Concrete edge breakout for multiple anchors in a single row (drag load) 2

[StrEng007]

I don’t know the best way to go about introducing this topic, so I’ll give a specific scenario before I try to explain the applicable part of ACI Chapter 17.

Let’s say you have a load that is being dragged along a concrete beam and you’re providing post installed anchors to resolve that loading. In this scenario the beam is an 8”x24” concrete beam, and the anchors are provided along the centerline of the 8” dimension (top of beam).

The governing load failure for a single anchor in shear (parallel to length of beam) ends up being concrete edge breakout. This can achieve a strength design reaction of 2,000 lb.

Per Hilti Profis analysis, when providing groups of anchors loaded in the same scenario, the load capacity is also 2,000 lb regardless of the number of anchors.

Single anchor fails from concrete edge breakout at 2,000 lb.

3-anchor group fails from concrete edge breakout at 2,000 lb.

5-anchor group fails from concrete edge breakout at 2,000 lb.


I’m not sure of the right way to interpret this information. For instance, if you had a single angle and single plate using this attachment, you’d develop the 2,000 lb reaction. If 2 ft away, you had another anchor on a separate plate, I believe you’d achieve another 2,000 lb reaction. However, when you put these anchors on the same plate, you get limited to a single breakout capacity based on a single anchor (regardless of the number of anchors). This is due to the limiting geometry perpendicular to the edge, all described through Avc and Avco per ACI 17.7.2.



So, what is the right answer? Do I base my numbers of the single anchor, and determine the required field spacing? Limiting multiples groups of anchors to a single anchor's concrete edge breakout seems too conservative.

The reason why I even attempted to put these anchors on the same plate was to make sure their proximity to each other didn’t have some sort of influence.

 

[StrEng007]

KootK:
But yes, longitudinal top bars can be used to restrain the shear breakout of the front anchors if the detailing checks out

How exactly does this work, or look? Profis seems to base the calculation on breakout of the first anchor row, unless I set the distance to infinity. Oddly enough, when it's not infinity but some really large number, the breakout provision still applies. For instance, when the edge distance is set to 60 in, that makes the initial ca1=60 in

If anchors are located in narrow sections of limited thickness such that both edge distances ca2 and thickness ha are less than 1.5ca1, the value of ca1 used to calculate
AVc in accordance with 17.7.2.1.1 as well as for the equations in 17.7.2.1 through 17.7.2.6 shall not exceed the greatest of (a) through (c)
•1.5ca1 = 1.5 x 60 in = 90 in
•ca2 (8 in wide beam) = 4 in, 4 in < 90 in
•ha = 24 in, 24 in < 90 in


(a) ca2/1.5, where ca2 is the greatest edge distance
(b) ha/1.5
(c) s/3, where s is the maximum spacing perpendicular to direction of shear, between anchors within a group
•ca2/1.5= 4 in / 1.5 = 2.67 in
•24 in / 1.5 = 16 in
•s/3, DOES NOT APPLY IN THIS CASE
So from all of this, the critical edge distance is 16 in per ACI, while the actual edge distance is 60 in. So how does this breakout actually look or occur?
Is it like this? If so, I have two follow-up questions:

1) If the entire section fails through the depth of the member like this, why then does it change when the edge distance is set to infinity versus 60 in or 200 in for that matter?
2) Would it be the longitudinal bars that provide the required shear friction to prevent this breakout plan from slipping past itself? (I know I have post-installed anchors, but I'm still interested in how the supplemental reinforcing works.)
Is it like this? If so, I have one follow-up question:


1) To me, only the stirrups would prevent this breakout plane. Even so, I don't know how these stirrups would actually develop bc there isn't enough width for the stirrup to develop as a hooked bar. (I know I have post-installed anchors, but I'm still interested in how the supplemental reinforcing works.)

Tomfh:
If we're concerned about brittle edge failure, I agree. But if we have reinforcement, then additional anchors can be mobilised.

To the point made by Nick6781, this reinforcement only applies for cast-in-place anchors. My situation is post installed so it wouldn't matter, right?

Tomfh:
The PROFIS software seems to distribute the load across multiple anchors when calculating the pryout load per anchor, as opposed to edge breakout where it spreads the load across the outer anchors only.

Correct, the calculation considers evenly distributed loads for all scenarios except the edge breakout. Even if I try to utilize the first (3) anchors instead of the first row, the governing case of ca1 applies at 16" due to section 17.7.2.1b

 

[Tomfh]

To the point made by @Nick6781, this reinforcement only applies for cast-in-place anchors. My situation is post installed so it wouldn't matter, right?

It’s stating that, in practice, this generally applies to cast-in anchors because, with drilled anchors, the exact location of the reinforcement isn’t always known.

It’s not suggesting that post-installed anchors are fundamentally incapable of benefiting from reinforcement—just that the effectiveness depends on reinforcement placement, which you don’t necessarily know when post installing anchors.

In your case, the longitudinal bars will intercept the edge breakout crack, as shown in post #3 by CDLD.

 

[StrEng007]

So, what you're saying, is if the longitudinal bar DID NOT EXIST the failure of the beam shown in the OP, with an long edge distance would cause the beam to break out like this?





Take this same scenario and rotate it so the load was pushing down to a footing. It still breaks out like this?

 

[icebloom]

If you want to ensure load sharing between all the anchors you can use HILTI filling washers or similar, which allow injection of mortar or epoxy into the gap between anchor and baseplate allowing engagement of all anchors. Then your shear force per anchor will be something like V/n where V is the total load and N is the number of anchors, rather than the shear force all being concentrated at the worst anchor for edge breakout purposes as per the code provisions.

 

[icebloom]

I was considering the edge breakout of the front anchor, as that appears to be the governing factor in this case, which we seem to agree on, regarding the longitudinal bars catching it.

However, regarding pryout, does the same principle of no load sharing apply? The PROFIS software seems to distribute the load across multiple anchors when calculating the pryout load per anchor, as opposed to edge breakout where it spreads the load across the outer anchors only. I take your point that in theory pryout is brittle too, like edge breakout is.

This kind of load sharing for pryout is something not really covered in the codes or in softwares like HILTI or Idea Statica as far as I know, however in the fib design guide 58 on concrete anchorage there is some more information on this. You can also find newer info in the following paper (open access, https://www.researchgate.net/public...R_ANCHORAGES_WITH_SUPPLEMENTARY_REINFORCEMENT). FIB bulletin 58 you can probably find with a google.

They both show that technically in the case where anchor load sharing occurs (eg welded anchors, filled holes as noted above, etc) or supplementary reo is used and failure cracks originate at the back anchors rather than anchors closest to the edge, some anchors near the edge will not be effective in the resistance of pryout failure. In fib 58 this is based on the idea that if the failure crack originates at the back anchors and supplementary reinforcement is activated, load transfer across this crack in terms of resisting pryout can no longer occur. The paper linked above updates this based on some more recent testing and says that a larger pryout capacity than what would be predicted by fib 58 is possible. In both however, some anchors are considered ineffective.