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Combined actions Anchor checks with anchor reinforcement 1

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Agent666

Structural
Jul 2, 2008
3,080
Hi all

In ACI318-19 chapter 17 (and earlier versions appendix D) :-

For checking combined actions on anchors if you are foregoing the shear and/or tension concrete breakout checks in lieu of adding specifically designed anchor reinforcement in accordance with ACI provisions. ACI isn't quite 100% clear in my mind on one thing...

Something I'm not quite 100% sure on is do you simply sub in your anchor reinforcement ratio for shear and/or tension in place of the concrete breakout and calculate combined actions check as normal if that reinforcement arrangement is still governing. The following suggests its a direct substitution and then presumably you carry on checking combined actions as normal?
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Or once you have anchor reinforcement you are only required to satisfy the anchor design being greater than the demand isolated from the other checks (ratio of demand over capacity <=1), and just check combined action according to ACI for the other concrete and steel failure mechanisms only?

So options
1 - simply sub in anchor reinf ratio directly in place of the relevant concrete breakout.
2 - calc anchor reinforcement ratio and compare separately to demand, calculated other combined actions without the concrete breakout mechanism included.

For the life of me I cannot find any specific design examples as they all seem to only concentrate on the concrete breakout failures and maybe use supplementary reinforcement enhancements but not designing/providing specifically designed anchor reinforcement. Software such as Simpson anchor software only allows you to neglect the tension or shear breakout, but no design of the anchor reinforcement can be specified.

Anyone aware of the intended way to treat anchor reinforcement, or aware of any published design examples, or offer any advice on whether we're interpreting things correctly if following 1?

Hopefully that all made sense. If not let me know and I'll try explain things better!
 
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I should also add, I'm talking about this with respect to headed studs welded to a cast in plate.
 

I have ACI 318 -14 .

17.4.2.9 Where anchor reinforcement is developed in accordance with Chapter 25 on both sides of the breakout surface said:
the anchor reinforcement shall be permitted to be used instead of the concrete breakout
strength[/b] in determining ϕNn. A]

My interpretation is that ,the reinforcement shall be designed for the required load and fully developed on both sides of the failure surfaces resulting from tensile or shear forces.

Consider a single anchor bolt and the checks for concrete failure modes,

- Tension failure of Concrete, where the failure surface due to applied tension load will be the surface of pyramid ,you checked and find that pull-out failure of concrete is critical and you provided tension reinforcement as per code ,

- Shear failure of concrete , The failure surface from the shear load is defined as a half pyramid with slopes 1.5 to 1,in all directions.. you checked and shear failure is also critical..

In this case, IMO , I will look to the other options ( Increasing spacing, Increasing concrete strength,Increase edge distance , increasing embedment depth since the providing reinforcement is not only the available option. ...) if not, provide separate shear reinforcement considering STI analogy ...

In my opinion, ductile failure shall be provided and brittle failure of concrete shall be avoided to all costs..






 
I read it the same as you that Option 1 is required, although I don't understand why. I also feel the same way about the other limit states. For example, combining concrete breakout in tension and steel strength in shear.

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Thanks, I'm guessing its all a bit of a fudge to fit within real world test results and come up with some logical but conservative method to evaluate an infinite number of scenarios!

From what I understand the provisions generally are intended to be conservative given the variation you might see in practice in terms of construction.
 
ASCE Anchorage Design for Petrochemical Facilities is the resource document you are looking for.
 
Yeah, sub directly into the interaction formula is what I've done. Quite conservative imo since how this (or concrete breakout) can possibly interact with steel failure in shear I don't know. You are eliminating breakout as a failure mode.
 
I generally follow the very conservative approach of considering the controlling shear and tension load capacities together in the interaction equation regardless of the controlling failure mechanism, but at times I have used some "engineering judgement" to disassociate, say, concrete breakout strength in shear with steel strength in tension. I'm not sure if the code technically allows that or not, but it seems like basic logic that these two effects are separate and should not HAVE to be combined.

For anchor reinforcement, think I've substituted in similar to your option 1 for the breakout failure more being reinforced. And then only checked interaction effects within relevant failure modes...think through the load path and use your engineering judgement.


 
Agent666 said:
...or offer any advice on whether we're interpreting things correctly if following 1?

In my opinion, once you rely on anchor reinforcement for any one aspect of concrete breakout resistance, it is then appropriate to rely on anchor reinforcement for all aspects of concrete breakout resistance. It makes no sense at all to me that one would go down this path:

a) Deal with one aspect of breakout -- lets say tension -- using reinforcement which inherently assumes that the concrete breakout failure has already occurred.

b) Deal with another aspect of breakout -- lets say shear -- assuming that the concrete is uncracked in the failure frustum sense.

I understand that failure frustums for different aspects of load resistance are often different but, then, can anyone claim to reliably understand how they interact such that one could be considered in isolation of the other? Not this cowboy.

In taking the approach that I've described above, I feel that the interaction is moot as far as breakout mechanisms go. Supply independent reinforcement for the various modes of concrete breakout and don't worry about interaction except in the unlikely even that you're using rebar for multiple purposes and double dipping so to speak.

 
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