Reinforced Concrete Breakout in Tension and Reinforced Concrete Breakout in Shear Interaction

[r-struct]

In designing baseplate connection to concrete utilizing the Widianto paper 'Design of Anchor Reinforcement in Concrete Pedestals' or Annex/Appendix D, is there a need to check the interaction of Tf/Tr + Vf/Vr <= 1.2, where Tr=tension resistance based on reinforced concrete breakout surface, Vr=shear resistance based on reinforced concrete breakout surface or by STM, Tf and Vf = factored tension and shear forces, respectively? Wondering how the more experienced people of this forum typically handle such cases where the design is governed by reinforced breakout surfaces in both tension and shear.

 

[Agent666]

In this case you still use the interaction equation you noted. It's use is not dependant on only one of either shear or tension breakout governing and another non breakout limit state governing (like a steel based failure).

https://engineervsheep.com

 

[KootK]

I've mostly been using the interaction equation for this thus far. However:

1) My sense of things is that the interaction equation is really intended to handle cases where both the tension and shear resistance mechanisms rely on concrete in tension. I question its applicability for something more like the Widianto stuff where everything is handled via rebar and STM. The interaction equation looks very much like the interaction equations developed for nelson studs and bolts way back when and, presumably, is based on similar research that did not include the effects of reinforcing.

2) If you are using reinforcing and STM-adjacent methods alone for both Tf & Vf, you'd have a hard time convincing me that the quantity of reinforcing specified needs to satisfy the interaction equation. For that, I'd just be using phi=0.75 or whatever jives with your local STM code. Of course, I'd also be keeping an eye out for rare situations where the tension and shear mechanisms made use of the same ties.

3) Were I to encounter a situation that I felt warranted it, I would not be opposed to designing the whole thing with STM, the STM phi factors on everything, and paying no heed to the the interaction equation. This could get a bit tricky in practice, however, since you'd then have to be careful about situations where the tension and shear resistance mechanisms would make use of the same struts, nodes, or ties. This is really just a different way of handling the interaction effects. That said, for a lot of basic anchorage problems there often is not a whole lot of double dipping on the STM components so there may well be some efficiency gains.

 

[r-struct]

Thanks Agent and Kootk for your responses!

For the usual case of columns on pedestals, IMHO, if both are relying on rebar, the tension breakout and shear breakout capacity could potentially (anyone reading this, please take this with a grain of salt) be treated without interaction, given that the following were checked or can be assumed:

1. Individual anchor pullout strength and anchor tensile strength, do not exceed capacity.
2. Individual anchor shear strength does not exceed capacity.
3. Utilization ratio between the two failure modes, whichever is greater, in Item 1 and utilization ratio of Item 2 does not add up beyond 1.2 (i.e. interaction is still checked on the individual anchor capacity in both tension and shear).
4. Tension sideface blowout (by providing enough edge distance and bearing plate area in between the double nuts at the bottom of anchor rod) and shear pryout are precluded consistent with the recommendations of the Widianto paper.

In the sketch below, it seems that the blue ties for shear breakout and the red vertical bars for tension breakout do not appear to involve double-dipping in capacities. I currently cannot find anything, however, that says interaction is not required for the reinforced breakout failure modes that it appears we may have to stay on the side of caution and make sure their combined utilization ratio does not exceed 1.2.

One other thing I would like to see more experienced views is on the handling of shear breakout reinforcement. I've seen various ways of accounting rebar to provide shear breakout strength: (a) do you count all of the blue ties that is crossed by the green shear breakout surface and assign them their full yield strength, similar to concrete beam stirrup design?, or (b) do you perform STM similar to the Widianto paper and limit the tensile stress of the blue ties with hooks to 20ksi or use whatever tensile pullout strength of the ties with hooks?, or (c) count only the blue ties that are close enough (i.e. 0.5ca1 or 0.3ca2, whichever is lower) to the anchors as per Appendix/Annex D?

Looking forward to your responses. Thanks!