Anchorage reinforcement ACI cl17

[azulazul]

Hi all,

Just a few questions below:

1. With the 2 U bars shown below on ACI, would that count as 4x Astxfy or 2x Ast x fy for tension capacity of the tension reo?
2. Can I just provide sufficient tension reo on one side of the failure plane (not both sides symmetrically). Using the same example from ACI below, the 2 u bars will be on one side of the bolt group only, taking the tension.
3. Instead of U bars, can we use straight bars in the top matt (perpendicular to bolts) that are developed beyond the failure zone. The load I assume would be N*/Tan35 ?

Thanks,

 

[RabitPete]

1. 2 U bars count as 4x Astxfy for tension capacity, assuming they are fully developed on both sides of the failure plane
2. Nope, that will not work well. Crack will open up on unreinforced side, and resulting moments will likely cause anchorage failure
3. Horizontal bars in the top reinforcement layer do little to nothing to resist anchor pullout forces, regardless of how far they extend beyond failure plane. Those are there to resist bending

 

[azulazul]

Thanks for your response @RabitPete. Would you mind explaining further the mechanism for item 3, with the straight bars? I was thinking if the vertical vector could transfer to a horizontal one and if we have the bar developed past the failure zone it could work?

 

[Agent666]

Tests show no/limited enhancement for #3 with bars perpendicular to the vertical force. So the anchor theory discounts any contribution because it is marginal at best.

https://engineervsheep.com

 

[azulazul]

Thanks for your response, I tried to look for tests/research on this but couldn't find it. Do you mind sending me a link for it please? @Agent666

 

[Agent666]

If you look into fib FASTENINGS TO CONCRETE AND MASONRY STRUCTURES (1997) report, they briefly discuss that closely spaced bars with stirrups do show an increased resistance to tension even if you're just past the bars with a mechanical anchor.

So fixing into a beam or column basically between longitudinal bars. But for say slabs with relatively widely spaced bars the effect is not worth worrying about and virtually non existent.

You're basically talking about shear friction right out at the edge of the concrete breakout surface right? I think this is a flawed approach given that the bars are generally located with min cover to concrete surface and you're not mobilising the bars before the concrete failure cone has occurred.

https://engineervsheep.com

 

[hardbutmild]

As Agent666 already mentioned you should not rely on horizontal reinforcement to transfer that load. How would that even work? If you try to follow the forces (imagine there are no vertical U pins) you'll see that when the diagonal force reaches top horizontal bar it needs to somehow be transferred. Horizontal component can be taken by horizontal reinforcement, but if you try to use the same reinforcement to transfer vertical load you're relying on a dowel action of that bar. This can not actually function because dowel action still requires concrete, bar needs concrete above (just imagine timber with a steel rod, rod can not be to close to the edge or the edge will simply rip. this is the same case and the edge will most definitely rip). This problem is actually similar to punching shear and in punching shear design if no vertical reinforcement is required, usually horizontal BOTTOM reinforcement is required to avoid progressive failure because top reinforcement will just unzip. This bottom reinforcement could maybe be able to resist this force, but vertical reinforcement is still better in this case.

 

[azulazul]

Thank you both for your response. So if we had a middle layer of reinforcement (for whatever reason) there will be sufficient concrete to mobilise the bar and provide shear friction to the failure plane?

@harbutmild For the punching shear example, wouldn’t the top bars provide the shear resistance as the failure cone is inverted compare to that for pullout for an anchor?

 

[azulazul]

Also, if we look at section AA in the ACI reference above, can we consider the layer perpendicular to the top most layer to resist in shear friction as the top layer will sort of restrain it from popping out?

 

[Agent666]

No.

https://engineervsheep.com