ACI 318 12.5.3(d) For Hooked Rebar For Anchors 1

[Tankie]

I don't want to derail the recent thread about the Section 12.5.3 modification factors for footing dowels, so pardon the semi-parallel subject.

The specific case that has me a bit confused is when a hooked bar (hairpin) is used to provide the needed Appendix D reinforcement for anchors in tension. The elevation view of Figure RD.5.2.9 in 318-11 illustrates this case perfectly.

The question that I have is as follows: Can the "excess reinforcement" provision of Section 12.5.3(d) be applied to this hooked anchor reinforcement in order to reduce the standard hook development length ldh?

I'm thinking that it CANNOT be used, since 12.5.3(d) states that this knockdown factor can only be used when "development for fy is not specifically required", and the Appendix D provisions, I believe, would require the anchor reinforcement to be designed based on yield.

Would appreciate your thoughts.

Thanks.

 

[jdgengineer]

I think this is an area that requires judgement. I'd say that I tend to agree with you, that it isn't wise to reduce the development length of reinforcement that is there to preclude the failure of a concrete breakout mechanism. However, in my opinion it may be acceptable by the code given some conditions.

A ductile failure of the anchor in Appendix D is not required, but it's encouraged. Per Section D3.3.6 you are allowed to take the design strength as 0.4 times the design strength as determined in D3.3.3 if the anchor is governed by a concrete failure mode. I would think that if you were to design for a reduced development length this failure mechanism would now be considered a "concrete failure mode" as the failure will be bar slip in the concrete. Therefore, in my opinion if you take 0.4 x design strength and calculated your development length based on this reduction it satisfies the intent of the code.

The language in D.5.2.9 states only that the anchor is developed in accordance with Chapter 12, therefore, in my opinion, the reduced development lengths are technically allowed.

However, I doubt you'd be reducing your development length much with this reduction in place, and I don't think except in some odd situation that I would ever pursue it.

 

[Tankie]

Thanks, jdgengineer. I agree, even if permitted by ACI there isn't much bang for the buck to be realized.