Pier-Footing Moment Connection, Appendix D?
Pier-Footing Moment Connection, Appendix D?
(OP)
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.






RE: Pier-Footing Moment Connection, Appendix D?
maybe a sketch would help.
RE: Pier-Footing Moment Connection, Appendix D?
RE: Pier-Footing Moment Connection, Appendix D?
RE: Pier-Footing Moment Connection, Appendix D?
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.
RE: Pier-Footing Moment Connection, Appendix D?
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.
RE: Pier-Footing Moment Connection, Appendix D?
RE: Pier-Footing Moment Connection, Appendix D?
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.
RE: Pier-Footing Moment Connection, Appendix D?
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.
RE: Pier-Footing Moment Connection, Appendix D?
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.
RE: Pier-Footing Moment Connection, Appendix D?
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.
RE: Pier-Footing Moment Connection, Appendix D?
RE: Pier-Footing Moment Connection, Appendix D?
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.