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]

Just talked to a Hilti Rep and I was informed that the values listed in the guide are per the ASD approach and the embedment value for yield strength was based on a bond strength approach.

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.

 

[JAX91]

I’m assuming the embedment required to develop yield strength is based off service design loads. In the Hilti technical guide, there are tables which give the capacities for the rebar. From my understanding, these tables are based off testing results and are to be used with service design loads. In the HIT-HY 150 MAX table, the embedment to develop yield is 4-1/2”. The ultimate bond strength at 4-1/2” is listed as 14840 pounds for 4000 psi concrete. The yield strength of a #4 is listed as 12,000 pounds (0.2*60,000). The minimum embedment depth that Hilti tested is 4-1/2”, so that is why they require 4-1/2” instead of the lower value that would give you 12,000 pounds of capacity.

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.