As Fy. More conservative, I suppose.

 

[WillisV]

Fy.

 

[sandman21]

You don’t design the supp. reinf. per App. D, you design per the rest of ACI, so Fy is the proper value

 

[Lion06]

I don't have ACI 318-08 in front of me, but I'm pretty certain that it's clear that you use Fy for the supplemental reinforcement.

And if it doesn't say it, then it should. Everyone else is right, you should use Fy.

 

[nuche1973]

I use Fy, because it will yield before it breaks, for the most part.

There are days when I wake up feeling like the dumbest man on the planet, then there are days when I confirm it.

 

[JAX91]

The general consensus is to use fy, so that is what I will use; however, I still don't know if I fully agree with the code if their intent is to use fy. If a piece of rebar were placed in the top of the concrete and set with epoxy, say Hilti HIT-RE 500-SD, the tensile capacity of the rebar would be calculated using the fu equation (D-3). Now when that same piece of rebar is pushed downward into the concrete, requiring the use of fy is basically saying the tensile capacity of the steel is reduced. For grade 60 rebar, the tensile capacity would be reduced by 50 percent. It seems very inconsistent to me. The only way this would make sense is if the development length of the bar was not sufficient to develop the full fu. If this is the case, there must be a modification that could be done to increase the required development length in lieu of adding 50 percent more steel.

 

[slickdeals]

If it yields and elongates, then there is nothing stopping the concrete cone from breaking out.

 

[Lion06]

I'm not sure I'm following. Rebar is always designed using Fy, at least in every part of the code that I've used. The anchors are allowed to use Fu, but it has an upper limit of 1.9fy.

Why do you say the tensile capacity of rebar is being reduced by 50%? Why do you say that when the rebar is pushed into the concrete that the tensile capacity is reduced?

 

[JAX91]

Lion – For rebar to be used as an anchor, it would have to be epoxied in place. As far as I know, Hilti is one of the only manufacturers that has had epoxy approved for use with the strength design methods of Appendix D, which is why I used them for a reference. From the Hilti ESR-2322 report for HIT-RE 500-SD epoxy, the tensile strength of the rebar is calculated using equation D-3, which utilizes the Fu value of 90 ksi (for Grade 60 rebar, Fu<1.9Fy). Using a #5 with equation D-3, the allowable tensile capacity of the rebar is 0.75*0.31*90 = 21 kip. Utilizing this same #5 as supplementary reinforcement and calculating the strength using Fy, the allowable tensile capacity of the steel is 0.75*0.31*60 = 14 kip, which is fifty percent less than the first equation. I agree with slick’s assessment. It’s probably not a matter of the strength of the rebar itself, but more a matter of how it will act under the loads and the type of failure that is allowed. The code obviously allows anchors to yield and stretch by using Fu. This ductility is actually preferred in seismic regions. Internally, however, elongation of the rebar would allow the concrete to break and defeat the purpose of the supplementary reinforcement. I see now that Fy is the proper value to use. Thank you all for your responses.

 

[Lion06]

Why would it need to be epoxied in place? Supplementary reinforcement in reinforcement provided in the vicinity of the primary anchors to preclude concrete breakout. You should take another read through App. D.

 

[JAX91]

The epoxied case was considering the rebar as an anchor, not as supplementary reinforcement. I was just using it as a comparison to state that the code gives two separate allowables for the tensile strength of rebar. As I just mentioned, I now see the need for the two different allowables. You should take another read through this thread.

 

[slickdeals]

Jax,
It appears your question is really independent of concrete. I believe you trying to ask why the strength of an anchor is calculated based on ultimate strength Fu and strength of rebar calculated based on yield strength Fy?

Traditionally rebar is used inside concrete and calculations are based on yield strength.

Imagine you have a reinforcing bar welded to a plate and a tension load hanging from it. How do you calculate the strength? Do you use 0.9 As Fy or do you use 0.75 As Fu?

I believe high strength fasteners are more brittle and hence Fu is used in calculating their capacities. Mild steel is more ductile and has a defined yield point and hence Fy is used in calculating capacities.

I am sure the sage folks here will chime in.

 

[Lion06]

Then that's a completely different case from using supplemental reinforcement as mentioned in the OP.

I didn't read anything in the thread where you talked about using the rebar as actual anchors. Can you point me to that? I may be missing it, but I don't see it, other than the post at 12:53.

 

[sandman21]

ACI 318-08 Section D.5.2.9 covers the design of reinforcement to take breakout. Supplementary reinforcement is a cover in the strength reduction factors, D4.4 or D.4.5.

A rebar used with epoxy should be design using fu, Simpson and USP make epoxies as well.

 

[JAX91]

Slick,
Your hypothetical question is a good one. The anchors would be included in the scope of Appendix D via D.2.1 as “anchors in concrete used to transmit structural loads by means of tension, shear, or a combination of tension and shear between connected structural elements.” At the same time, they would be excluded from Appendix D via D.2.2, which reads “Reinforcement used as part of the embedment shall be designed in accordance with other parts of this Code.” I have a feeling the scenario was not tested, and therefore would not be included in Appendix D. On the other hand, I don’t know how the embedded rebar would act any differently than headed studs welded to the back side of an embedded plate. I would probably use Appendix D for the design, and thus use 0.75*As*Fu for the tensile strength. Although rebar is typically used as reinforcement, in this case it is being used as an anchor. D.2.2 does, however, verify the requirement of using Fy for supplementary reinforcement. Sandman was correct when he said, "You don't design the supp. reinf. per App. D, you design per the rest of ACI, so Fy is the proper value."

 

[slickdeals]

Embedded rebar works on the principle of development length
Embedded stud anchor works on the principle of the headed end of the stud bearing against concrete.

Which is why embedded plates with deformed bar anchors (DBA's) have much higher capacities than embedded plates with headed studs.

 

[slickdeals]

http://www.gurmak.net/hiltison.pdf

And see attached

 

[slickdeals]

I was just checking what the Hilti Profis program does when you are using a rebar as an anchor.

Interestingly, it uses 0.75 Fu As to calculate the capacity of the rebar anchor.
For a #4 bar, it calculates 13.5 kip (.75*90*0.2)

 

[slickdeals]

Sorry for hijacking your thread JAX.

What does HILTI mean by embedment required to develop yield strength?

For example, in 4000 psi concrete a #4 bar requires 4.5" embedment to develop yield strength (12,000 lb). However, the tension development length (ld) of #4 bar is 19".

At 5" embedment, the bond strength (pull out) is 5500 lb and the concrete breakout strength is 6177 lb - nowhere close to the yield strength.

I am confused.