ACI 318-19 - Chapter 17 - Anchoring to concrete - Anchor reinforcement

[Pukao]

Hi everyone - I've done quite a bit of research on this forum about the topic I'm about to write about, but would appreciate others' advice on the matter.

As the subject reads, this is to do with anchor reinforcement. I'm currently working on a project where we are anchoring a bollard into a concrete beam with M56 anchors (big tension forces!). The project is located in Europe, so we are designing to Eurocode 2, Part 4. In principle, EC2-4 is equivalent to Chapter 17 of ACI 318-19.

Due to constraints with the geometry of the beam, I cannot mobilise a big enough concrete cone to resist the applied tension, therefore I am forced to use supplementary reinforcement (or anchor reinforcement, as defined in ACI). However, the bigger concrete cone projecting from the bottom of the supplementary reinforcement is still too small, so I'm looking to see what would be the best mechanism (if any), to transfer the tension into the main reinforcement in the beam. EC2-4 states that the bigger concrete breakout check can be precluded if this is achieved:

I guess my question boils down to: what do people here understand as "adequate lapping" in the context of this failure mechanism?

Bonus: I suspect I'll have to follow a similar approach with edge breakout due to shear.

Thank you!

Edit:

The following snippets are taken from the "Commentary to EN 1992-4" by the German Committee for Structural Concrete. You'll see that a reference to "a lap joint with existing reinforcement" is made:

 

[MotorCity]

The bars should be developed on both sides of the cone failure surface.

 

[Pukao]

Thanks MotorCity. If what you mean is that the anchor reinforcement should be developed beyond the cone failure surface by extending it further into the concrete, I don't think that would help. The reason for this, as I stated previously, is that the beam is relatively narrow, so the projected area of concrete on the surface of the beam will not increase. The only way this issue could be overcome is by precluding the concrete cone check by somehow lapping the vertical anchor reinforcement with the rebar in the bottom of the beam... Or that is at least my understanding of the code.

 

[KootK]

Or that is at least my understanding of the code.

That's my understanding too. The ACI anchorage provisions were originally developed considering pretty small scale stuff. Curtain wall anchors etc. For things of that scale, developing the reinforcement beyond the initial breakout frustums gets the job done by doubling or quadrupling the amount of concrete engaged. Logically, however, this simply moves the breakout frustum out beyond the rebar; it doesn't entirely preclude a breakout frustum. In many, small scale applications, this is good enough. For applications of a larger scale, or applications like yours that are highly constrained geometrically, more is required. And, just as you mentioned, the logical alternative is to revert to more conventional reinforced concrete design strategies where tension is passed into reinforcement further down the chain rather than relying on concrete in tension (breakout frustum).

 

[KootK]

I guess my question boils down to: what do people here understand as "adequate lapping" in the context of this failure mechanism?

Basically just whatever would constitute a legit way to transfer tension between reinforcement in your neck of the woods. In north America, this would be:

1) A regular, code compliant lap splice with a max 6" offset or;

2) Something like a strut and tie offset lap splice for offsets greater than 6". This article describes one author's take on how to do that.

 

[Pukao]

Thanks KootK.

My concern with the transfer mechanism is that the beam has a limited depth, so I'd have to go from the vertical plane to the horizontal plane, therefore I'm worried about concrete crushing within the bend of an L-bar. Not sure whether this makes total sense without a sketch?

 

[GC_Hopi]

You can use some of the geometry to come up with the compression strut and check it for concrete crushing. ACI has some direction of how to determine what the compression strut would look like. Not sure where you would find something similar in EC.

 

[Agent666]

Surely your beam has some stirrups to cross the breakout surface and take up the load??

https://engineervsheep.com