Tension and Shear on rebar

[StrEng007]

Does anyone know of an interaction equation for tensile and shear force on a rebar? The bar is being used to resist a force through it's tensile capacity, which is developed with the proper ld. Meanwhile, the bar also crosses a shear plane and acts as reinforcing per ACI 318-11 D.6.2.9.

 

[StrEng007]

I can't upload a sketch at this time.

 

[calvinandhobbes10]

Design---use separate bars for tension and for shear
Analysis of existing---unity equation

 

[KootK]

My suspicion is that your bars aren't actually being utilized in shear. I'll need to see that sketch to be sure however.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[StrEng007]

KootK, you are correct. The fv I'm looking at is actually a tensile force in the steel bar.

 

[StrEng007]

KootK,
I'm using a hairpin to engage slab reinforcing, or the slab ties shown below, in order to alleviate the lateral load from the "kick" on a PEMB. If my hairpin legs are long enough, I can engage enough slab length.

The same "kick" reaction from the PEMB is greater than the capacity of my anchors per ACI Appendix D. I need to consider anchor reinforcing, and want to use a hairpin to cross the shear plane. Can I use (1) hairpin to counter both of those loads? Would you say they are the same load and I need to check each scenario separately? Or would you consider an interaction between the two?

See figures B & D:

 

[StrEng007]

I'm considering:
Case a) 0.9*As*Fy to develop the tie-back into the slab.
Case b) 0.75*Av*Fyt to develop the shear resistance to concrete breakout

As and Av are the same member. I'm not sure if there needs to be an interaction equation between the two.

 

[KootK]

Your latest approach was precisely what I was planning to suggest. No interaction.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[StrEng007]

KootK,
One last question for you. Similar to the combined effects of ΦVc and ΦVs for shear in beams, can you rationalize a combination of ΦVcbg (shear breakout) and ΦVn (shear reinforcing)? Does ΦVn have to take 100% of the load if present?

I don't think ACI addresses this, possibly from a lack of research on the matter.

 

[KootK]

Similar to the combined effects of ΦVc and ΦVs for shear in beams, can you rationalize a combination of ΦVcbg (shear breakout) and ΦVn (shear reinforcing)? Does ΦVn have to take 100% of the load if present?

This is something that I've wondered a lot about by myself. I don't have a definitive answer but, rather, some thoughts to share:

1) I've posed a similar question here my self. See this thread:Other Thread. The clip below is from it.

2) I know lots of folks do take Vs + Vc in these situations. I've done it myself in the past.

3) I don't currently take Vc in addition to Vs. I've come to believe that there's something fundamentally different about shear and tensile anchorage applications. Clearly, when you grab the entire concrete section, it's conventional shear. And if anchorage provisions are to be believed then, when you only grab a little chunk of the section, it's not conventional shear. Where in between those two extremes doe it switch over? I don't know.

4) Frankly, I've never seen a justification for why it is that we can add Vc to Vs in normal beam applications. Mobilizing Vs implies that the shear crack has already formed so I would have thought that it would be Vc OR Vs. The best rationalization that I can come up with is that, so long as the shear crack is kept small, the Vc capacity must be maintained post-shear cracking. That and the fact that some mechanisms are independent of the cracking (dowel, friction across the compression block...). This thinking is a big part of why I don't include Vc for intermediate situations any more. I feel that, without definite moment reinforcement to keep cracks small and develop the friction mechanism, it may not be valid to count on Vc.

More than you ever wanted to know and still no answer, huh?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[StrEng007]

KootK,
Its surprising how many concrete texts flat out say something to the degree of "We are still trying to truly understand shear in concrete... but in the mean time do this"....

 

[KootK]

It is surprising.

If you ever want an interesting read on shear, and how little we seem to agree upon, check this out: Innovative Shear Design. When I read the preface etc, I knew I had to give it a read. Heresy all the way. Or the truth? Who the heck knows.

I actually believe that we do more or less understand concrete shear these days via the modified compression field theory: MPC. It just doesn't seem to be making it's way into the dogma of design office concrete design for some reason, perhaps due it's relative complexity. Dr. Adebar, from the linked paper, has basically dedicated his professional life to figuring out concrete shear. I asked him one time if he felt as though he truly understood concrete shear now and he said, a bit tepidly, that he did. Encouraging.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[sandman21]

The phi factor for reinforcing used to resist anchor loads should be .75 not .9. ACI 318-08 D5.2.9

 

[Mixtli]

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