Pier-Footing Moment Connection, Appendix D?

[pioneer09]

So I have a building column on an exposed 4' pier that is attached to a footing. Column connection to pier is only shear and design would be based on Appendix D of ACI. Where I am drawing confusion is with the pier to footing attachment. As I have rather large tensile forces that I am developing in the vertical bars of the pier, wondering how these bars should be checked both at the pier location and at the footing. Appendix D of ACI has sections for steel strength, concrete breakout, pullout strength, and side-faced blowout (which is not an issue in this case). Per App. D, these equations are really only for anchors. Does this point me to Chapter 12 for development lengths for the design of the "L" reinforcement bars that extend from the footing into the pier? Seems reasonable for tensile strength of steel and pullout, but what about breakout strength at the footing? Any help is appreciated.

 

[JStructsteel]

What do you mean would only bea shear design, but then you say you have rather larage tensile forces? Is this a column installed after the pier is poured?

maybe a sketch would help.

 

[pioneer09]

Column sits on top of pier with an assumed pinned connection. Tensile forces are developed in pier as it want to rotate. See attached sketch.

 

[sandman21]

Your pier does not fall under App. D. It would be designed like any other concrete element.

 

[KootK]

Seems reasonable for tensile strength of steel and pullout, but what about breakout strength at the footing?

I believe that you're spot on in your assessment here. The vast majority of engineers will miss this issue, mistaking this for a development situation when it's really an anchorage situation with potential for breakout (just like APP D stuff). It's a hotly debated issue here. Mostly other folk debating it hotly with me.

So, enough with the back story. What should one do? That part's harder. To my knowledge, there is no definitive method for assessing the breakout potential of embedded rebar. Which, when you think about it, seems utterly ridiculous given that it's surely the worlds most common anchorage problem.

In the course of a previous debate here, I created a mathcad sheet to evaluate a single bar being pulled out of a mass of concrete with no edge distance or spacing issues. I played around with treating it like an App D hooked bolt, a headed anchor, and a post installed straight anchor. At least I think that's what I did. It's been a few years. My conclusion was that you'll pretty much always yield a developed single bar prior initiating concrete breakout in this situation.

However, I believe that you can induce breakout prior to yield when you're dealing with tightly spaced groups of bars. And that makes nothing but sense. Lots more force delivered to an only marginally improved breakout frustum. This concern will be an issue wherever bars are tightly spaced. Shear wall zones are a good example. A densely reinforced pier with moment might be another.



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.

 

[KootK]

Link

That's one of the better discussions on the issue. As you'll see, people tend to feel threatened by this. Obviously, nobody wants to feel as though they may have been doing it wrong all along. I'll do my best to refrain from a protracted debate here as I'm pretty sure that I expressed all of my opinions on the matter in the other post, in spades.

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.

 

[JStructsteel]

I cant say I have ever seen that failure mode. Is it we as engineers have been lucky, or that it does not fail that way, if properly designed, tight spacing or not? I dont think there are code requirements that need to be satisfied. If out of curiosity you want to check as KootK suggests, by all means you should.

 

[KootK]

I was able to track down the result of my MathCAD work from this thread: Link. That was a mightily long thread so I'll save you some effort by just reproducing it here. It was for a slightly different situation: retaining wall bars "achored" rather than being designed/detailed as RC concrete. For what it's worth, it's not a practice that I endorse.

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.

 

[KootK]

Based on your sketch, your compression force will land on top of your breakout cone. That helps although not as much as one might expect. The source European docs for App D provide a method for accounting for that. I'm not sure if it's made it's way into 318 yet.

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.

 

[Once20036]

KootK - I share your skepticism the geometry you posted.
To make things a little more confusing, it seems that you can now also ignore these failure mechanisms with post installed/epoxied bars.

I had a situation recently where the contractor poured a footing but omitted the top reinforcing bars (column experienced a net uplift and the bars were necessary).
We worked with them to develop a solution that involved adding the bars and thickening the existing footing. I needed some shear friction bars, fully developed on each side of the joint. I called Hilti's engineers and they pointed me to their ICC report for HY-200. In that report is table 25, which provides a required development length, in accordance with ACI 318-11 12.2.3.

So if I took a threaded rod and embedded it 14.4" into concrete, I`d have 6 pages of calculations and a horrible capacity if I was near any edges/other bars/etc.
Per Hilti, if I did the same thing with a #4 bar, I'd have the full tensile capacity of the bar.
How does that make any sense!?

On the other hand, there is an enormous history of suitable performance of reinforced concrete structures based on a trust of development length.
In my situation, I ignored Hilti and used appendix D.

 

[WARose]

Like anything else, I think it boils down to load path. For the re-bar the OP shows (in the 3rd post on this thread).....that (in my opinion) does NOT fall under Appendix D. For starters, it's a different failure mechanism. The force is transferred along the bar's length......as opposed to anchors where theoretically there is no bond along the length and it transfers by bearing at the ends. That's a whole other ball game for initiating failure. If the concrete you are going into is properly reinforced and detailed, there should be no issue.

That being said, I have wondered over the years about Appendix D being applicable to vertical re-bars transferring shear near an edge. That's very close to the same failure mechanism as the anchor bolts. So I'm careful to have horizontal steel or check it as per Appendix D.

 

[pioneer09]

Thanks for all the helpful opinions on this matter. It seems this subject area is up for debate among many engineers. I can see both sides of the spectrum and will probably use a hybrid to develop a design that per engineering judgment seems reasonable.

 

[KootK]

@Once: if your situation was a shear friction problem, then I may know the answer to that. Shear friction is a bit different. The bars deliver tension into the concrete either side of the SF joint. Concrete in tension in the breakout areas is able to react against the concrete on the other side of the joint, however, so concrete breakout strength isn't required to keep things in check. I worried about this for years before getting a handle on it. Without this, pretty much every infill shear wall retrofit would be junk.

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.