Headed stud vs. deformed bar as an anchor 1

[jimmytwotimes]

Here is a related former thread that I couldn't extract a clear answer from:

https://www.eng-tips.com/viewthread.cfm?qid=243978

Here's a link to illustrate my question:

https://imgur.com/a/BrR3i6n

On the left is a headed stud that we can say is governed by concrete tension breakout. On the right is a deformed rebar with an embedded length equal to ld per 25.4.2.3 of ACI 318-14.

Let's say I wanted to design an anchor for a wall. Why would I ever use headed studs if I can use rebar to get WAY more strength in tension. Furthermore, in thinner slabs, the edge distance effects kill the capacity of headed studs in tension. Meanwhile, the embedded deformed bar could maintain huge capacities even at very small edge distance (cb value in the ACI equation). This logic seemed crazy, so I reached out to a manufacturer that uses embedded rebar tails and under tension loading the capacity is based on concrete breakout (determined by testing).

So now my understanding is the development length formula for deformed bars is only valid to prove the full capacity of the rebar can be transferred into the concrete (ie. bond). But a separate check would still be required to determine if the local area of the concrete can handle the load imposed upon it by the rebar. I think using the checks for an adhesive anchor make more sense for embedded deformed bars, but due to lack of knowledge I'd probably take the worst case between headed stud and adhesive. Thoughts?

 

[dik]

Headed studs are convenient... also, with welding rebar, the rebar has to be 'special', so it is weldable, and there is a distinct welding procedure.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[jimmytwotimes]

dik - fair points. I suppose I should have done a better job of framing this as a question relating to just concrete strength. For my particular application the extra fab work is very small compared to having to beef up concrete members.

 

[dik]

They both work, and I've used both.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[KootK]

So now my understanding is the development length formula for deformed bars is only valid to prove the full capacity of the rebar can be transferred into the concrete (ie. bond). But a separate check would still be required to determine if the local area of the concrete can handle the load imposed upon it by the rebar. I think using the checks for an adhesive anchor make more sense for embedded deformed bars, but due to lack of knowledge I'd probably take the worst case between headed stud and adhesive.

That is the truth of things. Development <> anchorage in all situations. Somehow that misconception pervades our profession still.

 

[CURVEB]

You might consider if it is possible to lap-splice the deformed bar anchor onto a typical bar in the wall or slab. That may help with the breakout issue.

 

[kjm93]

@jimmytwotimes and @KootK - So you would embed the deformed bar a minimum of ld, and then still run concrete breakout calcs? It seems like the code is pretty clear those checks only apply to headed studs and adhesive anchors in tension, although I don't quite understand why there aren't breakout checks in the code for deformed bars in tension - a quick search shows that plenty of research has been done indicating that DBAs can indeed be controlled by concrete breakout. What about using strut and tie?

On a side note, I work with ASCE 41 a lot for seismic retrofits and this becomes especially challenging for force-controlled column anchorages, the breakout strength almost always controls and requires some pretty heavy strengthening.

 

[jimmytwotimes]

@CURVEB That's probably a good solution, but as a specialty engineer I am about 99% certain the building EOR would not be willing to coordinate that sort of engineering.

@BennyTheBeaver Yes I have a hard time understanding why breakout calcs are not included either. My initial thought was load transfer through a continuous bond vs. bearing at the head may be the main reason, but the breakout checks required for adhesive anchors stopped that line of thinking.

I'm looking at anchors for wall systems. These are typically at the edge of building slabs. I am not overly familiar with applying strut and tie method...maybe a stiff pour stop could be helpful. But that seems a bit academic at this point to me to be able to actually apply on a project.

 

[KootK]

So you would embed the deformed bar a minimum of ld, and then still run concrete breakout calcs?

Yes, I'd give some consideration to breakout unless the deformed bar was actually passing its tension to another reinforcing bar.

It seems like the code is pretty clear those checks only apply to headed studs and adhesive anchors in tension

I disagree:

1) The code pretty much silent on how breakout applies to embedded rebar. That's unfortunate, in my opinion, and probably because it's assumed that most developed rebar is used in true RC concrete designs where the bar tension just gets passed to another bar or resisted by a compression strut. If anything, I'd say that the code is ambiguous on whether or not breakout applies to embedded bars. I wouldn't call that a clear endorsement that embedded bars don't need breakout considerations when they're not passing their tension along to other reinforcing.

2) There are provisions for post-installed bars used this way and they do include breakout provisions. I'd call that something of a soft endorsement that breakout failure modes do, in fact, apply to embedded rebar.

What about using strut and tie?

Works for me. That said, in a lot of these situations, it's difficult to generate a complete STM without adding reinforcement that wouldn't normally be there. This is because anchorage relys on concrete tensile resistance and STM ignores that almost entirely. You kinda have to choose your own adventure in that regard.

 

[bones206]

There are also ESR reports for DBA anchors, which provide allowable loads. For example:

https://studweldprod.com/Downloads/ESR-2823 D-Bar.pdf

 

[slickdeals]

See attached document that describes the issue of breakout even when reinforcing is developed into concrete. There needs to be more work in this area, and looks like some of it is happening.

 

[bones206]

I believe the ESR reports are also based on tests. See below excerpt from ESR-2823:

 

[KootK]

They'd pretty much have to be based on testing that reflected concrete breakout modes. Otherwise there wouldn't be much basis for using spacing and edge distances as parameters in their tables.

 

[CivilStructural01]

I would be careful using rebar like that. Notice that they use tests to prove capacity, thats for a reason