ACI 318-14 Anchor Strength Concrete Breakout Calculation

[jreit]

Hello. I am checking a calculation where the concrete breakout strength is being calculated for an anchor group.
The image shows a sample plan view but the concrete section extends in the vertical direction.
The section is resisting a moment that is being resolved as a force couple with the concrete on the tension face being evaluated and hef = 32".

My issue is that the minimum concrete distance appears to be 3" which is much less than hef. As per ACI 17.4.2.3, if anchors are located as less than 1.5hef from three or more edges, then hef should be calculated as the smaller of max. edge distance/1.5 or max. spacing/3. This gives me a hef of 2" or 4" respectively.

However, using this value in Eq. 17.4.2.2a gives a Nb value of ~ 6.8 kips which is extremely small compared to the loading.

How should I treat this condition? Should my concrete breakout area be considered as the distance from the tension face rebar to the near edge (3") and far edge (19") and ignore the compression face rebar altogether?

https://imgur.com/uhL70P1

 

[Celt83]

with Hef = 32" sounds like they are using reinforcement to restrain the breakout. If that is the case then they need to make sure and develop the bars above and below the break out plane. In the reinforced instance it is my understanding that you don't reduce Hef per 17.4.2.3, which would greatly limit the ability to develop the reinf. beyond the breakout plane, because you are no longer relying on the concrete failure modes for the capacity and reading the commentary associated with 17.4.2.3 the reduction on Hef is actually intended to get you a better capacity because how the equations are currently set up you would get a conservative value as your Anc/Anco ratio becomes very small when influenced by 3 or more edges with deep embedments.

Edit: or if they are truly using #6 bars as the anchors they are playing with the development loop hole and providing full tension development of the bars into the concrete in lieu of going thru appendix D.

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[jreit]

Thanks Celt83. I should have mentioned that this is an existing backwall on a new abutment. The new rebar in the CIP abutment will be vertically drilled and grouted into the backwall. I am concerned about the breakout since there is only 3" of cover and large tensile forces. It is fully developed in the existing backwall.

Is designing and detailing the bar as a typical concrete section enough to forgo the ACI 318-14 Chapter 17 checks for pullot/breakout etc?