Concrete edge (breakout) failure in shear on anchor group - Eurocode 1992-4 vs ACI 318 2

[BridgeEngineer21]

I am working on an anchorage design in accordance with Eurocode. In cases where Eurocode is unclear, I can refer to other sources, and in extreme cases I can apply for a waiver of a code requirement if I have a good justification to do it an alternate way. So, here is the situation at issue:



The relevant Eurocode clauses for concrete edge failure:

From 6.2.2.2:


I think its pretty clear Eurocode requires me to check this edge failure area against the full shear force:

Now if I look at ACI, this situation would warrant the following 2 cases to be checked:



Clearly, the Eurocode check is much more conservative, and in this case is causing serious problems.

Now, I'm also referring to a very helpful reference book: Anchorage in Concrete Construction, First edition (R. Eligehausen, R. Mallee, J. F. Silva). I don't know the history exactly, but it seems that a lot of the research and explanations adopted in this document are incorporated in both Eurocode 1992-4 and ACI 318. (In general, ACI seems to more explicitly state some of the explanations and reasoning from this reference in the commentary column).

I'm sharing a couple of pages attached in case anyone is interested in some more reading, but essentially the book agrees with the ACI reference manual. This figure below is particularly illustrative, and this part of the text was persuasive to me: "If the shear load is initially resisted by the near-edge anchor alone, it will generate a concrete breakout crack. Following displacement sufficient to bring the rear anchor into a bearing condition, that anchor will then resist the entire shear load.


Now, the question I have for all of you is if you can think of a persuasive argument why the Eurocode assumption on shear distribution should be adopted over the ACI/reference manual assumption, other than "just cause" it's more conservative.

Appreciate any input!

 

[icebloom]

Personally I think introducing undue eccentricity via a connection that seems a bit convoluted into the design simply to somehow try justify full engagement of the bolts is not the way to go. Even under moderate loads if you consider bending of the bolts in shear as the Eurocode tells you to (in the shear with lever arm section) your bolt will for sure start to fail with the kind of eccentricity you'd need to physically fabricate the hollow section connection. You're also going to introduce tension in your anchors which will blow up the interaction check for combined concrete utilisation in tension and shear. If the choice is filling washers or all this extra fabrication and welding for a connection I'd suggest will not work, then its by far the cheapest and easiest option for the builder. The washers will cost a couple hundred bucks and will by far be the best structural option other than having anchors welded to the endplate.

 

[BridgeEngineer21]

Completely understand your point icebloom, but I should clarify that the hollow section is a non-structural requirement and a built-in part of this construction device that's being attached, not something I came up with. Shear lever arm and tensile capacity (including interaction with shear) verifications are already confirmed to be fine - due to the size, quantity, and embedment length of the anchor bolts we have. The problem is just with concrete edge failure, and since I already have this pre-defined lever arm, I am just making use it for this case.

The filling washers are a great solution which I'm going to keep in my back pocket for the next time I come across this issue.