Appendix D Supplementary Reinforcement - fu versus fy
Appendix D Supplementary Reinforcement - fu versus fy
(OP)
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?






RE: Appendix D Supplementary Reinforcement - fu versus fy
RE: Appendix D Supplementary Reinforcement - fu versus fy
RE: Appendix D Supplementary Reinforcement - fu versus fy
RE: Appendix D Supplementary Reinforcement - fu versus fy
And if it doesn't say it, then it should. Everyone else is right, you should use Fy.
RE: Appendix D Supplementary Reinforcement - fu versus fy
There are days when I wake up feeling like the dumbest man on the planet, then there are days when I confirm it.
RE: Appendix D Supplementary Reinforcement - fu versus fy
RE: Appendix D Supplementary Reinforcement - fu versus fy
RE: Appendix D Supplementary Reinforcement - fu versus fy
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?
RE: Appendix D Supplementary Reinforcement - fu versus fy
RE: Appendix D Supplementary Reinforcement - fu versus fy
RE: Appendix D Supplementary Reinforcement - fu versus fy
RE: Appendix D Supplementary Reinforcement - fu versus fy
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.
RE: Appendix D Supplementary Reinforcement - fu versus fy
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.
RE: Appendix D Supplementary Reinforcement - fu versus fy
A rebar used with epoxy should be design using fu, Simpson and USP make epoxies as well.
RE: Appendix D Supplementary Reinforcement - fu versus fy
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."
RE: Appendix D Supplementary Reinforcement - fu versus fy
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.
RE: Appendix D Supplementary Reinforcement - fu versus fy
And see attached
RE: Appendix D Supplementary Reinforcement - fu versus fy
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)
RE: Appendix D Supplementary Reinforcement - fu versus fy
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.
RE: Appendix D Supplementary Reinforcement - fu versus fy
The new strength design approach values are tabulated in the ICC report 2262, which considers bond strength, breakout strength and designs it per Appendix D.
RE: Appendix D Supplementary Reinforcement - fu versus fy
As far as the embedment depth is concerned, the 4.5" embedment length is the embedment into the epoxy. Using equation 12-1 from ACI 318-08, the tension development length of a #4 bar is 12". The epoxy has a stronger bond to the concrete than a cast-in-place bar, and therefore does not need its full development length to reach its yield strength.
Are the bond strengths and concrete breakout strengths you show calculated with Hilti Profis? If so, they were probably calculated using strength design methods. When using the strength design methods, you need to use the Hilti ESR reports. I have attached a copy of 3013, which is used for Hilti HIT-HY 150 MAX-SD.