The Age Old eng-tips Debate of Rebar Anchorage vs Rebar Development Length 7

[dauwerda]

As many on this forum know, there are a couple of previous threads that address the debate of anchorage vs development length for rebar that KootK has done a great job of spearheading. Here are a few of them:
thread507-401855
thread507-379988

What I didn't know is that ACI 318-19 now addresses this to a certain extent, I don't think this has been noted here before (I didn't find anything in a quick search) so I thought I would share.

17.1.6 Reinforcement used as part of an embedment
shall have development length established in accordance
with other parts of this Code. If reinforcement is used as
anchorage, concrete breakout failure shall be considered.
Alternatively, anchor reinforcement in accordance with
17.5.2.1 shall be provided.

R17.1.6 Concrete breakout strength in tension and shear
should be considered for reinforcing bars in a group used
as anchorage. Concrete breakout behavior can occur even
if reinforcement is fully developed in accordance with
Chapter 25. Breakout behavior of straight reinforcement as
a group is analogous to tension and shear breakout behavior
of adhesive anchors whereby hef is taken as equal to or less
than the embedded bar length. Similarly, breakout behavior
of hooked and headed reinforcement groups is similar to
tension and shear breakout behavior of headed anchors.
Consideration should be given to extending bars beyond the
development length.

​

As an alternative to explicit determination of the concrete
breakout strength of a group, anchor reinforcement provided
in accordance with 17.5.2.1 may be used, or the reinforcement
should be extended.

 

[Greenalleycat]

Am I correct that this is only really relevant for the case where you have an unreinforced block of concrete with a bunch of rebar sticking out of it?
I would have thought that any normal application would have a bunch of reinforcement in the slab (or whatever it is) that completes the load path?

 

[KootK]

Yes!!! Hell yes!! And I've been waiting for vindication on this for the better part of a decade now given that most of those threads tended to have a "KootK vs The World" character to them. Celt83 also turned me on to the 2020 paper below which basically supports the same conclusions. One of the sketches in that paper looks so very identical to the "ridiculous" sketch that I posted here back in 2015 that I almost wonder if researchers don't troll this forum for good research ideas.

Suck it world!! Sorry, couldn't resist...

 

[skeletron]

KootK: did you ever catch one of your sketches (and a thread) being included in an AISC presentation? I think it was on detailing or something, I'll try to go back into my way-back-timemachine(photographic memory) and dig it up.

 

[KootK]

KootK: did you ever catch one of your sketches (and a thread) being included in an AISC presentation?

Really? No, I did not catch that.

A friend recently forwarded the promotional email below to me, pointing out that the subject matter seemed eerily similar to a recent discussion here: post tension zone of influence.

Possible explanations for this stuff:

1) Total coincidence.

2) We're all somehow attuned to a common zeitgeist of sorts.

3) Lurkers get some good ideas from Eng-Tips.

One thing that I can say with certainty is that the population of lurkers on this forum grossly out numbers the population of active contributors. And structural is, by a wide margin, the most active forum on this website.

 

[WARose]

Great stuff. I kind of always suspected the shear part of that (near a edge)......just wasn't sure about the tension part.

The "embedment" vs. "anchorage" part has me confused though. (I.e. what is the difference?) Is one the transfer of forces between two concrete parts and the other is anchoring something else (like a steel connection)?

Code verbage throws me for a loop sometimes.

 

[DayRooster]

I’m with KootK on this one.

That being said if you have a long skinny column with rebar developed into it then I hope you also have primary rebar that would be developed and cross the concrete breakout plane. In addition that column should hopefully have ties that would cross the shear breakout plane.

 

[Greenalleycat]

@WARose, in my head I would use anchorage for describing rebar, and embedment for describing cast-in fixings or post-fixed anchors. Think it probably comes from the way codes describe these situations? They’re pretty equitable situations but with different maths etc governing them I guess

 

[Settingsun]

At the risk of over-simplifying:

I didn't read those other discussions to see if this had been said already.

 

[KootK]

The "embedment" vs. "anchorage" part has me confused though. (I.e. what is the difference?) Is one the transfer of forces between two concrete parts and the other is anchoring something else (like a steel connection)?

Much to my chagrin, the ACI provision doesn't address the transfer of forces between two concrete parts. It only covers embedded hardware with attached rebar and divides that into two subcategories:

1) Rebar not used for anchorage and, therefore, not subject to concrete breakout checks and;

2) Rebar used for anchorage and, therefore, subject to concrete breakout checks.

As you can imagine, I would wish for a broader statement that does cover the transfer of forces between concrete parts. You gotta start somewhere though, right?

It will be interesting to see how this plays out for connections like that shown below which are common in a lot of applications, including the nuclear, precast, and tilt-up worlds. I would think that the range of application for the deformed anchors in such spaces would diminish significantly given that:

a) One can no longer use such anchors to obviate the need for tension breakout checks and;

b) We don't really have provisions allowing us to assess groups of anchors containing different types of anchors.

In most applications like that shown below, I suspect that the deformed bar anchors will just get replaced with more studs even if, in reality, that produces a weaker connection. It may wind up being a bit like the current situation with through bolting: we all like it, but we don't really know what to do with it calculation-wise. At least I don't.

 

[WARose]

Good post Kootk.

 

[bones206]

I typically use the DBA values per the mfr's ESR report. For example:

https://icc-es.org/report-listing/esr-2907/

I haven't seen any that address bent or hooked DBA's, unfortunately, even though they do mfr them.

 

[Brad805]

This seems helpful to Koot, but beyond that what jumps out to me is "shall be", and "analogous to." I have modeled connections like koot's sketch in NLFEA software, and can generate different load distributions based on a few key assumptions. Find me 10 engineers and I will get you three to five similarish opinions and the others will all be different on this topic. I don't think this moved the topic much. I am baffled why it is not studied more given the importance to so many industries.

Bones, do you have a link to the pre-bent DBA's? I would like to see the accessories needed for the stud gun, and different lengths. The ones we have used get bent after, and that does not always work the way we think.

 

[bones206]

Nelson catalog pgs 29, 30 offer pre-bent DBA's. They say they use a special chuck for welding.

https://www.stanleyengineeredfasten...ud_welding_stud_ferrule_accessory_catalog.pdf

 

[Brad805]

Thanks bones.

 

[bones206]

One workaround for the bent DBA is to calc it as you would for a J-bolt anchor, with respect with Appendix D requirements… Btw I enjoy that the fact I keep stubbornly referring to it as Appendix D

 

[slickdeals]

Amit Varma and his colleagues have done a lot more research and testing related to this. I wouldn't be surprised if something new comes out of that.

https://www.youtube.com/watch?v=CwLqnGACq_8&t=1543s

 

[bones206]

Interesting research, thanks.

On a nuke project I worked on there were standard designs for embeds with DBA’s. The anchor capacities were based on the Nelson ESR allowables, but were also enveloped with Appendix D calcs and were long enough to develop Ld as well. The backup calcs were super rigorous, even had a full blown Ansys model to look at local spalling under shear load. The funny thing is the drawings had an “alternate detail” showing 12” long bent DBAs for walls that were less than 26” thick (most walls). Yet there was no backup calc whatsoever for that alternate detail. Curious if this new research will prompt any design audits at these plants where the hooked DBA’s were used.

 

[Brad805]

Great study slickdeals. Tks.